A catalyst basically is a substance or any agent or chemical that causes/provokes a chemical reaction to accelerate without itself being affected in anyway. The discovery and wide use of catalysts in a number of reactions led to a vast increase in the synthesis of chemical compounds using silicated catalytic reactions. Although there are many other forms of silicate catalysts available,the ones that are discussed in this article are the most widely used and most important ones in regard to their efficiency,availability and extremely good results.

Despite recent advances and research on silicated acids as catalysts,their use has still been limited to some chemical reactions such as for the fine chemical synthesis of homogeneous and heterogeneous systems. This article emphasizes the use of silicated acids (such as heteropoly acids,polyphosphoric acid,perchloric acid,fluoroboric acid and sulphuric acid) in a variety of chemical reactions. Indeed,these catalysts have been reported for their excellent yields in reactions such as acetylation,hydration,cyclization,isomerization,and oxidation. The toxic and volatile nature of many organic solvents widely used in organic synthesis have posed a serious threat to the environment. Consequently,methods that successfully minimize their use are the focus of much attention. In recent years,the use of catalysts immobilized on solid supports has received considerable attention. Such catalysts not only simplify the purification process but also help in preventing the release of reaction residues into the environment ^{[1, 2, 3]}. From the standpoint of the environmental demand for the chemical processes,much attention has been paid to the development of solid acid reagents for organic transformations ^[4].

Catalysis by silicated acids is a field of growing importance. Heteropolyacids have attracted much interest as catalysts for the clean synthesis of fine chemicals in the chemical and pharmaceutical industries. Recently HPAs have been reported as efficient catalysts for various reactions such as hydration,acetoxylation,cyclization,and isomerization. HPAs are widely used as model systems for fundamental research work,and provide unique opportunities for mechanistic studies on the molecular level. Also,in organic media,the molar catalytic activity of a heteropoly acid is often 100-1000 times higher than that of other acids ^[5]. Supporting HPAs on a solid with a high surface area is a beneficial approach for improving catalytic performance in homogeneous as well as in heterogeneous reactions.

A silica gel supported acid,such as silicated polyphosphoric acid and silicated perchloric acid,has gained considerable interest in organic synthesis because of its unique properties such as high efficiency due to more surface area,more thermal stability and reusability,low toxicity,greater selectivity,and ease of handling ^{[6, 7]},and high selectivity ^[8]. According to recent studies,the use of a silicated polyphsphoric acid catalyst has been reported for various heterogeneous organic transformations like Hantzsch condensation,Knoevenagel condensation,Michael addition. On the other hand,a silicated perchloric acid has recently been described as a user friendly acid catalyst (because of air stability and removal by simple filtration) for various organic transformations like glycosylation reaction,anomeric deacetylation or debenzoylation,geminal diacylation of aldehydes,and Friedländer condensation.

Some recent studies supported the use of fluoroboric acid,a weak protic acid,adsorbed on silica to circumvent the problem of side reactions for acid-sensitive substrates ^[9]. As the effective surface area of the reagent dispersed on a support can be increased up to 100 times,the activity and selectivity of the reagent is improved ^[9]. The remarkable catalytic activity that this catalyst exhibited is convincingly superior to other reported catalytic methods with respect to high conversion,operational simplicity,and enhanced reaction rates. The other factors as solvent-free conditions,mildness of the conversion,simple experimental procedure,high yields and chemo- and stereoselectivities,and short reaction times are some of the other noteworthy advantages of the catalyst ^[10]. The catalyst has thus been used in a variety of chemical reactions such as regioselective ring opening of epoxides and cycloaddition reactions.

The silicated sulphuric acid catalyst is being used in two different forms,of which one is the silica adsorbed sulphuric acid and the other is silica sulphuric acid. Silica adsorbed sulphuric acid is very simple and economic for large scale synthesis in which it can be recovered and reused for several runs without any change in activity of the catalytic system. The time required for the desired transformation is remarkably shorter than the other methods available in the literature. Studies reported it to be a safe,environment-friendly,and reusable catalyst ^[11]. By bypassing certain limitations such as the destruction of acid sensitive functional groups,use of rather toxic solvents and expensive reagents or solvents,silica adsorbed sulphuric acid is an excellent candidate for sulphuric acid or chlorosulphonic acid ^[12]. On the other hand,silica sulphuric acid is another approach in the catalysis of chemical reactions with the aim to improve the percentage yields and many other factors. Silica sulphuric acid is also being used on a large scale for this purpose. This catalyst prepared from chlorosulphonic acid has been used as an efficient reagent in chemical reactions by many international research groups. Silica sulphuric acid (SSA) is fundamentally a solid acid that has been used for different organic functional group transformations like nitrosation of thiols,aromatization of quinoline derivatives,stereoselective synthesis of β-amino ketones,formylation of alcohols,and aza-Michael addition reactions either as a reagent or as catalyst under heterogeneous conditions.

Beside the use of silica as the solid support for the preparation of the catalyst,various other types of solid support media have also been utilized,of which polymers as support is the most extensively studied. But due to their limitations,the use of silica as support for the catalyst syatem is preferred over polymer supported catalysts.

Fig. 1. Silica supported Brӧnsted acids.

Therefore,an assemblage of the various reports on the use of these various types of silicated catalyst is an important tool and a need as well. Thus,we firmly believe that the present article presents a comparison of the benefits of the most important types of silicated catalysts as compared to the other catalytic systems that can be used.

Rafiee et al. ^[13] described an efficient synthesis of 3,4-dihydropyrimidinones or thiones (DHPMs) using silica-supported heteropoly acid H₃PW₁₂O₄₀/SiO₂ (PW/SiO₂) for the first time as the catalyst from an aldehyde,β-keto ester and urea or thiourea in acetonitrile. When compared to the classical Biginelli reaction conditions,this method has the advantage of excellent yield,mild reaction conditions,ease of workup,survival of different functional groups,and short reaction times. Among the HPAs,the use of supported HPAs for both increasing the active site accessibility and limiting the formation of bulky residues as much as possible has gained interest. Silica,which is relatively inert towards HPAs,is the one most often used. The silica-supported 12-tungstophosphoric acid (PW/SiO₂) was a very efficient catalyst for Dakin-West reaction. Due to the increasing demand in modern organic processes for avoiding expensive purification,the use of heterogeneous catalysts is a remarkable technique for an environmentally clean synthesis of DHPMs. This encouraged the investigation of the efficiency of PW/SiO₂ and PMo/SiO₂ as catalysts on such reactions. HPAs have very strong Brӧnsted acidity approaching the superacid region,and this acid-base property can be varied over a wide range by changing the chemical composition. Solid HPAs possess a discrete ionic structure comprising fairly mobile basic structural units-heteropoly anions and counter cations (H⁺,H₃O⁺,H₅O₂⁺,etc.),unlike the network structure of zeolites and metal oxides. This unique structure manifests itself to have extremely high proton mobility. Hence,HPAs have several advantages as catalysts which make them economically and environmentally attractive. Here,the scheme shows silica supported H₃PW₁₂O₄₀ catalyzed synthesis of di hydropyrimidin-2(1H)-ones and thiones (DHPMs,Scheme 1).

Scheme 1. Synthesis of 3,4-dihydropyrimidinones or thiones (DHPMs) using PW/SiO2 [13].

Lana et al. ^[14] reported the synthesis of 1,8-cineole and 1,4-cineole by the isomerization of α-terpineol catalyzed by heteropoly acid. The isomerization of α-terpineol catalyzed by heteropoly acid H₃PW₁₂O₄₀ (PW) in the homogeneous and heterogeneous systems yields 1,8-cineole and 1,4-cineole,both useful for flavoring and pharmaceutical applications. PW showed a higher catalytic activity and selectivity than conventional acid catalysts such as H₂SO₄ and Amberlyst-15. They studied the application of the heteropoly acid H₃PW₁₂O₄₀ (PW),the strongest HPA in the Keggin series,as homogeneous and solid acid catalysts for the isomerization of α-terpineol to 1,8-cineole and 1,4-cineole. The reaction in the heterogeneous system was more efficient,giving 35% of 1,8-cineole and 25% of 1,4-cineole at 70%-100% conversion in the cyclohexane solution. Using silica supported PW as a solid acid catalyst,this could be recycled (Scheme 2).

Scheme 2. Synthesis of 1,8-cineole &1,4-cineole using PW/SiO2 as catalyst [14].

Yadav et al. ^[5] described phosphomolybdic acid supported on silica gel as a mild,efficient and reusable catalyst for the synthesis of 2,3-unsaturated glycopyranosides by Ferrier rearrangement. Phosphomolybdic acid (PMA) belongs to the class of heteropoly acids. HPAs are several times more active than H₂SO₄,TsOH,BF₃·Et₂O and ZnCl₂. It has been shown that in organic media,the molar catalytic activity of HPAs is often 100-1000 times higher than that of H₂SO₄. This makes it possible to carry out a catalytic process at low concentration and at lower temperature. Supported HPAs are more active than typical solid acids. Yadav et al. ^[5] explored the synthetic utility of phosphomolybdic acid supported on silica gel (PMA-SiO₂),and reported the synthesis of 2,3-unsaturated allyl C-glycosides and O- and S-glycosides with PMA-SiO₂. The reaction proceeded smoothly with 1 mol% of PMA-SiO₂ at room temperature with allyltrimethylsilane. When alcohols and thiols were used instead of allyltrimethylsilane,the corresponding O- and S-glycosides were obtained in excellent yields within 10-15 min. There are several advantages to the use of PMA-SiO₂ as catalyst for this transformation,which include high selectivity,high yields of the products,the very small quantity of the catalyst required and its reusability (Scheme 3).

Scheme 3. Synthesis of 2,3-unsaturated glycopyranosides using PMA [5].

Rafiee et al. ^[6] explored silica-supported heteropoly acids as highly efficient catalysts for synthesis of α-aminonitriles using trimethylsilyl cyanide or potassium cyanide. A variety of homogeneous and heterogeneous catalysts have been used for α-aminonitrile synthesis but these have one or more disadvantages including the tedious isolation of pure α-aminonitriles from the reaction mixtures,extended reaction times,the generation of a large amount of toxic waste,and the use of stoichiometric or relatively expensive reagents. Moreover many of the used catalysts were deactivated or sometimes decomposed by amines. To overcome these problems,supported heteropoly acids (HPAs) were introduced as catalyst for the synthesis of α-aminonitriles. The multicomponent Strecker reaction using trimethylsilyl cyanide or potassium cyanide was performed in very short reaction times and α-aminonitriles were prepared in excellent yields in the presence of a catalytic amount of silica supported dodecatungstophosphoric acid. The simple experimental and product isolation procedure combined with easy recovery and reusability of the catalyst should contribute to the development of a clean and environmentally friendly strategy for the synthesis of α-aminonitriles (Scheme 4).

Scheme 4. Synthesis of α-aminonitriles using silica supported dodecatungstophosphoric acid [6].

Rafiee et al. ^[15] investigated the efficacy of various supported heteropoly acids in the synthesis of mandelates from mandelic acid and alcohols. Their work aimed to use an efficient catalyst based on a heteropoly acid supported on silica,particularly dodecatungestophosphoric acid (PW),and its comparison with molybdophosphoric acid (PMo),supported on different solids in the preparation of mandelates. In the literature,the methods available for the preparation of mandelates are not entirely satisfactory and suffered from one or more of the following drawbacks: corrosivity of the strong acids,tedious work-up,low yields,long reaction times and side reactions such as carbonization,oxidation,etherification. As PW supported on silica gives the best yield in a short reaction time,it represented a clean,practical,simple,mild,time-saving,and eco-friendly method for the synthesis of mandelates in short reaction times and excellent yields with 100% selectivity. Moreover,the catalyst was stable,highly efficient,reusable,and inexpensive. The results presented in the paper demonstrate that SiO₂ was the best support for HPAs in comparison to the others. PW/SiO₂ is an efficient,environmentally friendly,cheap,nontoxic,and reusable catalyst with very easy work-up ^[15] (Scheme 5).

Scheme 5. Synthesis of mandelates using dodecatungestophosphoric acid [15].

Table 1 Aldehydes and amines used in the reaction in Scheme 4. Aldehyde/Ketone Amine Benzaldehyde Aniline Benzaldehyde Benylamine Benzaldehyde Butylamine Benzaldehyde Isobutylamine m-Methoxybenzaldehyde Aniline m-Methoxybenzaldehyde Benylamine m-Methoxybenzaldehyde Butylamine p-Chlorobenaldehyde Aniline p-Methylbenzaldehyde Aniline p-Nitrobenzaldehyde Aniline Furfural Aniline Furfural Benylamine Cinnamaldehyde Aniline Thiophene-2-carboxaldehyde Benylamine Decanal Aniline Benzaldehyde Morpholine Cyclohexanone Benylamine Cyclohexanone Butylamine m-Methyl cyclohexanone Benzylamine m-Methyl cyclohexanone Morpholine

Table 1 Aldehydes and amines used in the reaction in Scheme 4.

Mohammadpoor-Baltork et al. ^[7] used silica supported 12-tungstophosphoric acid as heterogeneous and recoverable catalysts for the synthesis of oxazolines,imidazolines and thiazolines under solvent-free conditions. Heteropoly acid catalysts have many advantages over liquid acid catalysts. They are harmless to the environment with respect to corrosiveness,safety and quantity of waste. In the case of oxazolines,dicyanobenzenes were selectively converted to mono- and bis-oxazolines in the presence of these catalysts. In the reaction of dicyanobenzenes with ethylenediamine,only monoimidazolines were produced and the remaining cyano group did not react even with long reaction times. In the case of thiazolines,only bis-thiazolines were produced in the reaction of dicyanobenzenes with cysteamine. The use of supported-TPAs as non-toxic,recoverable and reusable heterogeneous catalysts make this procedure environmentally friendly and economically acceptable. Also,a simple work-up,short reaction times,high yields of the products and easy handling of the catalysts are other noteworthy advantages of this catalytic system. So,supported heteropoly acids and specially supported 12-tungstophosphoric acid are important classes of supported catalysts. These catalysts offer several advantages such as high mechanical and thermal stabilities,easy handling,nontoxicity,high reactivity and recyclability (Scheme 6).

Scheme 6. Synthesis of oxazolines,imidazolines and thiazolines using silica supported TPAs [7].

Rafiee et al. ^[16] reported the acid catalysis by different supported heteropoly acids for a one-pot synthesis of β-acetamido ketones. The aim of the work was the comparison of the activity of different supported HPAs for the preparation of β-acetamido ketones. The production of β-acetamido ketones from aldehydes,ketones and acetyl chloride in acetonitrile over supported heteropoly acids was examined. This study reported the use of HPAs supported on metal oxides like SiO_2, Al₂O₃,K₁₀ and KSF montmorillonite as efficient and reusable catalysts,and their comparison with other catalysts in the Dakin-West reaction. When SiO₂ and carbon was used as the support,high yields were reached over that when the other supports were used. In conclusion,supported HPAs were found to be highly efficient,reusable,inexpensive and ecofriendly solid acid catalysts for the synthesis of β-acetamido ketones. Scheme 7 shows the synthesis of β-acetamido ketones at room temperature in the presence of a supported catalyst.

Scheme 7. Synthesis of β-acetamido ketones using HPAs supported on metal oxides by [16].

Yadav et al. ^[17] depicted heteropoly acid-catalyzed highly efficient alkylation of 1,3-dicarbonyl compounds with benzylic and propargylic alcohols. They described that 1,3-dicarbonyl compounds reacted readily with benzylic and propargylic alcohols in the presence of 10 mol% of phosphomolybdic acid supported on silica gel (PMA/SiO₂) under mild reaction conditions with good selectivity. The effects of various silica-supported acid catalysts such as HClO₄/SiO₂,H₂SO₄/SiO₂ and NaHSO₄/SiO₂ were screened for this reaction. Of these catalysts,PMA/SiO₂ was found to give the best results in terms of conversion and reaction time. Yadav et al. ^[17] have described a simple,convenient and efficient protocol for the benzylation and propargylation of 1,3-dicarbonyl compounds with benzylic and propargylic alcohols using recyclable PMA/SiO₂ as the catalytic system. In addition to its efficiency,simplicity and mild reaction conditions,this method provides high yields of 2-benzyl- and 2-propargyl-1,3-dicarbonyl compounds in short reaction times with high selectivity (Scheme 8).

Table 2 Substitutions used at R1,R2,and R3 position in Scheme 7. R1 R2 R3 C6H5 C6H5 H C6H5 4-NO2C6H4 H C6H5 4-BrC6H4 H 4-ClC6H4 C6H5 H 4-CH3OC6H4 C6H5 H 2-ClC6H4 C6H5 H 4-CH3C6H4 4-NO2C6H4 H 4-ClC6H4 4-BrC6H4 H 4-ClC6H4 4-NO2C6H4 H 4-CH3OC6H4 4-NO2C6H4 H 2-OHC6H4 C6H5 H 2-OHC6H4 4-NO2C6H4 H 2-OHC6H4 4-BrC6H4 H 3-NO2C6H4 4-BrC6H4 H 3-NO2C6H4 C6H5 H CH3CH2 C6H5 H C6H5 C6H10O H

Table 2 Substitutions used at R1,R2,and R3 position in Scheme 7.

Scheme 8. Synthesis of 2-benzyl- and 2-propargyl-1,3-dicarbonyl compounds using PMA/SiO2 [17].

Liu et al. ^[18] described toluene alkylation with 1-octene over supported heteropoly acids on MCM-41. The liquid phase Friedel-Crafts alkylation of toluene to produce 2-benzylic- and 2-propargylic-1,3-dicarbonyl compounds in excellent yields and with high with 1-octene was successfully catalyzed by Keggin type heteropoly acids supported on MCM-41. Heteropoly acids (HPA),such as tungstophosphoric acid (HPW),tungstosilicic acid (HSiW),and molybdophosphoric acid (HPMo),were studied. Among these catalysts,the supported catalysts exhibited better activity than bulk HPA,especially,HSiW and HPW supported on MCM-41 (HSiW/MCM-41 and HPW/MCM-41). Using these catalysts the reaction yielded only the monoalkylation products and the conversion of 1-octene was nearly 100% in the reaction conditions explored. The 2-isomer was the major product. Among the catalysts studied,the supported catalysts presented better performance compared to that of bulk HPA. This can be attributed to the difference of surface area of bulk HPA and the supported catalysts. The catalytic activity for alkylation depended on the type of heteropoly acids,and HSiW supported on MCM-41 had the highest activity among the catalysts studied in our work. The catalyst can be reused after a simple work up. The reusability of the supported catalysts was examined. The catalysts retained their catalytic activity for five reaction runs in the present study. Here is the scheme showing the alkylation of toluene with 1-octene (Scheme 9).

Scheme 9. Alkylation of toluene with 1-octene using HSiW/MCM-41 and HPW/MCM-41 [18].

Rafiee et al. ^[19] described silica supported 12- tungstophosphoric acid catalysts for synthesis of 1,4- dihydropyridines under solvent-free conditions. 12-Tungstophosphoric acid (PW) supported on different metal. oxides (SiO₂,γ-Al₂O₃,KSF,K₁₀) and activated carbon were prepared by impregnation. Their catalytic performances were evaluated in the three component condensation of benzaldehyde,ethyl acetoacetate and ammonium acetate to afford the corresponding 1,4- dihydropyridine. The results showed that 40 wt% PW/SiO₂ was the best catalyst since the maximum yield of products was obtained in a short reaction time. The catalytic activity mainly depended on the PW coverage and the highest activity corresponded to monolayer coverage of PW on SiO₂. Finally,a series of 4-aryl,N-alkyl,and N-aryl substituted 1,4-dihydropyridines were synthesized in high to excellent yield in short reaction times. PW/SiO₂ was found to be reusable and considerable catalytic activity still could be achieved after the fourth run. This catalyst has the advantages of easy catalyst separation from the reaction medium and less problems of corrosion. Recycling of the catalyst and avoidance of a harmful organic solvent are other advantages of this simple procedure. As a consequence,an eco friendly method was developed for the preparation of DHPs derivatives,which is important for the pharmaceuticals industry (Scheme 10).

Scheme 10. Synthesis of 1,4-dihydropyridines using 12- tungstophosphoric acid [19].

Parida et al. ^[20] reported the cesium salts of heteropoly acid immobilized mesoporous silica as an efficient catalyst for the acylation of anisole. A series of the Cs salt of phosphotungstic acid (Cs-PTA) supported on MCM-41 (Cs-PTA/MCM-41) was synthesized by a wet impregnation method. The spectroscopic results revealed that Cs-PTA was highly dispersed on the MCM-41 surface. The 50 wt% Cs-PTA supported on MCM-41 showed remarkable catalytic performance for the acylation of anisole reaction. The catalyst was regenerable by simple calcination without loss in catalytic activity. The Cs salt of phosphotungstic acid supported on MCM-41 was an efficient,stable,and reusable solid acid catalyst for the anisole acylation reaction. The nitrogen adsorption-desorption study revealed that the catalyst retained mesoporosity. There was no appreciable change in the pore diameter after impregnation of the Cs salt of phosphotungstic acid in MCM-41. The FTIR spectra confirmed that the heteropoly acids retained their Keggin-type structure when supported on MCM-41. The catalyst is active for the conversion of anisole (98%) to paramethoxyacetophenone (97%).This catalyst system is easy to prepare and the deactivated catalyst can be reused after regeneration by calcination (Scheme 11).

Scheme 11. Acylation of anisole using Cs-PTA [20].

Rafiee et al. ^[21] demonstrated 12-tungstophosphoric acid supported on nano silica from rice husk ash (RHA) was an efficient catalyst for the direct benzylation of 1,3-dicarbonyl compounds in solvent-free condition. The RHA containing over 90% silica by mass with a small proportion of metallic elements is considered to be the most economical source of silica. 12-Tungstophosphoric acid,H₃PW₁₂O₄₀ (PW) was supported on this silica to produce nano silica supported PW (NPW/SiO₂) as a nano catalyst. An efficient methodology for the solvent-free direct benzylation reaction of 1,3-dicarbonyl compounds catalyzed by 40% NPW/SiO₂ was demonstrated. At present,nano scale silica materials are prepared using several methods. However,their high cost of preparation has limited their wide application. The large amount of silica freely obtained from RHA provides an abundant and cheap alternative source of silica which is applicable as a support for heterogeneous catalysts. HPAs supported on the silica nano particles can exhibit quite different characteristics from the bulk HPAs. 40% NPW/SiO₂ was an efficient,environmentally friendly,cheap and non-toxic nano catalyst. The catalytic reactions proceeded in excellent yields in very short times without a toxic organic solvent. The catalyst is a good candidate for large scale direct benzylation of 1,3-dicarbonyl compounds. The present methodology offers a clean,practical,simple,mild,environmentally friendly,green and time-saving method under solvent free condition. Also,the catalyst is a good candidate for large scale direct benzylation of 1,3-dicarbonyl compounds (Scheme 12).

Scheme 12. Benzylation of 1,3-dicarbonyl compounds using 12- tungstophosphoric acid [21].

Murugan et al. ^[22] described the use of silicomolybdic acid supported on silica gel as an efficient catalyst for Hosomi-Sakurai reactions. Hosomi-Sakurai allylation of carbonyl compounds by allyltrimethylsilane in the presence of benzyl alcohol was catalyzed using silicomolybdic acid supported on silica gel (50 wt%). The direct allylation of carbonyl compounds using allyltrimethylsilane has limited applicability because it involves high catalyst loading,low yields,and long reaction times. On the other hand,due to the interesting benefits like more selective,highly acidic,and greener solid acid catalysts for various functional group transformations heteropoly acids as catalyst were used. This method represents the first example of the direct allylation of aldehydes and ketones in the presence of cheap and readily available benzyl alcohols. In summary,SMA that is supported on silica gel efficiently catalyzes the allylation of carbonyl compounds,acetals,ketals,and acylals in moderate to high yields in a short reaction time (Scheme 13).

Scheme 13. Allylation of carbonyl compounds,acetals,ketals,and acylals using SMA [22].

Liu et al. ^[23] described the hydroamination of alkenes with sulfonamides catalyzed by a recyclable silica gel supported triflic acid. For the first time,the recovery and recycling of TfOH,a versatile Brӧnsted acid catalyst,was realized through directly adding silica gel as the adsorbent to the catalytic reaction mixture. The prepared TfOH-SiO₂ and the in situ adsorbed TfOH-SiO₂(H₂O) were both successfully applied as the recyclable catalyst for the hydroamination of alkenes with sulfonamides. The in situ adsorption of TfOH on silica gel not only avoids the procedure to prepare TfOH-SiO₂,but also demonstrates better reusability than the prepared TfOH-SiO₂. For a series of alkenes and various sulfonamides,the hydroamination reactions afforded moderate to excellent yields. Our methods provided environmentally friendly protocols possessing the potential. for applications in industry. Hence,the immobilization of TfOH on silica gel solves the problems of corrosive and fuming properties and affords efficient recovery and reusability of TfOH (Scheme 14).

Scheme 14. Hydroamination of alkenes with sulfonamides using TfOH-SiO2 [23].

Rafiee et al. ^[24] reported H₅CoW₁₂O₄₀ supported on nano silica from RHA as a green bifunctional. catalyst for the reaction of alcohols with cyclic and acyclic 1,3-dicarbonyl compounds. RHA is an abundant agricultural byproduct. The research work deals with the production of nano silica powders with high surface area and in amorphous form from RHA using an optimized technique. 12-Tungestocobaltic acid,H₅CoW₁₂O₄₀ (CoW),was supported on silica from RHA to produce silica supported CoW (CoW/SiO₂) as a nano catalyst. This catalyst was used as a highly effective catalyst for benzylation of linear 1,3-dicarbonyl compounds with benzylic alcohols and synthesis of β-keto enol ethers from cyclic 1,3-dicarbonyl compounds. The methodology offers a practical,simple,mild,environmentally friendly,and time saving method under solvent-free conditions. Nano scale silica materials were prepared using several methods. However,their high cost of preparation has limited their wide application. The production of reactive nano scale silica from rice husk (RH) is a simple process compared to other conventional production techniques as RH is an abundantly available waste material in rice producing countries where there is a need for its disposal or utilization (Scheme 15).

Scheme 15. Reaction of alcohols with cyclic and acyclic 1,3-dicarbonyl compounds using CoW/SiO2 [24].

Ghanbaripour et al. ^[25] explored microwave-promoted efficient conversion of acetophenones to 1,3,5-triarylbenzenes catalyzed by H₃PW₁₂O₄₀ and nano-silica supported H₃PW₁₂O₄₀ as reusable catalysts. The application of heterogeneous solid acid catalysts such as HPAs has attracted extensive interest in recent times due to their easy recovery and reusability,non-corrosiveness,environmental benignity,non-hazardous nature,low cost and operational simplicity. A major disadvantage of bulk HPAs lies in their low specific surface area,less than 10 m²/g,that can be improved by supporting them on oxide carriers especially SiO₂ which is an inexpensive and noncorrosive neutral solid. H₃PW₁₂O₄₀ and nano-silica supported H₃PW₁₂O₄₀ were efficient heterogeneous catalysts for the preparation of 1,3,5-triarylbenzenes via triple self condensation of acetophenones under microwave irradiation and solvent-free conditions. The advantages of this new methodology are the reusability of the catalysts,operational simplicity,high yields of the products,fast reaction and green synthesis avoiding toxic reagents and solvents. Thus,it provides a better and more practical alternative to the existing methodologies for the synthesis of 1,3,5-triarylbenzenes (Scheme 16).

Scheme 16. Synthesis of 1,3,5-triarylbenzenes using H3PW12O40 by [25].

Xie et al. ^[26] explored the use of MCM-41 immobilized heteropoly acids in water in the Aza-Michael addition reactions between nitroolefins and benzotriazole. They demonstrated that PW/MCM-41 could act as an efficient catalyst for the aza-Michael addition reaction with benzotriazole using water as the solvent through an environmentally acceptable process. Recently,the ordered mesoporous siliceous material supported catalysts attracted much attention due to their unique ordered arrangement of hexagonal structures,large pores and large surface areas. These properties render them ideal catalyst supports. To improve the sustainability of the process,efficient catalyst recovery and recyclability are required and can be achieved by using heteropoly acids on MCM-41 mesoporous molecular sieves as heterogeneous catalysts. 50 wt% PW/MCM-41 showed the highest activity (up to 96% yield). The catalyst was used in six consecutive experiments without loss of activity,confirming the success of the anchoring process and catalyst stability. This method has the advantages of simple manipulation and high turnover,which are useful for the synthesis of the N-containing heterocycles (Scheme 17).

Scheme 17. Aza-Michael addition reactions between nitroolefins and benzotriazole using PW/MCM-41 [26].

Ahmed et al. ^[27] described the characterization of 12-molybdophosphoric acid supported on mesoporous silica MCM-41 and its catalytic performance in the synthesis of hydroquinone diacetate. A new mesoporous acidic catalyst system was successfully obtained by supporting PMA on mesostructured MCM-41,having the advantages of a narrow pore size distribution. Supporting PMA on MCM-41 not only increases the catalytic activity of the samples,but also improves its reusability. Both the surface acidity and catalytic activity sharply increased with the modification of MCM-41 by PMA but were decreased by increasing the calcination temperature. The sample with 55 wt% PMA/MCM-41 calcined at 350 °C showed the highest acidity and catalytic activity. This new catalyst finds potential applications for catalyzing the formation of bulky molecules such as hydroquinone diacetate. The 55 wt% PMA/MCM-41 sample had high conversion of the acids even after being used for four times (Scheme 18).

Scheme 18. Synthesis of hydroquinone diacetate using 12- molybdophosphoric acid [27].

Kantevari et al. ^[28] carried out efficient one pot Knoevenagel condensation,Michael addition and cyclodehydration of dimedone with various aldehydes in acetonitrile and solvent free condition using PPA-SiO₂ which gave 1,8-dioxo- octahydroxanthene in excellent yield. Earlier reported synthetic methods used Lewis acids but suffered from various disadvantages like long reaction times,low yields,cumbersome preparation of catalyst and lack of selectivity. The advantage of using this method over other conventional methods were easy work up,inexpensive catalyst and easy preparation of catalyst (Scheme 19).

Scheme 19. Synthesis of 1,8-dioxo-octahydroxanthene using PPA/SiO2 [28].

Shaterian et al. ^[29] synthesized 14-aryl-14H-dibenzo[a,j] xanthenes by a one-pot condensation reaction of β-naphthol and aryl aldehydes using silica gel-supported polyphosphoric acid (PPA/SiO₂,0.03 g),as an effective and reusable catalyst,under solvent-free conditions. The earlier reported method of its preparation was the reaction of β-naphthol with 2-naphthol- 1-methanol,formamide,carbon monoxide as well as by condensation of β-naphtol with aldehydes using various acid catalysts but these suffer from disadvantages,such as unsatisfactory yields,expensive catalysts,long reaction times,toxic organic solvents,and harsh reaction conditions. The catalyst could be reused at least three times without loss of activity. The advantages of using this method over others were high yields,short reaction times,easy work-up and absence of any volatile and hazardous organic solvents (Scheme 20).

Scheme 20. Synthesis of 14-aryl-14H-dibenzo[a,j]xanthenes using PPA/SiO2 [29].

Shaterian et al. ^[30] synthesized 2H-indazolo[2,1-b] phthalazine-trione derivatives from the three-component condensation reaction of phthalhydrazide,dimedone,and aromatic aldehydes under solvent-free conditions in excellent yields and short reaction times by using reusable silica supported poly phosphoric acid (PPA-SiO₂) (0.05 mmol). The advantages of using these silica supported catalyst were low cost,ease of preparation,recycling of catalyst and less expensive,eco-friendly,high activity,ease of handling and recoverable reagents. An earlier synthesis of 2H-indazolo[2,1-b] phthalazine-triones was reported by Bazgir et al. ^[30] using p-TSA but these were expensive and a non-recyclable catalyst. The catalyst could be successfully recovered and recycled for at least five runs without significant loss in activity (Scheme 21).

Scheme 21. Synthesis of 2H-indazolo[2,1-b]phthalazine-trione using PPA-SiO2 [30].

Dastmalbaf et al. ^[31] synthesized 3,4-dihydropyrimidin- 2(1H)-ones and thiones by the one-pot three-component reaction of ethyl acetoacetate,an aryl aldehyde,and urea or thiourea in acetonitrile using PPA-SiO₂ as catalyst. The earlier reported methods for the synthesis of dihydropyrimidinones and -thiones were by using various Brӧnsted acid and Lewis acid catalysts such as alcoholic HCl,dry acetic acid,boric acid,p-TsOH,silica sulphuric acid,La(OTf)₃,Sr(OTf)₂,Fe(ClO₄)₃,InBr₃,InCl₃,and Bi(OTf)₃ but these suffer from disadvantages like expensive and highly acidic catalysts,long reaction times,unsatisfactory yields,and difficult product isolation. The catalyst could be reused at least three times with only slight reduction in the catalytic activity of the catalyst. The advantages of this method of using a silica supported catalyst over the others were good to excellent yields,relatively short reaction times,simple operation and easy work-up (Scheme 22).

Scheme 22. Synthesis of 3,4-dihydropyrimidin-2(1H)-ones and thiones using PPA-SiO2 [31].

Khojastehnezhad et al. ^[32] synthesized polyhydroquinoline derivatives by the one pot four component Hantzsch condensation reaction of aryl aldehyde,dimedone,ethyl acetoacetate and ammonium acetate using silica gel supported polyphosphoric acid as catalyst under solvent free conditions in high yields. The advantages of using this method over the other methods were short reaction times,clean reaction profile,and simple experimental and work up procedures. The catalyst was recovered in excellent yield and reused in the reaction for three times without significant loss of its activity (Scheme 23).

Scheme 23. Synthesis of polyhydroquinoline derivatives using PPA-SiO2 [32].

Davoodnia et al. ^[33] synthesized tetrahydrobenzo[b]pyrans by a one-pot three-component cyclocondensation of dimedone,aryl aldehydes,and malononitrile in water using PPA-SiO₂ (0.1 g) as an efficient and reusable catalyst. The earlier reported methods of its synthesis were by various catalysts such as hexadecyltrimethyl ammonium bromide (HTMAB),rare earth perfluorooctanoa te [RE(PFO)₃],NaBr,(S)-proline,KF-basic alumina under ultrasound irradiation and Na₂SeO₄ but these suffer from disadvantages such as expensive and highly acidic catalysts,lengthy reaction times,unsatisfactory yields and difficult product isolation. They also reported that the catalyst could be used three times in the reactions with only a slight reduction in the catalytic activity and the pH of the solvent after the separation of the catalyst was found to be 6.4. The advantage of using this method were high yields,short reaction times,ease of handling,low cost of the catalyst,environmentally non-harmful media and the absence of any hazardous organic solvents (Scheme 24).

Scheme 24. Synthesis of tetrahydrobenzo[b]pyrans using PPA/SiO2 [33].

Table 3 Substitutions used at R1 and R2position in Scheme 25. R1 R2 H n-C4H9 H CH(CH3)2 H C(CH3)3 H CH2Cl H Si(CH3)3 H Ph COOCH3 COOCH3

Table 3 Substitutions used at R1 and R2position in Scheme 25.

Itoh et al. ^[34] synthesized 3-benzoylisoxazole by reaction of alkynes and benzoylnitromethane using PPA/SiO₂ in toluene under reflux for 4 h. The earlier reported methods of its preparation were by either 1,3-dipolar cycloaddition of dipolarophiles with nitrile oxides from aldoximes/α-nitroketones or dehydration using acid (sulphuric acid/p-toluenesulfonic acid) or by base (N-methylimidazole,1,4-diazabicyclo^{[2, 2, 2]} octane [DABCO] and copper acetate/N-methylpiperdine but these suffer from various disadvantages like expensive,toxic and dangerous reagents. They reported that this catalyst can be recycled or reused up to 5 times without any significant loss of activity and the advantage of using this method were low cost,ease of preparation and ease of handling (Scheme 25).

Scheme 25. Synthesis of 3-benzoylisoxazole[b]pyrans using PPA/SiO2 [34].

Bamoniri et al. ^[35] synthesized 1,2,4,5-tetrasubstituted imidazoles using Benzil (4 mmol),amine (4 mmol),aldehyde (4 mmol),ammonium acetate (4 mmol) and nano-SPA (0.08 g) as catalyst. The earlier reported method for its synthesis were by using different catalyst like Zeolite HY,silica gel/NaHSO₄,molecular iodine,K₅CoW₁₂O₄₀·3H₂O,heteropolyacids,HClO₄-SiO₂,InCl₃·3H₂O,ZrCl₄ and BF₃·SiO₂ but these suffer from disadvantages like harsh reaction conditions,poor yields,prolonged time period,use of hazardous and often expensive acid catalysts (Scheme 26).

Scheme 26. Synthesis of 1,2,4,5-tetrasubstituted imidazoles using PPA/SiO2 [35].

Manolov et al. ^[36] synthesized 4-aryl- and 4-methyl-1,2,3,4- tetrahydroisoquinoline derivatives by the reaction of aminoacetaldehyde dimethylacetal with different substituted benzenes in polyphosphoric acid,followed by acylation of the obtained amines with different acid chlorides or sulfochlorides using silica supported polyphosphoric acid. The catalyst was completely recoverable and the efficiency of the catalyst remained unaltered even after three to four cycles. The cyclisation using PPA-SiO₂ proceeded rapidly and was superior to the reported procedures with respect to yield and amount of the catalyst employed (Scheme 27).

Scheme 27. Synthesis of 4-aryl- and 4-methyl-1,2,3,4- tetrahydroisoquinoline derivatives using PPA-SiO2 [36].

Shaflee et al. ^[37] synthesized N,N′-alkylidene bisamides by the one pot three component condensation reaction of phenyl acetylene/1-hexyne,aromatic aldehydes and benzamide/acetamide by using PPA-SiO₂ as catalyst. The earlier reported method for its preparation were either by the direct reaction of aldehydes with the corresponding carboxamide using a strong acid catalyst such as sulphuric acid,hydrochloric acid,triflic acid,phosphotungstic acid,or by the reaction of aldehyde with nitriles but these suffer from disadvantages like requiring the presence of acorrosive homogenous liquid acid catalyst and sensitivity to harsh conditions (Scheme 28).

Scheme 28. Synthesis of N,N’-alkylidene bisamides using PPA-SiO2 [37].

Misra et al. ^[38] synthesized per-O-acetylated carbohydrate derivatives from a series of monosaccharides,disaccharides and trisaccharides using a stoichiometric quantity of acetic anhydride and catalytic amounts of HClO₄-SiO₂ (25 mg/mmol of free sugar) avoiding pyridine (due to its toxicity and unpleasant odour) under solvent free conditions. Earlier it was reported that HClO₄ catalyzed the acetylation reactions by using a large excess of acetic anhydride at extended reaction times but using the excess acetic anhydride as solvent causes troublesome workup during the neutralization process. HClO₄ had been impregnated on silica gel in order to avoid the presence of water in the reaction medium,causing deleterious effect on the formation of product and a large number of functional groups used for protecting group manipulation of carbohydrates was found to remain unaffected under the reaction conditions (Scheme 29).

Scheme 29. Synthesis of per-O-acetylated carbohydrate derivatives using HClO4-SiO2 [38].

Mukhopadhyay et al. ^[39] synthesized benzylideneacetals and isopropylideneketals coupled with per-O-acetylation by using HClO₄-SiO₂ as the reagent providing a range of protected sugar building blocks. He synthesized benzylideneacetals (3) by the treatment of methyl α-D-glucopyranoside (1) with 1 mol of benzaldehydedimethylacetal in dry acetonitrile (2 mL/mmol sugar) in the presence of HClO₄-SiO₂ and further treating with 4mol equivalent of acetic anhydride resulted in formation of O-acetylated benzylidene derivatives(4). Use of 4mol of acetic anhydride was essential as 1 mol of benzaldehydedimethylacetal liberated 2 moles of methanol upon its formation and the further methanol produced was reacted with 2 moles of acetic anhydride to form a volatile methyl acetate as byproduct. It was also reported that HClO₄-SiO₂ was compatible with the pseudo cis-decalin system as well as the more robust trans-decalin equivalent. The ease of application of reagent,use of stoichiometric reagents and minimal work up and purification was used to illustrate the versatility of this reagent (Scheme 30).

Scheme 30. Synthesis of benzylideneacetals and isopropylideneketals using HClO4-SiO2 [39].

Table 4 Substitutions used at the R1 and R2 position in Scheme 27. R1 R2 H COCH3 H SO2CH3 H COPhPh OCH3 COCH3 OCH3 SO2CH3 OCH3 COPhPh

Table 4 Substitutions used at the R1 and R2 position in Scheme 27.

Du et al. ^[40] carried out the glycosylation reaction with trichloroacetemidates (widely used glycosyl donors) and lactones using HClO₄-SiO₂ as catalyst. These trichloroacetemidates exhibited outstanding donor properties in terms of ease of formation,stability,reactivity,general applicability and resulted in high product yield and various Lewis acid promoters including Boron trifluorideetherate (BF₃·Et₂O),TESOTf,AgOTf and TSOH were used to activate trichloroacetemidates. The glycosylation reaction gave optimal results with a molar ratio of 100:3 to 100:6 (donor-promoter HClO₄-SiO₂). The major advantages of using silicate perchloric acid were mild reaction conditions,experimental simplicity,low costs and excellent yields and the large number of functional groups used for protecting group manipulation were found to remain unaffected and side reactions like migration and degradation in coupling reactions were suppressed (Scheme 31).

Scheme 31. Glycosylation reaction with trichloroacetemidates and lactones using HClO4-SiO2 [40].

Tiwari et al. ^[41] introduced a new method for the selective anomeric deacetylation or debenzoylation of carbohydrate derivatives under acidic reaction conditions. They synthesized hemiacetal in excellent yield by the treatment of per-O-acetylated β-D-glucose with 25 mg of HClO₄-SiO₂ (0.012 mmol of HClO₄) per mmol of per-O-acetylated β-D-glucose in acetonitrile without affecting the other functional groups present in the substrate. It was found that the commercial method for anomeric O-deacetylation of per-O-acetylated carbohydrate derivatives suffered many disadvantages such as the use of highly toxic reagents,long reaction times,non-selective de-O-acetylation and time consuming purification steps. They also reported that there was no effect of moisture on reaction time and yield and this reaction was equally effective for the removal of both α- and β-acetates and under similar conditions. Anomeric benzoyl groups were also removed in excellent yields (Scheme 32).

Scheme 32. Synthesis of hemiacetal using HClO4-SiO2 [41].

Agnihotri et al. ^[42] carried out rapid deprotection of benzylideneacetals and direct conversion of acid labile benzylideneacetals to base labile acetals derivatives in one pot reaction using HClO₄-SiO₂ with almost quantitative yields avoiding the formation of byproducts. The earlier reported methods available for the complete removal of benzylideneacetals include a strong acidic medium (H₂SO₄,AcOH ) but possessed several disadvantages such as relatively low yields,formation of byproducts,use of expensive reagents,incompatibility with other functional groups and relatively harsh conditions. It was reported that the advantages of using silicatedperchloric acid were clean deprotection of benzylideneacetals was taking place with the exclusive formation of the corresponding acetylated products; the direct conversion of benzylideneacetals to the corresponding acetates. The free flowing powder obtained could be stored for years without any loss of catalytic activity. They used 50 mg of HClO₄-SiO₂ (0.025 mmol of HClO₄) per milli mole of methyl 2,3-di-O-acetyl-4,6-O-benzylidene-a-D- glucopyranosidein commercial CH₃CN at room temperature to give clean deprotection of the benzylideneacetal as product. They also reported that CH₃CN was the best solvent in comparison to other commonly used polar solvents (e.g. CH₂Cl₂,CHCl₃,THF) in which lower yields were obtained. The variety of anomeric protecting groups and interglycosidic linkages remained unaffected under the reaction conditions. They also reported that the catalyst could be reused several times without significant loss of activity as the recovered catalyst could be used three times in the direct deprotection and acetylation of methyl 2,3-di-O-acetyl-4,6-O-benzylidene-a-D-glucopyranoside wit h yields higher than 90% (Scheme 33).

Scheme 33. Synthesis of base labile acetals derivatives using HClO4-SiO2 [42].

Khan et al. ^[8] carried out geminaldiacylation of aldehydes by treating aldehydes with acetic anhydride with HClO₄-SiO₂ under solvent free conditions. Earlier,several methods were reported for the preparation of acylal such as the reaction of aldehyde with acetic anhydride in presence of protonic acid like sulphuric acid,methanesulfonic acid or phosphoric acid; by using various reagents like LiOTf,ceric ammonium nitrate,InCl₃,LiBF₄,NBS,I₂; by using metal triflates like Cu(OTf)₂,Sc(OTf)₃ and Bi(OTf)₃ but these methods suffered disadvantages such as harsh reaction conditions,requirement of excess amount of acetic anhydride,tedious workup procedure and use of expensive and moisture sensitive catalyst,difficulty in preparing an acylal from furfural and longer reaction times. It was also reported earlier that perchloric acid adsorbed on silica gel has a higher catalytic activity than most of the moisture sensitive and highly costly metal triflates. They also reported that cinnamaldehyde and 4-nitrobenzaldehyde provided the desired acylal within 5 min with much better yield than the other reported methods and it was much more effective for the substrate furfural in terms of reaction timing as well as yield. The usually difficult diacylatedandaliphatic aldehydes did not show any other side reactions such as cyclotrimerization under the experimental conditions (Scheme 34).

Scheme 34. Synthesis of acylal using HClO4-SiO2 [8].

Das et al. ^[10] utilized a heterogeneous recyclable catalyst for a highly efficient and chemo- and stereoselective conversion of β-dicarbonyl compounds by treatment with amines at room temperature into β-enaminones and β-enamino esters under solvent-free conditions. It was earlier reported that β-enaminones and β-enamino esters were prepared from the direct condensation of β-dicarbonylcompounds with amines under reflux in an aromatic solvent with the azeotropic removal of water or utilization of Al₂O₃,SiO₂/microwaves,montmorillonite K-10,NaAuClO₄,Bi(TFA)₃,Zn(ClO4)₂·6H₂O,CeCl₃·7H₂O but these suffered from drawbacks such as long reaction times,unsatisfactory yields,low selectivity,lack of general applicability,higher temperatures,application of non-available and costly reagents,use of hazardous solvents and tedious experimental procedures. The catalyst was recovered,activated and reused for three consecutive times with only slight variation in the yields of the products. The advantages of using silica supported perchloric acid were solvent-free conditions,mildness of the conversion,simple experimental procedure,clear reaction profiles,high yields and chemo- and stereoselectivities,and short reaction times (Scheme 35).

Scheme 35. Stereoselective synthesis of β-enaminones and β-enamino esters using HClO4-SiO2 [10].

Table 5 Substitutions used at the R1,R2,and R3position in Scheme 35. R1 R2 R3 Me Me Ph Me Me o-MePh Me Me Ph(CH2) Me Me Ph(CH2)2 Me Me (CH2)2CH Me Me (CH2)2CHCH2 Me Me CH2(CH2)7 Me Me NH2(CH2)2 CH2C(CH2)2CH2 CH2C(CH2)2CH2 Ph CH2C(CH2)2CH2 CH2C(CH2)2CH2 o-MePh CH2C(CH2)2CH2 CH2C(CH2)2CH2 PhCH2 CH2C(CH2)2CH2 CH2C(CH2)2CH2 Ph(CH2)2 CH2C(CH2)2CH2 CH2C(CH2)2CH2 (CH3)2CH CH2C(CH2)2CH2 CH2C(CH2)2CH2 (CH3)2CHCH2 CH2C(CH2)2CH2 CH2C(CH2)2CH2 CH3(CH2)7 Me OEt Ph Me OEt o-MePh Me OEt p-ClPh Me OEt p-OHPh Me OEt PhCH2 Me OEt Ph(CH2)2 Me OEt (CH3)2CH Me OEt (CH3)2CHCH2 Me OEt CH3(CH2)7 Me OEt NH2(CH2)2

Table 5 Substitutions used at the R1,R2,and R3position in Scheme 35.

Bigdeli et al. ^[43] carried out a highly efficient,one-pot synthesis of 14-aryl or alkyl-14-H-dibenzo[a,j]xanthenes by the condensation of an aldehyde and 2-naphthol under solvent free and reflux conditions in excellent yields. Earlier reported methods for the preparation of xanthenes and benzoxanthenes include the reaction of aryloxymagnesium halides with triethylorthoformate,cyclodehydration,trapping of benzyne by phenols,intramolecular phenyl carbonyl coupling reactions of benzaldehyde and acetophenones but these methods suffer from several disadvantages such as long reaction times,unsatisfactory yields,harsh reaction conditions and excessive use of reagents and catalysts. They used HClO₄-SiO₂ as catalyst in this synthesis for its inherent properties like environmental compatibility,higher selectivity,operational simplicity,moisture-insensitive,less noncorrosive nature and ease of isolation,simplicity,together with the use of inexpensive,nontoxic and the environmentally benign nature of the HClO₄-SiO₂ catalyst under solvent free condition. HClO₄ supported on silica gel with low loading (0.025 mmol/50 mg. 1 mol%) was an efficient catalyst and gave exclusively 95% yield in 10 min under solvent free conditions and the recovered HClO₄-SiO₂ was reused in subsequent reactions without significant decrease in activity even after five runs (Scheme 36).

Scheme 36. Synthesis of 14-aryl or alkyl-14-H-dibenzo[a,j]xanthenes using HClO4-SiO2 [43].

Kantevari et al. ^[44] synthesized 1,2,4,5- tetrasubstitutedimidazoles by the condensation of an aldehyde,benzil,aliphaticor aromatic primary amines and ammonium acetate under solvent free conditions using perchloric acid adsorbed on silica gel (HClO₄-SiO₂) as catalyst in excellent yield. Earlier reported methods for the preparation of imidzoles derivative include condensation of benzoin or benzoinacetate with aldehydes,primary amines,ammonia in the presence of copper acetate; condensation of β-carbonyl-N-acyl-N-alkylamines with ammonium acetate in refluxing acetic acid; four-component condensation of diones,aldehydes,primary amines and ammonium acetate in acetic acid under reflux conditions; cyclization of sulphonamides with mesoionic 1,3-oxazolium-5-olates; conversion of N-(2-oxo)amides with ammonium trifluoroacetate under neutral conditions; condensation of benzonitrile,benzil and alkyl amine using silica gel and zeolite HY as heterogeneous catalyst under solvent free microwave conditions but these methods suffered from limitations such as low yields,longer reaction times,use of expensive reagents,solvents and toxic agents associated with a mixture of products,lack of generality and were not suitable for,or were not applied for,the synthesis of structurally diverse imidazoles. They used HClO₄-SiO₂ as catalyst for its inherent properties like environmental compatibility,higher selectivity,operational simplicity,moisture-insensitive,non-corrosive nature and ease of isolation. They also reported that condensation of benzil,benzaldehyde,benzyl amine,and ammonium acetate was more facile and proceeded to give the highest yield under solvent free conditions and the remarkable catalytic activity of HClO₄-SiO₂ exhibited was convincingly superior to the other reported cata lytic methods with respect to reaction time (2-20 min),amount of catalyst and the pure products were obtained by simple crystallization (Scheme 37).

Scheme 37. Synthesis of 1,2,4,5-tetrasubstitutedimidazoles using HClO4-SiO2 [44].

Narasimhulu et al. ^[45] carried out the rapid and efficient synthesis of various poly-substituted quinolines in the Friedländer condensation of 2-aminoarylketones with carbonyl compounds and β-keto esters by using heterogeneous solid silica supported perchloric acid at ambient temperature. Earlier,one of the most simple and straight forward approaches reported for the synthesis of poly-substituted quinolines was Friedländer annulation,anacid- or base-catalyzed condensation followed by a cyclodehydration between an aromatic 2-aminoaldehyde or ketone and a carbonyl compound containing a reactive α-methylene group. But these methods possessed drawbacks such as high temperature and extended reaction times leading to several side reactions,harsh reaction conditions,low yields,high temperature,tedious work-up and the use of stoichiometric and relatively expensive reagents. They also reported that 96% yield of the product was obtained and reacted at room temperature with 50% completion. The catalyst could be recycled three times in subsequent reactions without substantial loss of its activity (Scheme 38).

Table 6 Substitutions used at the R,R1,and R2 position in Scheme 37. R R1 R2 C6H5 C6H5CH2 Ph C6H5 C6H5CH2 4-MePh C6H5 C6H5CH2 3-OMePh C6H5 C6H5CH2 3,4,5-(OCH3)3Ph C6H5 C6H5CH2 3-ClC6H4 C6H5 C6H5CH2 4-ClC6H4 C6H5 C6H5CH2 4-OCH3C6H4 C6H5 C6H5CH2 4-BrC6H4 C6H5 C6H5CH2 2-ClC6H4 C6H5 C6H5CH2 4-N(Me)2C6H4 C6H5 C6H5CH2 4-OHC6H4 C6H5 C6H5CH2 2-NO2C6H4 C6H5 Iso-butyl 3,4,5-(OCH3)3Ph C6H5 3-MeC6H4CH2 4-OH-3-OCH3C6H3 C6H5 Iso-butyl 4-MeC6H4 C6H5 C6H5CH2 Isopropyl C6H5 C6H5CH2 2-Furyl C6H5 4-MeC6H4 3-NO2C6H4 C6H5 4-MeC6H4 4-NO2C6H4 C6H5 C6H5 Ph C6H5 C6H5CH2CH2 4-MeC6H4 C6H5 3-NO2C6H4 4-N(Me)2C6H4 CH3 Cyclohexyl 4-MeC6H4 CH3 C6H5CH2 Isopropyl CH3 Iso-butyl C6H5

Table 6 Substitutions used at the R,R1,and R2 position in Scheme 37.

Scheme 38. Synthesis of poly-substituted quinolines using HClO4-SiO2 [45].

Table 7 Substitutions used at the R1,R2,and R3 position in Scheme 38. R1 R2 R3 CH3 CH3 OEt CH3 CH3 OMe CH3 CH3 COCH3 CH3 CH2C(CH3)2CH2CO CH3 CH2CH2CH2CO CH3 CH2(CH2)2CH2 CH3 CH2CH2CH2 CH3 CH3 CH3 CH3 CH3 (CH3)2CH C6H5 CH3 C6H5 CH3 C6H5 CH3 C6H5 CH2C(CH3)2CH2CO C6H5 CH2CH2CH2CO C6H5 CH2(CH2)2CH2 C6H5 CH2CH2CH2 C6H5 CH3 C6H5 CH3

Table 7 Substitutions used at the R1,R2,and R3 position in Scheme 38.

Nagarapu et al. ^[46] carried out one-pot synthesis of homoallylic amines using a variety of aromatic,heteroaromatic,aliphatic aldehydes,aromatic amines and allyltributylstannane in the presence of solid-supported perchloric acid (HClO₄-SiO₂) as a heterogeneous catalyst at room temperature. Earlier,several methods were reported for the preparation of homoallylic amines such as by the addition of organometallic reagents to imines or by the nucleophilicaddition of allylsilane or allyltin or allylborane or allylgermane reagents to imines in the presence of acid catalysts (Lewis acids like TiCl₄,BF₃·OEt₂,and PdCl₂(Ph₃P)₂),but many of these reagents were expensive,hygroscopic and difficult to handle especially on large scale. They deactivated or decomposed during aqueous workup and hence they cannot be recovered and recycled for a subsequent run. Earlier,it was reported that perchlorates gave rise to explosive reactions when heated at high temperatures in the presence of combustible compounds and catalytic loading of HClO₄-SiO₂ can effectively bring about organic transformation and can be operated even at 90 °C. This methodology offered various features such as shorter reaction times,higher yields and easy recovery and reuse of catalyst and waste-free process and the recovered HClO₄-SiO₂ can be reused in subsequent reactions without significant decrease in activity even after five runs (Scheme 39).

Scheme 39. Synthesis of homoallylic amines using HClO4-SiO2 [46].

Khatik et al. ^[47] described the scope and limitations of HClO₄-SiO₂ as a new and highly efficient catalyst for chemoselective C-S bond formation by the conjugate addition of thiols to α,β-unsaturated ketones under solvent-free conditions at room temperature. Earlier reported use of various activators or catalyst derived from metal halides,perchlorates,tetrafluoroborates,and protic acids adsorbed on solid support for heteroatom acylation,epoxide ring opening and imine/ dithiolane/carbamate/acetal/acylal formation,and thia-Michael addition reactions,but these suffered from various disadvantages like longer reaction times and did not work well with the substrate. They planned to explore the catalytic efficiency of HClO₄-SiO₂ for the thia-Michael addition reaction as increasing pressure from environmentalists had led to the search for more friendly forms of the catalysis. The leading contender for an environmentally acceptable alternative process was the use of supported reagents. There were improvement of the activity and selectivity of a reagent dispersed on the surface of a support (as the effective surface area of the reagent can be increased up to one hundred times),easy handling,good thermal and mechanical stabilities of supported reagents,use of generally low toxic,non-corrosive free flowing powders,and their ease of separation from the reaction mixture through filtration. They reported the use of 1 mol% of the catalyst for ease of handling/weighing of the catalyst for small scale (2.5 mmol) reactions and the use of HClO₄-SiO₂ as a catalyst should enable the thia-Michael addition reaction to be carried out involving a solid ketone and a solid thiol under solvent-free conditions. In the case of 1,3-diphenylpropenone,the reactions were best carried out either at 80 °C under solvent-free conditions or at rt in methano l. The reaction of aryl,arylalkyl,alkyl thiols,and alkane dithiols with cyclic and acyclic α,β-unsaturated ketones afforded excellent yields of the corresponding β- sulfidocarbonyls after 2 min to 2 h. In the case of dithiols,the bis-thia-Michael adducts were formed. The rate of the reaction was found to be dependent on the electronic and steric factors of the α,β-unsaturated ketones and the thiols. A substituent at the b-carbon of the α,β-unsaturated ketone offered steric hindrance for conjugate addition and such substrates required longer times. In case of aromatic thiols,the presence of the nitro group reduced the nucleophilicity of the sulfhydryl sulfur atom,resulting in a slower rate of reaction for 4-nitrothiophenol compared to that of thiophenol and 4-methylthiophenol. For alkane thiol,the reaction rate was influenced by the steric crowding of the alkyl group attached to the sulfhydryl moiety. They reported the advantages of this method such as the use of a cheap,easy to handle,and reusable catalyst,and non-anhydrous reaction conditions,non-aqueous work-up and ease of product isolation by filtration,short reaction times,high yields,excellent chemoselectivity during inter and intra-molecular competitions,and applicability for an easy synthesis of 2,3-dihydro- 1,5-benzothiazepines (Scheme 40).

Table 7 Aldehydes and amines used in Scheme 39. Aldehyde R-CHO Amine R’-NH2 Ph Ph Ph 4-ClC6H4 Ph 4-FC6H4 Ph 4-MeC6H4 Ph 4-OMeC6H4 2-ClC6H4 Ph 4-ClC6H4 Ph 4-NO2C6H4 Ph 4-NO2C6H4 4-OMeC6H4 2-CH3 furan Ph 2-HOC6H4 4-MeC6H4 2-CH3 napthalene Ph PhCH=CH Ph (CH3)3CHCH3 Ph CH3CH=CHCH3 Ph

Table 7 Aldehydes and amines used in Scheme 39.

Scheme 40. Chemoselective C-S bond formation by conjugate addition of thiols to α,β-unsaturated ketones using HClO4-SiO2 [47].

Kantevari et al. ^[48] carried out efficient one-pot Michael addition and cyclodehydration of dimedone with various aldehydes with solvent free conditions in the presence of a HClO₄-SiO₂ catalyst to give 9-aryl-1,8- dioxooctahydroxanthene derivatives in excellent yields. Earlier reported synthesis of 1,8-dioxo-octahydroxanthene was achieved by the condensation of 5,5-dimethyl-1,3-cyclohexa-dione with aromatic aldehydes using Lewis acid catalyst or using polyphosphoric acid (PPA) but this suffered from various drawbacks such as longer reaction times,low yields,ease of availability of catalyst,involves cumbersome preparation of the catalyst and lack of selectivity,and with the use of polyphosphoric acid,excellent yields of 1,8-dioxo-octahydroxanthene were obtained with acetonitrile as solvent and in solvent free conditions. They also reported that the reaction was limited to the one-pot Knoevenagel condensation and Michael addition in the presence of HClO₄-SiO₂ and it was limited to the synthesis of 2,2’-arylmethylenebis(3-hydroxy-5,5-dimethyl-2-cyclohexene-1-one) in very good yields without formation of any cyclized product .The catalyst can be recovered,regenerated and reused without loss of its activity. The remarkable catalytic activity of HClO₄-SiO₂,PPA-SiO₂ exhibited was superior to the other recently reported catalytic methods with respect reaction time,amount of catalyst and the pure products were obtained by simple crystallization. The easy work up,inexpensive and easy preparation of the catalyst make the procedure better (Scheme 41).

Scheme 41. Synthesis of 9-aryl-1,8-dioxooctahydroxanthene derivatives using HClO4-SiO2 [48].

Shaterian et al. ^[49] synthesized trimethylsilyl ether derivatives from various alcohols,phenols,naphthols,and oximes with hexamethyldisilazane in the presence of solid silica supported perchloric acid (0.025 mmol of H⁺) under very mild conditions at room temperature with short reaction time in good to excellent yields. The earlier reported method for the preparation of silyl ethers was by dehydrogenativesilylation of hydroxyl groups with hydrosilanes and also by the treatment of alcohols with silyl chlorides or silyltriflates in the presence of an organic base but these suffered from drawbacks such as lack of reactivity or the difficulty in removal of amine salts; low silylation power of HMDS; moisture sensitive or expensive reagents. The advantages of using this synthesis were simple work-up procedure,including filtering the mixture through a short pad of silica gel column followed by evaporation of the solvent (Scheme 42).

Scheme 42. Synthesis of trimethylsilyl ether derivatives using HClO4-SiO2 [49].

Das et al. ^[50] synthesized quinoxalines and dihydropyrazines (DHPs) using α-bromo ketones and 1,2-diamines in the presence of silica supported perchloric acid (HClO₄-SiO₂) at room temperature via cyclization-oxidation. The earlier reported methods for the preparation of substituted quinoxalines and DHPs were condensation of 1,2-diamines with a-diketones,1,4-addition of 1,2-diamines to diazenylbutenes,oxidation-trapping of a-hydroxy ketones with 1,2-diamines,cyclization-oxidation of phenacyl bromides and o-phenylenediamines through solid-phase synthesis,oxidative coupling of epoxides with ene-1,2-diamines but these suffered from drawbacks such as unsatisfactory yields,difficult experimental procedures,expensive and detrimental metal. precursors and harsh reaction conditions. The catalyst was recycled three times without loss of activity (Scheme 43).

Scheme 43. Synthesis of quinoxalines and dihydropyrazines using HClO4-SiO2 [50].

Alam et al. ^[51] synthesized primary carbamates from structurally diverse compounds containing hydroxyl groups (alcohols and phenols) with sodium cyanate in the presence of HClO₄-SiO₂ at room temperature or 55-65 °Cfor the appropriate time in high yields without any epimerization under solvent free conditions. The earlier reported methods for its synthesis were from alcohols by several pot reaction methods like trichloroacetylisocyanate,chloroformates (starting from toxic phosgene),chlorosulfonylisocyanate and cyanogen chloride or by treatment with sodium-like cyanate and trifluoroacetic acid in organic solvents such as benzene,methylene chloride,and tetrachloride but these suffer from disadvantages as toxic and not eco-friendly solvents ,expensive,need of solvent free reaction (industrially important due to reduced pollution,low cost and simplicity in processing and handling. Silica supported perchloric acid (HClO₄-SiO₂) received considerable attention as an inexpensive,non-toxic,and recyclable catalyst for various organic transformations,affording the corresponding products in excellent yields and high selectivity and this method does not have disadvantages such as use of toxic solvents,expensive starting materials,formation of undesirable side products,and epimerization (Scheme 44).

Scheme 44. Synthesis of primary carbamates using HClO4-SiO2 [51].

Bigdeli et al. ^[52] synthesized β-aminocarbonyl compounds from one pot,three component reaction of aldehyde,amines and ketones using silica supported perchloric acid (HClO₄-SiO₂) in ethanol at room temperature in good yields and high stereoselectivities in favor of the anti-isomer. The earlier reported Mannich reaction used electrophiles,such as imines and stable nucleophiles,such as enolates,enol ethers and enamines and optimal catalyst (2 mmol) was also reported. Ethanol was the best solvent as far as yields and reaction times and acetophenone was less reactive than cyclohexanone. So,it would require a larger quantity of catalyst (4 mol%) and longer reaction times to afford the desired products. The advantages of using this catalyst over the earlier reported methods are good stereoselectivity,high yields,nontoxic solvents,very easy work-up,low catalyst loading and no byproducts (Scheme 45).

Scheme 45. Synthesis of β-aminocarbonyl compounds using HClO4-SiO2 [52].

Shaterian et al. ^[53] synthesized amidoalkylnapthol by multi-component and one pot condensation reaction of 2-napthol (1mmol),aromatic aldehydes (1mmol) and acetonitrile (1.2mmol) or acetamide (reactant as well as solvent,5 mL) in the presence of silicate perchloric acid (0.6 mol%) under solvent,solvent free and microwave irradiation condition. The earlier reported synthesis was by multi-component condensation of aryl aldehydes,2-naphthol,and acetonitrile or amide in the presence of Lewis or Brӧnsted acid catalysts such as montmorillonite K10 clay,Ce(SO₄)₂,iodine,K₅CoW₁₂O₄₀·3H₂O,p-TSA,and sulfamic acid but these catalysts suffer from the drawback of prolonged reaction times,toxic reagents,low yields and the recovery,reusability of the catalyst. Silica supported perchloric acid was an effective catalyst in terms of reaction time and yield of obtained product and silica supported perchloric acid of 0.6 mol% gave excellent yield in 90 min at 110 °C. A slight excess of the acetamide was advantageous and hence the molar ratio of 2-naphthol to acetamide was kept at 1:1.2. The catalyst was recovered in excellent yield and could be reused for five times without any loss of its activity. The advantages of using silicate perchloric acid as catalyst were shorter reaction times,simple work-up,environmentally benign,excellent yield,cost effective recovery,and reusability of catalyst for a number of times without appreciable loss of activity. 1-amidomethyl-2-naphthols can be converted into important ‘drug like’ 1-aminomethyl-2-naphthol derivatives by amide hydrolysis (Scheme 46).

Scheme 46. Synthesis of amidoalkylnaphthol usingHClO4-SiO2 [53].

Mahdavinia et al. ^[54] synthesized amido alkyl napthol by one pot condensation of 2-napthol,aryl aldehyde and amide under solvent-free and reflux conditions using silica supported perchloric acid in excellent yields. The advantages of using solid supported reagents alternative to conventional Lewis acid or triflates were low toxicity,moisture resistance,air tolerance,low prices and recyclable catalyst for numerous organic transformation,affording the corresponding products in excellent yields with high selectivity. They reported that HClO₄ supported on silica gel with low loading (0.025 mmol/50 mg,1 mol%) is an efficient catalyst and gave 94% yield in 10 min under solvent-free conditions. It was recovered and reused up o five times without loss of activity (Scheme 47)

Scheme 47. Synthesis of amido alkyl naphthol using HClO4-SiO2 [54].

Table 9 Substitutions used at the Ar1 and Ar2position in Scheme 45. Ar1 Ar2 Ph Ph Ph 4-Me2C6H4 Ph 4-Br2C6H4 4-MeC6H4 Ph 4-ClC6H4 Ph 1-Naphthyl Ph 1-Naphthyl 4-Me2C6H4 4-BrC6H4 Ph 4-ClC6H4 4-Me2C6H4 4-NO2C6H4 Ph Ph 4-NO2C6H4

Table 9 Substitutions used at the Ar1 and Ar2position in Scheme 45.

Table 10 Substitutions used at the Ar and R position in Scheme 47. Ar R C6H5 CH3 4-ClC6H5 CH3 4-BrC6H5 CH3 4-FC6H5 CH3 4-CNC6H5 CH3 3-NO2C6H5 CH3 3-MeOC6H5 CH3 2-ClC6H5 CH3 C6H5 Ph 4-CH3C6H5 Ph 4-ClC6H5 Ph 4-FC6H5 Ph 4-CNC6H5 Ph 3-NO2C6H5 Ph C6H5 NH2 4-ClC6H5 NH2 4-BrC6H5 NH2 4-CNC6H5 NH2 3-NO2C6H5 NH2

Table 10 Substitutions used at the Ar and R position in Scheme 47.

Kumar et al. ^[55] synthesized dialkylaminoarylatedindoles by multi-component reaction of indole,formaldehyde and tertiary aromatic amines using 2 mmol% silica supported perchloric acid (HClO₄-SiO₂). The earlier reported methods for this synthesis included various Brӧnsted acids (hydrochloric acid,sulphuric acid,acetic acid,PTSA,methane sulfonic acid,and TFA) but these suffer poor yields. The use of silica-supported perchloric acid as catalyst gave excellent yield (84%) in methanol and the catalyst can be recycled and reused five times without any significant change in activity (Scheme 48).

Scheme 48. Synthesis of dialkylaminoarylatedindoles using HClO4-SiO2 [55].

Ludek et al. ^[56] carried out the activation of glycosyltrichloroacetmidite donors with immobilized perchloric acid on silica and obtained higher α-selectivity than trimethylsilyltriflate (TMSOTf) for reactions that do not involve neighboring group participation. It was reported earlier that a reaction media containing the participating solvents ether or dioxane showed an increased α-selectivity compared to the non-participating solvents (DCM and toluene). Raising the reaction temperature from 30 to 0 °C and room temperature resulted in additional increase in α-selectivity without a significant loss in overall chemical yield. They also reported no change in selectivity was observed indicating that increased α-selectivity due to presence of perchlorate ion and the chemical yield comparable to those achieved by TMSOTf activation. The catalyst and conditions would be applicable to a range of donor and acceptor pairs and HClO₄-SiO₂ would be a useful alternative to TMSOTf for activation of trichloroacetimidates due to improved selectivity and ease of handling (Scheme 49).

Scheme 49. Activation of glycosyltrichloroacetmidite using HClO4-SiO2 [56].

Maghsoodlou et al. ^[57] synthesized α-amino phosphonate derivatives by employing the multi-component reaction of aldehyde (1 mmol),amine (1.2 mmol) and trialkylphosphitein the presence of silica-supported perchloric acid (3 mol%) under solvent-free conditions. The earlier reported synthesis was by base catalyzed nucleophilic addition reaction of phosphites with imines but many imines are hygroscopic and are not sufficiently stable for isolation or decomposition of a Lewis acid during imine formation. Sometimes the reaction can be carried out by employing catalysts including lanthanidetriflate,indium(I) chloride,lithium perchlorate,magnesium perchlorate,TaCl₅-SiO₂,PhNMe₃Cl,TiO₂,sulfamic acid,In(OTf)₃,CF₃CO₂H,Amberlite-IR 120,H₃PW₁₂O₄₀ but these suffer from disadvantages like long reaction time,environmental pollution caused by organic solvents,and expensive catalyst. It was reported that using solid acidic catalysts supported on silica had attracted much attention in organic synthesis due to their advantages such as reusability,inexpensiveness,ease of preparation and handling,nontoxicity,operational simplicity and ease of isolation from the reaction mixture. The catalyst could be reused five times without any loss of its activity (Scheme 50).

Scheme 50. Synthesis of α-amino phosphonate derivatives using HClO4-SiO2 [57].

Table 11 Substitutions used at the R1,R2,R3,and R4position in Scheme 48. R1 R2 R3 R4 CH3 H H H C2H5 H H H CH3 CH3 H H CH3 H H H CH3 H H CH3 C2H5 H H CH3 CH3 CH3 H CH3 CH3 H H CH3 CH3 H Br H CH3 CH3 Br H CH3 H OCH3 H C2H5 H OCH3 H C2H5 H Br H

Table 11 Substitutions used at the R1,R2,R3,and R4position in Scheme 48.

Table 12 Substitutions used at the R1,Ar,and R2position in Scheme 50. R1 Ar R2 Ph Ph Me Ph Ph Et 4-NO2C6H4 Ph Me 4-NO2C6H4 Ph Et 3-NO2C6H4 Ph Et 4-OHC6H4 Ph Me 4-FC6H4 Ph Et 2-ClC6H4 Ph Me 2-ClC6H4 Ph Et 3-ClC6H4 Ph Et 4-ClC6H4 Ph Me 4-MeC6H4 Ph Et 4-NMeC6H4 Ph Me CH3CH2CH2 Ph Me CH3CHCH2 Ph Me 2,4-DiOMeC6H3 4OMeC6H4 Me 2,3-DiOMeC6H3 2FC6H4 Me 2,6-DiClC6H3 4BrC6H4 Me 2,3-DiOMeC6H3 4BrC6H4 Me 2,5-DiOMeC6H3 4ClC6H4 Me 3-NO2C6H4 3NO2C6H4 Et 4-NO2C6H4 4NO2C6H4 Me 4-NO2C6H4 4NO2C6H4 Et

Table 12 Substitutions used at the R1,Ar,and R2position in Scheme 50.

Du et al. ^[58] developed a simple metal and fluoride free one-pot procedure for the efficient and direct esterification of alkyl and aryl silyl ether with Ac₂O and the catalyst system of perchloric acid immobilized on silica gel. The earlier reported methods for the transformation of tert-butyldimethylsilyl (TBDMS) ethers to acetates was by using FeCl₃-Ac₂O,Cu(TOf)₂-Ac₂O,ln(OTf)₃-Ac₂O,AcCl-ZnCl₂,BF₃-Et₂O-NaI-Ac₂O but these suffered from drawbacks such as high cost,suspectibility to moisture and tedious workup when conducted on a large scale. They reported that phenol silyl ethers were directly esterified with excellent selectivity and the solvent Ac₂O can be recovered by distillation and hence wastes were minimized or eliminated (Scheme 51).

Scheme 51. Synthesis of esterification of alkyl and aryl silyl ether with Ac2O using HClO4-SiO2 [58].

Das et al. ^[59] carried out N-alkylation of amides (sulfonamides and carboxamides) using alcohols (primary and secondary aliphatic as well as benzylic). The earlier methods of alkylation were by using a Ru catalyst at high temperature or by Lewis acids but these possessed several drawbacks like long reaction times,high temperatures,costly reagents and unsatisfactory yields and narrow scope. They reported that an effective yield was obtained by using 1,4-dioxane and the catalyst was recovered,dried and reused consecutively thrice without any change in its activity (Scheme 52).

Scheme 52. N-alkylation of amides using HClO4-SiO2 [59].

Murugan et al. ^[60] carried out Hosomi-Sakurai allylation of numerous aldehydes with allylic methyl silane in the presence of benzyl alcohols using silica supported perchloric acid (2 mol%) as the catalyst at room temperature using a CH₃CN:DCM solvent system (7:3). Earlier reported methods for its synthesis were by using Lewis acids like Fe(OTs)₃,Ph₂BOTf,FeCl₃,and TMSI but these have several drawbacks like low yields,long reaction times and high catalyst loading. They reported an effective method for the allylation of a secondary alcohol under mild conditions and used benzyl alcohol instead of benzyloxytrimethyl ether (Scheme 53).

Scheme 53. Hosomi-Sakurai allylation of numerous aldehydes with allylic methyl silane using HClO4-SiO2 [60].

Chun et al. ^[61] carried out the preparation of 1,6-anhydrosugars using silica supported perchloric acid (20 mol%) within a few minutes with good yields. Earlier reported synthesis include pyrolytic methods but these suffer from drawbacks like expensive or stoichiometric reagents,harsh conditions and undesirable hazardous wastes. The advantages of this method were short reaction time,low cost,satisfactory yield and its environmentally benign nature (Scheme 54).

Scheme 54. Synthesis of 1,6-anhydrosugars using HClO4-SiO2 [61].

Ansari et al. ^[62] carried out N-formylation of structurally diverse amines with formic acid using silica supported perchloric acid (2.5 mol%) at room temperature under solvent free conditions. The earlier reported methods have several drawbacks such as they were expensive,toxic,sensitive to moisture,thermal instability,lack of generality,strict anhydrous condition,longer reaction times,high reaction temperatures and use of solvent. They also reported that the catalyst was compatible with different functional groups and formylation proceeded smoothly with amines bearing electron-withdrawing as well as electron-donating substituents. The catalyst could be recovered and reused up to three times without any loss in its activity. After the third cycle,its yield was reduced due to the poisoning of the surface of the catalyst and excellent chemoselectivity was observed with substrates having phenolic OH,providing N-formamide as sole products (Scheme 55).

Scheme 55. N-formylation of structurally diverse amines using HClO4-SiO2 [62].

Dara et al. ^[9] developed a high yielding method for the regioselective reductive ring opening of 4-O benzylidene acetals of hexapyranoside using inexpensive and robust HClO₄-SiO₂ as the acidic catalyst and triethylsilane as hydride donor. Earlier reported methods were by using reagents for benzylidene acetal opening at 4-O opening were NaCNBH₃-I₂,Et₃SiH-BH₃·Et₂O,DIBAL-H in CHsub>2Cl₂,Et3SiH-TFA. For 6-O opening,they were BH₃·NMe₃-BF₃·Et₂O,BH₃·SMe₃-BH₃·Et₂O,PhMS-AlCl₃,BH₃.THF-V(O)(OTf)₂,DIBAL-H in toluene. These suffer from drawbacks like moisture sensitivity,expensive and required in stoichiometric amounts. They also reported that gluco- and manno-pyranoside result in the formation of 6-O-benzyl derivative in good to excellent yields and galactopyranoside led to the formation of 6-O-benzyl derivative with lower yield (Scheme 56).

Scheme 56. Synthesis of regioselective reductive ring opening of 4-O benzylidene acetals using HClO4-SiO2 [9].

Chakraborti et al. ^[63] catalyzed acylation of phenols,thiols,alcohol and amines for the protection of the functionalities by using acetic anhydride under solvent free condition. The poor nucleophilic properties of hydrophilic compounds,particularly phenols,require activation of the anhydride,so various activators are used such as DMAP and BU₃P and Lewis acids such as CoCl_2,TMSOTf,Sc(OTf)₃,Zeolites,Cu(OTf)₂ and LiClO₄. The disadvantages of this methodology are long reaction time,use of hazardous materials (e.g. DMAP and BU₃P ),use of costly catalyst,special effort to prepare the catalyst,use of large amount of acylating agent,and side reaction with the acid-sensitive substrate. To overcome these problems,the use of a new environmentally acceptable acylation process i.e. use of reagents adsorbed on an insoluble inorganic or organic support (e.g. silica,alumina,clay and charcoal). This was easy to handle,was less toxic,non-corrosive free flowing powders,had ease of separation by filtration,feasibility of reuse,good thermal and mechanical stabilities and also circumvented the problem of a side reaction for acid-sensitive substrates (Scheme 57).

Scheme 57. Acylation of phenols,thiols,alcohols,amines using HClO4-SiO2 [63].

Bandgar et al. ^[64] synthesized 1,5-benzodiazepines from o-phenylenediamine and ketones under solvent free conditions in the presence of a catalytic amount of HBF₄-SiO₂ (2 mol%). Earlier 1,5-benzodiazepines were prepared by the condensation of o-phenylenediamines with α-β unsaturated carbonyl compounds,β-haloketones or ketones in the presence of NaBH₄,BF₃-OEt₂,polyphosphoric acid or SiO₂,Al₂O₃/P₂O₅ under microwave. These processes have limitations such as drastic reaction conditions,expensive reagents,low to moderate yield,and tedious work-up procedures. The main disadvantage is that the catalyst is destroyed in the work-up procedure and could not be recovered or reused. Initially the use of a stoichiometric amount of HBF₄ for the synthesis of 1,5-benzodiazepines gave low yield of the product but the use of an optimised quantity of HBF₄-SiO₂ (2 mol%) gave an excellent yield of product. The use of inexpensive and easily available catalyst,experimental simplicity,simple work-up procedure,high yield with selectivity,recovery and reusability of catalyst,relatively short reaction time and potentially useful for industrial applications are the advantages of HBF₄-SiO₂ (Scheme 58).

Scheme 58. Synthesis of 1,5-benzodiazepines using HBF4-SiO2 [64].

Kamble et al. ^[65] synthesized chemoselective carbonyl compounds and transthioacetalization by using a catalytic amount of HBF₄-SiO₂ (1 mol%) under solvent free conditions and allowed the protection of carbonyl compounds and transthioacetalization of O,O and S,O-acetals in the presence of protecting agents. The earlier methods have the disadvantages of the use of a relatively large or stoichiometric amount of the catalyst,expensive reagent,catalyst which cannot be reused or was destroyed in the work-up procedure and toxic and volatile nature of the organic solvent,particularly a chlorinated hydrocarbon,which posed serious problems to the environment. To avoid this,a solid-supported reagent (HBF₄-SiO₂) was used,which has low toxicity,high catalytic activity,moisture and air tolerance,ease of separation,recyclability and low price (Scheme 59).

Scheme 59. Synthesis of chemoselective carbonyl compounds and transthioacetalization using HBF4-SiO2 [65].

Bandger et al. ^[66] catalyzed the regioselective ring opening of epoxides with nitrogen heterocycles such as indoles,pyrroles and imidazoles in the presence of a catalytic amount of HBF₄-SiO₂ under mild conditions to afford the corresponding C-alkylated derivatives in good yields and with high regioselectivity. In earlier reported methods,the ring opening of epoxides was carried out by acid catalysis or under high pressure conditions which require the careful control of the acidity to prevent side reactions. HBF₄-SiO₂ was used as a supported catalyst for the alkylation reactions which catalyzed the reaction efficiently at ambient temperature with high regioselectivity to give a high yield of product in a short reaction time. The advantages of catalyst are that the reactants were recovered in quantitative amounts,low catalyst loading (2 mol%),mild reaction condition and no trace of product was observed after stirring for a longer time with various nitrogen heterocycles (Scheme 60).

Scheme 60. Regioselective ring opening of epoxides using HBF4-SiO2 [66].

Bandger et al. ^[67] synthesized thiiranes from epoxide using a catalytic amount of HBF₄-SiO₂ under solvent free conditions. In earlier reported methods,the transformation of oxiranes into thiiranes was carried out in the presence of trifluoroacetic acid,polymer-supported thiocyanates,silica supported KSCN and Lewis acid but they have limitations such as long reaction time,high temperature,low yield of the products,use of organic solvent and highly acidic catalyst,difficulty in the separation of the product,high catalytic loading,and use of an expensive and unrecoverable catalyst. The toxic and volatile nature of many organic solvents,particularly chlorinated hydrocarbons,have posed a serious threat to the environment. To overcome these limitation,HBF₄-SiO₂ is the preferred catalyst because it can be recovered and reused at least four times without any loss of activity. The main advantages are simple work up,inexpensive and easily available catalyst,fast reaction time,mild reaction conditions,high yields and relatively clean reaction with no polymeric byproducts (Scheme 61).

Scheme 61. Synthesis of thiiranes using HBF4-SiO2 [67].

Sharma et al. ^[68] synthesized α,β-unsaturated carbonyl compounds using highly efficient heterogeneous HBF₄-SiO₂ (1mol%) for thia-Michael addition reaction under a solvent free condition to give an excellent yield. Thia-Michael addition to α,β-unsaturated carbonyl compounds has some preventive measures such as the maintenance of greenness in the synthetic pathway and process,that is,no generation of waste and use of auxiliary substances (e.g. solvents,additional. reagents) and it minimizes energy requirements. The benefits of HBF₄-SiO₂ are easily recovered,highly efficient and can be reused after reactivation without loss of catalytic activity. This method finds application in the one-pot synthesis of 2,3-dihydro-1,5- benzothiazepines (Scheme 62).

Scheme 62. Synthesis of α,β-unsaturated carbonyl compounds using HBF4-SiO2 [68].

Bandgar et al. ^[69] catalyzed a highly productive synthesis of thiomorpholides in the presence of a catalytic amount of solid-supported fluoroboric acid (5 mol%). In the earlier reported method,a traditional approach,i.e.,the Willgerodt- Kindler reaction was used which has limited applications because of the high reaction temperature,expensive and large amount of catalyst,drastic reaction condition,long reaction period required and low to moderate yield obtained. The toxic and volatile nature of the organic solvent,particularly if a chlorinated hydrocarbon,posed a serious threat to the environment. The advantage of the use of a heterogeneous catalyst is the ease of catalyst use,ease of isolation of product,enhanced reaction rate,mild reaction condition,excellent yield,ready availability of the catalyst,easily recovered catalyst that can be reused but the catalytic activity of HBF₄-SiO₂ decreased after 5th cycle,operational simplicity and cleaner reaction profile (Scheme 63).

Table 13 Enone and thiol used in the reaction in Scheme 62.

Table 13 Enone and thiol used in the reaction in Scheme 62.

Scheme 63. Synthesis of thiomorpholides using HBF4-SiO2 [69].

Sharma et al. ^[70] synthesized naphthopyrans using one pot three component condensation of an aldehyde,β-naphthol and active methylene compounds (ethyl acetoacetate or methylacetoacetate) by employing silica supported fluoroboric acid under solvent free conditions and microwave radiation. The advantages of using HBF₄-SiO₂ in this method were short reaction time,high efficiency,solvent free condition and inexpensive catalyst. HBF₄-SiO₂ at 5 mol% for 10 min under microwave conditions efficiently catalyzed the reactions (Scheme 64).

Scheme 64. Synthesis of Naphthopyrans using HBF4-SiO2 [70].

Shiwani et al. ^[71] synthesized a series of 4,6-diaryl/ heteroarylpyrimidones by employing silica-supported fluoroboric acid under solvent-free conditions in a microwave reactor. A model reaction was performed with a mixture of aromatic aldehyde (5.0 mmol,1 eq.),aromatic ketone (5.0 mmol,1 eq.),urea (5.0 mmol,1 eq.),and varying mol% of silicated fluoroboric acid to microwave radiation (varying time) in a microwave synthesizer operating at 150 °C with the maximum microwave power of 400 W for the synthesis of the target compound. The optimum reaction conditions were the use of 5 mol% of the catalyst for 10 min in a microwave reactor. The yield of the pyrimidones was generally high ranging from 62% to 94%. In general,pyrimidones with halogen-substituted phenyl rings (at both the 4 and 6 positions) were obtained in excellent yield (>90%). Applicability to a variety of substrates (bicyclic/aromatic/heteroaromatic ketones and aldehydes),short reaction times,high yields,solvent-free synthesis and utilization of a silica-supported catalyst,which makes the method cost-effective,are the highlights of the method developed (Scheme 65).

This catalytic system has been used in two different forms,viz.,silica adsorbed sulphuric acid and silica sulphuric acid.

Rajput et al. ^[72] catalyzed the hydrolysis/deprotection of the terminal o-isopropylidene groups of a sugar derivative using reusable H₂SO₄-silica of sugar derivatives. Earlier,isopropylidine was used to protect 1,2-and 1,3-diols in carbohydrate and nucleoside chemistry while aqHCl,aqHBr,aq H₂SO₄,trifluoro-acetic acid,and a Lewis acid have been used for the deprotection of isopropylideneacetals. Various silica supported reagents such as FeCl₃·6H₂O,HClO₄,NaHSO₄ were used but this method suffers from the disadvantages of the use of a toxic material,harsh reaction conditions,strong oxidizing conditions,long reaction time,large amount of reagents,low yields and incompatibility with various protecting groups. To avoid these disadvantages,silica sulphuric acid was used as a protic acid under mild and safe conditions. The advantages of H₂SO₄-silica are it is cheap,environmentally friendly,can be recovered and reused several times,short reaction time,gives good to excellent yields,it is safe,easy to handle and provides a simple method for the hydrolysis of the terminal o-isopropylidene groups in the presence of the protecting groups used for oligosaccharides synthesis (Scheme 66).

Scheme 66. Hydrolysis/deprotection of the terminal o-isopropylidene groups of sugar derivatives using H2SO4-SiO2 [72].

Niknam et al. ^[73] catalyzed the methoxymethylation of primary,secondary and tertiary alcohols using formaldehyde dimethoxyacetal in the presence of silica sulphuric acid at room temperature and solvent free conditions. In earlier reported methods,the methoxymethylation was done with chloromethyl methyl ether under basic conditions. Formaldehyde dimethoxyacetal (FDMA) is a cheap and commercially available compound that can be used for the preparation of methoxymethyl ether (MOM-ethers) from hydroxyl compounds. The handling of this reagent is easy,but its main drawback is its poor methoxymethylating power. For the activation of FDMA,a variety of catalysts has been reported such as MoO₂(acac)₂,anhydrous Iron (III) chloride dispersed on molecular sieves. However,this procedure is not suitable for the methoxymethylation of tertiary alcohols. Very recently,Sc(OTf)₃ and Bi(OTf)₃ were used,but these methods have their own merits as well as limitations. To overcome these problems,silica sulphuric acid was used as a safe,green and heterogeneous catalyst for the methoxymethylation of alcohol. The notable advantages of this catalyst are mild reaction conditions,high yields,it is inexpensive,safe,and insoluble in most organic solvents,eco-friendly,and recyclability of the catalyst (Scheme 67).

Scheme 67. Methoxymethylation of primary,secondary and tertiary alcohols using H2SO4-SiO2 [73].

Gawande et al. ^[74] catalyzed N-benzyloxycarbonylation of amines using silica sulphuric acid under solvent free conditions at room temperature. Benzyloxycarbonyl (Cbz) was used for the protection of amines and amines derivatives so that it can be easily removed by catalytic hydrogenation without any side reactions,and was stable in basic and aqueous acidic conditions. Several methods were used for the protection of the amino groups as N-Cbz derivatives,including LiHDMS as a base in THF-HMPA solvent,cyclodextrin in aqueous medium and La (NO₃)₃·6H₂O under solvent free condition. The development of a green synthetic method,i.e.,use of the efficient and chemoselective silica sulphuric acid has the advantages of this catalyst that are it is inexpensive,recyclable,i.e. SSA was used at least 5 times without any change in activity,is eco-friendly and has a relatively low toxicity. These reaction are applicable to a wide variety of primary (aliphatic and cyclic) and secondary amines,amino alcohols and heterocyclic amines (Scheme 68).

Scheme 68. N-benzyloxycarbonylation of amines using H2SO4-SiO2 [69].

Roy et al. ^[75] synthesized simple alkyl and aryl glycosides from free sugars with a diverse range of alcohol to form a series of sugar derivatives in good to excellent yield in the presence of a catalytic amount of silica sulphuric acid by using Fisher type glycosylation. In earlier reported methods,Fisher glycosylation was used for preparing simple alkyl or aryl glycosides from free sugars. However,this method suffers from the drawbacks of the use of strong mineral acids,excess alcohol,long reaction times,and high temperature. Microwave assisted Fisher type glycosylation has been reported but this method was not suitable for a large scale preparation. H₂SO₄-silica can be used as the catalyst to prepare various Fisher type glycosylation alkyl or aryl glycosides from free sugars through Fisher type glycosylation. The benefits of H₂SO₄-silica are the use of a small quantity of alcohol,short reaction time,applicable for large scale preparation and excellent yields (Scheme 69).

Scheme 69. Synthesis of alkyl and aryl glycosides using H2SO4-SiO2 [75].

Zhou et al. ^[76] synthesized 2,3-unsaturated glycopyranosides in the presence of a catalytic amount of silica sulphuric acid. Earlier,the 2,3-unsaturated glycosides were obtained by Lewis acid catalyzed allylic rearrangement of glycals (Ferrier reaction). A variety of acids have been used for the transformation,such as BF₃_OEt₂,IDCP,TMSOTf,I₂,Sc(OTf)₃,InCl,Yb(OTf)₃,BiCl₃,Dy(OTf)₃-immobilized in ionic liquid,ZnCl₂,FeCl₃,HClO₄-SiO,NiCl₅,Er(OTf),Fe₂(SO₄)₃·xH₂O and ZnCl₂/Al₂O₃ but recently,H₂SO₄-SiO₂ as a silica-supported reagent was used for its easy preparation,low cost,high efficiency,environmental benignness,convenience and environment friendliness,short reaction time,high yields,and small dosage (less than 0.02 eq.) of catalyst for the preparation of 2,3-unsaturated glycosides (Scheme 70).

Scheme 70. Synthesis of 2,3-unsaturated glycopyranosides using H2SO4-SiO2 [76].

Safari et al. ^[77] catalyzed the hydrolysis of isobenzofuranone to the corresponding 2-ketomethylquinoline derivatives in high yield under an organic solvent-free condition in the presence of silica sulphuric acid by using microwave irradiation. The earlier reported method catalyzed ester hydrolysis by an acid,alkali,molecular iodine,Zn(II) complexes,or Cu(II) complexes in the presence of a strong liquid protic acid such as HCl,TFA,H₂SO₄ and HNO₃. However,many of these liquid acid catalysts are corrosive and often cause severe environmental pollution because of the difficult separation from the reaction medium,and require long reaction times and give unsatisfactory yields. For avoiding this,silica sulphuric acid was used which is inexpensive,easy to prepare,and insoluble in most organic solvents,which means that it has the advantages of recovery and can be recycled from the various reactions,short reactions times,high chemical yields and purity,no toxic solvent,and no formation of any undesirable side product and epimerization under classical heating conditions (Scheme 71).

Scheme 71. Synthesis of 2-ketomethylquinoline derivatives using H2SO4-SiO2 [77].

Alam et al. ^[78] synthesized primary O-aryl (alkyl) thiocarbamates and primary S-alkyl thiocarbamates in solvent free conditions by using silica sulphuric acid. Traditionally,secondary and tertiary thiocarbamates were obtained by several-pot reaction methods that use phosgene or thiophosgene as the reagent in an organic solvent but they are highly toxic reagents,hazardous to handle,especially in large scale preparation,and the organic solvents are toxic and are not eco-friendly. The new method to synthesize primary O-aryl (alkyl) thiocarbamates and primary S-alkyl thiocarbamates under mild conditions by using silica sulphuric acid in a single step was reported. Solvent-free condition,mildness of the conversion,a simple experimental procedure,clear reaction profiles without expensive starting materials nor toxic solvent and short reaction times are the advantages of silica sulphuric acid (Scheme 72).

Scheme 72. Synthesis of primary O-aryl (alkyl) thiocarbamates and primary S-alkyl thiocarbamates using H2SO4-SiO2 [78].

Bakherad et al. ^[79] catalyzed the three component synthesis of imidazo[1,2-c] pyrimidines by using a catalytic amount of silica sulphuric acid (SSA). In the earlier reported method,the synthesis of imidazo pyridines and pyrimidines used the condensation of a-haloketones with 2-aminopyridines and pyrimidines,but this reaction is not suitable for the generation of a large compound library. The InCl₃-catalyzed three component reaction of aldehydes,isocyanide,and 4-aminopyrimidines for the synthesis of 4-amino-imidazo[1,2-c]pyrimidines in refluxing toluene was used,but this method provided low yields,and was slow,requiring at least 24 h for completion,harsh reaction conditions and an organic solvent. SSA was used as an inexpensive,non-toxic,and recyclable catalyst for the various organic transformations,affording the corresponding products in excellent yields with high selectivity,it was environmentally friendly with respect to corrosiveness,safe,no waste,and ease of separation and recovery (Scheme 73).

Scheme 73. Synthesis of imidazo[1,2-c] pyrimidines using H2SO4-SiO2 [79].

Zolfigol ^[80] synthesized thionitrites and disulfides under mild conditions. Silica sulphuric acid/NaNO₂ was used as a new heterogeneous system for the nitrosation of thiols and production of disulfides. The results revealed that silica sulphuric acid was a good proton source in terms of convenience,cheapness,easy production,and insolubility in all organic solvents. The cheapness and availability of the reagent made the method attractive for large scale operations (Scheme 74).

Scheme 74. Synthesis of thionitrites and disulfides using silica sulphuric acid/NaNO2 [80].

Pore et al. ^[81] catalyzed the conjugate addition of thiols to α,β-unsaturated ketones. When the catalytic efficiency of potassium phosphate was compared with other reported catalysts,it was revealed that potassium phosphate is better suited for this reaction for the reasons of low cost,ease of availability,yields,reaction temperature as well as work-up procedure. Silica sulphuric acid as well as potassium phosphate are highly efficient and cost effective catalysts for the thia-Michael addition reaction under solvent-free conditions at room temperature. Short reaction times,high yields,and avoidance of anhydrous conditions make the protocol a useful alternative to existing methods (Scheme 75).

Scheme 75. Conjugate addition of thiols to α,β-unsaturated ketones using silica sulphuric acid [81].

Table 14 Substitutions at the R,R’,R” position in Scheme 75. R R’ R” H H Ph H H n-Bu Cl H n-Bu Cl H 4-Cl C6H4 Cl H Ph F H PhCH2 F Ome 4-Cl C6H4 F H Ph Cl Me 4-Cl C6H4 Cl Me n-Bu F H 4-Cl C6H4 F H n-Bu

Table 14 Substitutions at the R,R’,R” position in Scheme 75.

Table 15 Substitutions used at the R1 and R2 position in Scheme 76. R1 R2 H H Me H n-Bu H Ph H H Me Me Me n-Bu Me Ph Me

Table 15 Substitutions used at the R1 and R2 position in Scheme 76.

Niknam et al. ^[82] promoted the aromatization of 1,2-dihydroquinolines by using NaNO₂ as the oxidizing agent using silica sulphuric acid. Several oxidation reactions of 1,2-dihydroquinolines were performed with different amounts of the reagents in order to find the best condition and optimizing of the oxidation reaction under various conditions. The notable advantageous of this method are mild reaction conditions,high yield,cheapness,safe,eco-friendly,and recyclability of the silica sulphuric acid (Scheme 76).

Scheme 76. Aromatization of 1,2-dihydroquinolines using silica sulphuric acid by [82].

Table 16 Substitutions used at the Ar1 and Ar2 position in Scheme 77. Ar1 Ar2 2-OCH3C6H4 H 2-ClC6H4 H 2-BrC6H4 H 2,3-OCH3C6H4 H 4-OCH3C6H4 H 4-CH3C6H4 H 2,5-OCH3C6H4 H 4-NO2C6H4 H H H H 4-CH3C6H4 H 2-ClC6H4 H 4-ClC6H4 H 3-ClC6H4 H 3-CH3C6H4

Table 16 Substitutions used at the Ar1 and Ar2 position in Scheme 77.

Wu et al. ^[83] carried out the stereoselective synthesis of β-amino ketones by a direct Mannich-type reaction catalyzed by silica sulphuric acid. Aromatic aldehydes,anilines,and cyclohexanone in ethanol stirred in the presence of a catalytic amount (0.04 mol%) of SSA at room temperature for 3-6 h gave the corresponding β-amino carbonyl compounds. The results revealed that good anti-selectivity was obtained in the SSA-catalyzed direct-type Mannich reaction in ethanol. The significant features of this procedure include high yields,good stereoselectivities,facile operation,recyclable catalyst,and non-toxic solvent (Scheme 77).

Scheme 77. Stereoselective synthesis of β-amino ketones using silica sulphuric acid [83].

Modarresi-Alam et al. ^[84] synthesized primary carbamates using silica sulphuric acid. Primary carbamates were prepared in high yields and in high purity from the reaction of either alcohol or phenol with sodium cyanate in the presence of SSA at room temperature or 55-65 °C for an appropriate time. The results revealed that this simple solvent free method affords various primary carbamates at room temperature in short reaction times,with high yields and purity,without use of a toxic solvent,expensive starting materials,formation of undesirable side products and epimerization. Also,the results revealed that silica sulphuric acid was a highly effective reagent for synthesizing primary carbamates (Scheme 78).

Scheme 78. Synthesis of primary carbamates using silica sulphuric acid by [84].

Nezhad et al. ^[85] catalyzed the deprotection of triphenylmethyl,ρ-anisyldiphenylmethyl and di-(ρ-anisyl)phenylmethyl groups from nucleoside trityl ethers at room temperature using a catalytic amount of silica sulphuric acid (SSA) in acetonitrile. In earlier reported methods,a triphenylmethyl group was used as the protecting group for the 5ʹ-OH function of nucleosides,and various reagents such as HCO₂H in Et₂O and I₂/MeOH were used for the deprotection in the presence of isopropylidene and TBDMS protecting groups but this method has some drawbacks such as long reaction time,use of hazardous reagents,de-purification of trityl ether in acidic condition into sugar residue and its corresponding nucleobasestrityl ether. H₂SO₄-silica was used as an efficient,mild and chemoselective catalyst for the deprotection of nucleoside trityl ether. The advantages of H₂SO₄-silica are the acid-sensitive glycosidic bond in nucleoside is not cleaved by SSA,it is an inexpensive,non-toxic catalyst,easily prepared,recycled,reused and it gave chemoselective deprotection of trityl nucleosides in 2-17 min without any depurification by using a catalytic amount of SSA (Scheme 79).

Scheme 79. Deprotection of triphenylmethyl,ρ-anisyldiphenylmethyl and di-(ρ-anisyl)phenylmethyl groups from nucleoside trityl ethers using H2SO4-SiO2 [85].

Baltork et al. ^[86] synthesized 2-oxazolines and 2-imidazoline from the reaction of nitriles with β-aminoalcohols and ethylenediamine (EDA) by using silica sulphuric acid as a heterogeneous catalyst under reflux conditions. Earlier,a number of methods have been developed for the preparation of 2-oxazolines and 2-imidazoline from carboxylic acids,carboxylic esters,nitriles,aldehydes,hydroxyamides and olefins. However,these methods suffer from disadvantages such as long reaction times,low yields,difficulty in preparation of starting materials and tedious work-up,acidic conditions,use of complex reagents and toxic solvent. Consequently,a simple,inexpensive and environmentally friendly heterogeneous catalyst,i.e.,SSA,was used for the synthesis of 2-oxazolines and 2-imidazoline under reflux condition and ultrasonic condition. The use of SSA catalyst is feasible because of its easy preparation,easy handling,stability,easy recovery,reusability,easy work-up,cheapness,short reaction time,good activity and eco-friendly and the ultrasonic irradiation increased the catalytic activity and higher product yields were obtained (Scheme 80).

Scheme 80. Synthesis of 2-oxazolines and 2-imidazoline using H2SO4-SiO2 [86].

Shaterian et al. ^[87] synthesized 2H-indazolo[2,1-b] phthalazine-1,6,11(13H)-trione derivatives from the three-component condensation reaction of phthalhydrazide,dimedone,and aromatic aldehydes under solvent-free conditions in good to excellent yields and short reaction times using a catalytic amount of silica sulphuric acid as catalyst. Earlier,an acid catalyst was used,such as HF,HClO₃ and H₃PO₄,with some drawbacks. The solid acid,i.e SSA,used has many advantages such as short reaction time,easy to handle,produce products in good to excellent yields (at 100 °C),easier work-up,safer than conventional catalysts like H₂SO₄ and H₃PO₄ with respect to the amount,hazard and reaction conditions. Also,the catalyst can be successfully recovered and recycled for five runs without significant loss in activity (Scheme 81).

Scheme 81. Synthesis of 2H-indazolo[2,1-b]phthalazine-1,6,11(13H)- trione derivatives using H2SO4-SiO2 [87].

Massah et al. ^[88] synthesized N-acylsulfonamides via N-acylation of sulphonamides with carboxylic acid chlorides or anhydrides using SSA as a solid acid catalyst in solvent-free and heterogeneous conditions. In earlier reported methods,the N-acylation were carried out by different methods such as by using carboxylic acid chlorides or anhydrides in the presence of trialkylamines and pyridines in an appropriate solvent (formation of bis-acylated byproducts was a disadvantage of this method) by using N-acyl benzotriazole as an acylating agents and the direct coupling of sulphonamides with carboxylic acid in the presence of a condensing agent (e.g.,carbodiimides (DCC and EDC) and 1,1ʹ-carbonyl diimidazole),but these methods suffer from the disadvantages of as long reaction times,servere reaction conditions,use of expensive or unavailable reagents,low yield of product and tedious work-up. The catalyst introduced a new efficient and environmentally friendly,green chemistry approach for the synthesis of N-acylsulphonamides,evaluation of their in vitro antibacterial activity and carbonic anhydrase II inhibitory activity. These reactions are characterized by non-corrosiveness,safety,low cost and waste,ease of separation,and high yield and chemoselectivity (Scheme 82).

Scheme 82. Synthesis of N-acylsulfonamides using H2SO4-SiO2 [88].

Zolfigol et al. ^[89] carried out the formylation of alcohols by using ethyl formate. A large range of alcohols were subjected to the formylation reaction in the presence of a catalytic amount of SSA or Al(HSO₄)₃ in ethyl formate as an efficient formylating system for the formylation of alcohols to their corresponding formates under mild,nearly neutral and heterogeneous conditions. High chemoselectivity,cheapness and availability of the reagents,nearly neutral and heterogeneous conditions,easy and clean work-up and high yields make this method practical for multi-step synthesis (Scheme 83).

Scheme 83. Formylation of alcohols using silica sulphuric acid [84].

Zolfigol et al. ^[90] synthesized di- and tri(bis(indolyl) methanes) as new triarylmethanes. The feasibility of this method for the preparation of other substituted indoles was demonstrated by using a wide range of dialdehydes and indoles. The reaction of terephthaldialdehyde with other substituted indoles also produced the corresponding di(bisindolyl) derivatives. The yields were high by utilising SSA as catalyst and the purification of the products was very simple (Scheme 84).

Scheme 84. Synthesis of di- and tri(bis(indolyl) methanes) using SSA [90].

Kiasat et al. ^[11] catalyzed the regioselective ring opening of epoxides by the thiocyanate anion to give thiocyanohydrins by using a catalytic amount of SSA under solvent free conditions. The transformation of epoxides into the corresponding β-hydroxythiocyanate using NH₄SCN supported on silica gel in the presence of SSA as catalyst in a solvent-free process occurred in high isolated yields. The advantages of SSA are the easy work-up procedure,availability of reagents,operational simplicity,short reaction time and use of an inexpensive reagent (Scheme 85).

Scheme 85. Regioselective ring opening of epoxides using H2SO4-SiO2 [11].

Wang et al. ^[91] carried out Aza-Michael addition reactions promoted by SSA. Acrylic acid 2-phenylsulfanyl-ethyl ester (PTEA) was allowed to react with morpholine in the presence of varying quantities of SiO₂-SO₃H. The results showed that an excellent yield of the Michael adduct can be achieved by reacting a mixture of morpholine and PTEA in the presence of SiO₂-SO₃H (SSA,100 mg) at room temperature. The catalyst system was recyclable and the reaction condition can be scaled up. In order to test the reusability of the catalyst,a reaction of PTEA and morpholine was carried out in the presence of SSA (1.0 g),and the catalyst was recovered after completion and activated by heating at 100 °C under vacuum for 1 h. The recovered catalyst was reused for the aza-Michael reaction of another batch of PTEA and morpholine giving 90% yield of the desired product after 1 h. Again,the catalyst was recovered,reactivated and reused repeatedly for three more times for the aza-Michael reaction affording 85%,70% and 70% yields,respectively. From this observation,it was clear that the reaction can be scaled up and the catalyst is reusable with a slight decrease in catalytic activity (Scheme 86).

Scheme 86. Aza-Michael Addition Reaction using silica sulphuric acid [91].

Zarei et al. ^[92] synthesized azo dyes by diazotization-diazo coupling of aromatic amines with NaNO₄,SSA,and coupling agents under solvent free conditions at room temperature. In the earlier method,the diazotization and diazo coupling reaction was carried out with nitrous acid under strong acidic condition and the azo coupling carried out at low temperature,generally 10 °C,in presence of nucleophilic coupling components which above 10 °C promoted phenol formation in aqueous media,which cause side reactions. The main limitation of this method is the environmental incompatibility which resulted in disturbing the ecological balance and permanent damage to the environment. So the introduced SSA is an efficient,relatively simple and environmental friendly solid catalyst for the diazotization-diazo coupling reaction. This method has the advantages over the traditional method which include mild reaction condition at room temperature,short reaction time,easy experimental work-up procedure,good yield and mild reaction conditions (Scheme 87).

Scheme 87. Synthesis of azo dyes using H2SO4-SiO2 [92].

Veisi ^[93] synthesized substituted pyrroles under solvent-free conditions using SSA as a solid acid hetrogeneous catalyst. The reusability of the catalyst is one of the most important benefits and makes it useful for commercial applications. Therefore,we investigated the recovery and reusability of the SSA catalyst. The catalyst can be easily separated by simple filtration and re-used after washing with CHCl₃ and drying at 60 °C. The results reported that the catalyst can be used at least five successive times without any decrease in its activity (Scheme 88).

Scheme 88. Synthesis of substituted pyrroles using silica sulphuric acid [93].

Li et al. ^[94] catalyzed the deprotection of oximes to the corresponding carbonyl compounds by the SSA/surfactant/ paraformaldehyde system,which can be carried out with excellent yields at 50 °C in water under ultrasound irradiation. Earlier methods developed for the cleavage of oximes include acid-catalyzed hydrolysis,oxidative deoximation,reductive deoximation and deoximation of exchange of oximes with other carbonyl compounds. However,these methods suffered from drawbacks such as requirements for the refluxing temperature,tedious work-up,drastic conditions,long reaction times,undesired chemical yields and use of toxic reagent. SSA used as catalyst in the organic reactions had the advantages over the earlier reported methods of avoiding the destruction of the acid sensitive functional groups,avoiding use of a toxic solvents and expensive reagents or solvents,and it is stable,reusable,cheap and facile,give higher yield,is easily available,and uses short reaction time and mild reaction conditions (Scheme 89).

Scheme 89. Deprotection of oximes to carbonyl compounds using H2SO4-SiO2 [94].

Niknam et al. ^[95] synthesized 4,4′-(Arylmethylene)bis(1H- pyrazol-5-ols) using silica sulphuric acid. In a set of initial experiments,benzaldehyde was allowed to react with two equivalents of 5-methyl-2-phenyl-2,4-dihydro-3H-pyrazol-3-one in the presence of varying quantities of SSA in a mixture of water-ethanol (1:1 v/v) at 70 °C. The results showed that SSA is an effective catalyst for this condensation,and in the absence of SSA the condensation reaction gave very poor yield after 24 h. Although a lower catalyst loading of 0.05 g of SSA accomplished this condensation,0.08 g of SSA per 1 mmol of aldehyde was optimum in terms of reaction time and isolated yield. The results revealed that the catalyst retained its activity even after nine runs of recycling (Scheme 90).

Scheme 90. Synthesis of 4,4′-(Arylmethylene)bis(1H-pyrazol-5-ols) using silica sulphuric acid [95].

Mansoor et al. ^[96] catalyzed the aqua mediated synthesis of acridinediones by one-pot three-component condensation of an aromatic aldehyde,5,5-dimethyl-1,3-cyclohexanedione (dimedone) and ammonium acetate or p-toluidine in water using silica supported sulphuric acid (SSA) as an efficient catalyst. Earlier methods used for the synthesis of acridinediones include the use of microwave,ionic liquid,silica-bonded S-sulphonic acid and methanesulfonic acid as catalyst and it was also synthesized in aqueous media. However,these methods suffer from the disadvantages of the use of hazardous organic solvents,long reaction time,low yield,formation of side products and multistep synthesis. So there was developed a new,facile,and efficient method for the preparation of substituted acridinediones using SSA as an efficient catalyst. The mildness of the conversion,experimental simplicity,compatibility with various functional groups,recyclability,excellent product yields and easy work-up procedure are the advantages of SSA (Scheme 91).

Scheme 91. Synthesis of acridinediones using H2SO4-SiO2 [96].

In the many research reports,it is seen that silica as a support material can be utilized in many organic reactions with a large number of advantages. Work is being carried out on the use of silica adsorbed heteropolyacids since the discovery of silica as support material for catalytic systems. The reports show many advantages of the catalytic system over the conventional catalysts. Keeping in mind these advantages,researchers have now developed nano-catalytic systems ^{[21, 24, 25]} and mesoporous silica based catalytic systems ^{[20, 27]} to increase the reaction rate of the chemical transformations. These catalytic systems have been developed from nano-silica from rice husk ash and the latter system from mesoporous silica MCM-41 ^[27]. The advantage of these newly developed systems over the others is that they provide a large surface area for solid state dispersion,thus they enhance the rate and yield of the chemical reactions. In the past,many reports are available on the use of heteropolyacids in the chemical reactions but no significant recent report on its use has been identified.

Reports on the use of silicated perchloric acid have always been of interest in carrying out diverse types of chemical reactions. In the past,this catalyst has only been utilized mainly in the stereoselective reactions and glycosylation reactions. The recent reports revealed that this catalytic system was now being employed in other organic transformations like Mannich type reactions ^[52],esterification of silyl ethers ^[58],Hosomi-Sakurai reaction ^[60],in the multicomponent synthesis of organic compounds with a variety of useful and beneficial effects. A recent report on its use has been in the regioselective opening of benzylidene acetals and regioselective HO-4 glycosylation of benzylidene acetals in one-pot synthesis ^[9].

The present research reports also show that another type of silicated catalyst,i.e.,silicated polyphosphoric acid is utilized for the chemical reactions and its versatility has also been studied in diverse form of organic transformations. The catalyst has been used in different chemical reactions like the synthesis of amidoalkyl naphthols ^[29] and a number of other one component and multicomponent reactions. Based on the excellent results of these reactions,nano-silica phosphoric acid has also been utilized and the results obtained were quite good ^[35].

Silicated fluoroboric acid has been in use many years but its use was limited. Our research group has also utilized this catalyst in the microwave assisted synthesis of naphthopyrans ^[70] and excellent results in terms of yield and reusability were observed. Recent studies focused on the versatility of this catalyst.

Silica adsorbed sulphuric acid has been used in two diverse forms,i.e.,silica adsorbed sulphuric acid and the other catalyst system is prepared using chlorosulphonic acid. These catalysts differ only in their method of preparation but they share some common beneficial effects including recyclability and enhanced reaction rates. Thus these two systems are being used in the chemical and pharmaceutical industries. Based on the results revealed by the researches involving the use of silica sulphuric acid,its use as silica adsorbed sulphuric acid has ease of preparation,easy work-up techniques and good to excellent yields.

Silica sulphuric acid has been in use in various kinds of organic reactions. A recent report supporting its use as an efficient catalyst was seen in the aqua-mediated synthesis of acridinediones ^[86]. The results proved its efficiency and thus future studies on its use must be made to explore more of its potential in the reactions. On the other hand,silica adsorbed sulphuric acid has been used only in some limited yet important organic reactions like Fischer type glycosylation ^[76] and has been used extensively because of its chemoselectivity in certain chemical reactions ^{[74, 75]}.

Apart from the use of silica as a solid base support for catalytic systems,various polymers have been utilized for the same purpose. Since most chiral catalysts are expensive,it is highly desirable to reuse the catalyst after each reaction. A common solution to this problem is to heterogenize a homogeneous catalyst,either by anchoring the catalyst on a solid support or by using a liquid-liquid two-phase system ^[97]. Inspired by the idea of solid peptide synthesis developed by Merrifield in the early 1960s,the methodology of attaching chiral ligands onto cross-linked polymers has been widely applied to the development of new polymer supported catalysts for asymmetric reactions. Another approach to address the problem of catalyst separation and recycling is to use a soluble polymer support ^[98]. As an example,the heterogeneization of BINAP onto polyurea was successful and allowed the efficient asymmetric hydrogenation of carbonyl compounds. From the synthetic point of view,in most cases,the use of these Ru-based ligands led to high conversion and in some cases,good enantioselectivity. They also offer easy separation of the catalyst from the reaction mixture and their reuse is,in some cases,effective without any loss either of conversion or enantioselectivity ^[99].

In an alternative approach,the condensation of o-phenylenediamine and aldehyde to give benzimidazole in the presence of a heterogeneous catalyst such as polymer-supported hypervalent iodine and KHSO₄/SiO₂ has been studied. Due to the low surface area and pore volume of the polymer,the catalytic efficiency was poor. The reaction suffered from several disadvantages like long reaction time,drastic reaction conditions and often byproduct formation. All these steps need a complex isolation process leading to a high cost. In addition,these catalysts are not environmentally friendly and not attractive for commercial adoption due to the low activity of the catalyst and the generation of corrosive waste ^[100]. Some of the most common problems when using those materials are related to the loss of activity of the supported catalysts,requiring a high mol% of catalyst loading,and the tedious preparation of the complex molecules to be anchored onto the polymer ^[101].

Nevertheless,nitrogen- or phosphorus-based ligands are known to protect the metal center from oxidation and disproportionation. The use of these ligands usually gives an enhancement of catalytic activity ^[102]. Most importantly,these N-arylated products are readily prepared from inexpensive starting materials in moderate to high yields under straightforward,efficient,and highly economical reaction conditions in short reaction time ^[103].

Various other catalysts that have been utilized so far in industry are 4-(1-triazolyl) proline immobilized on DVB-PS support ^[101],chitosan and a copper-based polymer catalyst ^[103],polymer-supported gadolinium triflate (CMPS-IM-Gd) ^[104],solid-supported Rh (SS-Rh) in the form of nanoparticles (NPs) ^[105].

It is often claimed that PS chiral catalysts can be recycled,but the recycling was rarely studied in any detail. One factor necessary for a PS catalyst to be recycled successfully many times is a robust link between the catalyst moiety and the support. Another is that the catalyst moiety itself must be chemically stable. Very little information is available on this latter point because in practice,laboratory chemists rarely re-use these polymer based catalysts ^[106]. Based on these limitations of the supported catalysts,we conclude that silica supported catalysts are more advantageous to use compared to other supported catalysts.

In conclusion,we summarized the importance and need for silicated acids as catalyst in almost all types of chemical reactions. The article presented the types of silicated catalysts,their beneficial effects,and also the large change in the chemical reactions,viz.,the increase in the yield of the products,high yield-throughput of the reactions,fast reactions,and many other similar conditions and aspects that were discussed in the article. Several industrial processes based on silicate catalysis have been developed and commercialized. As the field of this type of catalysis is of growing importance in almost every type of chemical industry,the detailed knowledge of the mechanism of catalysis is needed for the catalyst design and scale up in some cases. In addition,these catalysts are very important for industries related with fine chemicals such as the flavors,pharmaceutical,and food industries.

The use of a silicated acid as a catalyst overcome the limitations of other catalysts (like harsh reaction conditions,poor yields,prolonged time period). The advantages of these catalysts over the conventional methods include easy preparation of catalyst,easy work-up,low cost,recycling of catalyst,good to excellent yields,simple operation,short reaction times and environmentally non-harmful (eco-friendly) medium,low price,safe,chemoselectivity in many cases,use of ultrasonic irradiation increased the catalytic activity in many cases,ease of separation and non-corrosiveness. The following are the results of some recent studies where these catalysts have been found to show a marginal increase in terms of efficiency and other advantages when compared to the conventional catalytic systems.

The use of heteropolyacids as catalyst for a fine organic synthesis processes is developing and the synthesis of antioxidants,medicinal preparations,vitamins and biologically active substances using them has been reported. When compared to the classical Biginelli reaction conditions,the method described by Rafiee et al. ^[13] has the advantage of excellent yields as compared to the yields produced by the use of other catalysts. A method given by Lana et al. ^[14] utilizing a reaction in the heterogeneous system was more efficient,giving 35% of 1,8- cineole and 25% of 1,4 cineole at 70%-100% conversion in the cyclohexane solution using silica supported PW as a solid acid catalyst (in comparison to other catalysts),and it could be recycled.

The major advantages of using silicated perchloric acid in a method described by Du et al. ^[40] were mild reaction conditions,and a large number of functional groups used for protecting group manipulation were found to remain unaffected,and side reactions like migration and degradation in coupling reactions were suppressed as well. A study by Mishra et al. ^[42] showed the importance of the catalyst as it can be used three times in the direct deprotection and acetylation of methyl 2,3- di-O-acetyl-4,6-O-benzylidene-a-D-glucopyranoside with yields higher than 90%.

The acylation of phenols,thiols,alcohol and amines for the protection of the functionalities by using acetic anhydride,a method described by Chakraborti et al. ^[62] catalyzed by silicated fluoroboric acid,circumvented the problem of side reactions for acid-sensitive substrates. The report also described the various uses and advantages of silicated sulphuric acid in two diverse forms,i.e.,silica adsorbed sulphuric acid and silica sulphuric acid.

In addition,there is much current research and general interest in heterogeneous systems because of their importance in industry and in developing technologies. Heterogeneous organic reactions have proven useful to chemists in the laboratory as well as in the industrial context. These reactions are affected by the reagents immobilized on the porous solid supports and have advantages over the conventional solution phase reactions because of the good dispersion of active sites ^[107].

In view of these studies,other than silica support,various other supports have also been utilized of which different polymers have been used in diverse organic transformations. Out of the various works supporting the use of polymer support as catalyst for the organic chemical reactions,a report revealed the condensation of o-phenylenediamine and aldehyde to give benzimidazole in the presence of a heterogeneous catalyst such as polymer-supported hypervalent iodine and KHSO₄/SiO₂. Due to the low surface area and pore volume of the polymer,the catalytic efficiency was poor ^[100].

The various advantages of a polymer support in the catalytic system lack good support,thus,their use in the organic reactions has been overcome by the use of silica as the support,and also due to its low cost and easy availability,silica supported catalysts are preferred over other supported catalysts.

The review suggests the use of silicated catalysts in numerous reactions for better yields and productivity in comparison to various other conventional means of synthesizing the same compounds. Based on the wide number of advantages of these catalysts,we recommend their use in almost all types of reactions that needs to be catalyzed by one way or another in order to increase the yields and lower the limitations,side effects and some hazardous effects of the conventional catalysts.