Epoxides are an important structural class of industrial chemicals that are regularly used as intermediates in organic synthesis,and the catalytic epoxidation of alkenes represents a useful method for the formation of epoxides [ 1 ]. Among the various catalysts available for the epoxidation of alkenes,chiral MnIII(Salen) is one of the most practical and efficient in terms of the yields and ee values of the resulting epoxides [ 2 ]. Environmental concerns associated with the separation and recycling of the catalysts used in organic synthesis,as well as the purification of products resulting from catalytic processes,have led to the deployment of considerable research efforts towards the development of solid supported MnIII(Salen) catalysts [ 3, 4, 5, 6 ]. Several supports have been developed for Salen catalysts,including high-molecular-weight polymer and inorganic supports,as well as supports based on dendrimers and oligomers [ 7, 8, 9, 10, 11, 12 ]. Although significant advances have been made in recent years towards the development and application of recoverable catalysts,reports pertaining to the effect of inorganic-organic hybrid supports and linker group to the activity of the catalyst have been scarce.
During the course of the last two decades,research within our group has been primarily directed towards the development and application of novel organic-inorganic hybrid materials [ 13, 14, 15, 16, 17, 18, 19 ]. Extremely high levels of catalytic activity were recently observed in the epoxidation of unfunctionalized olefins using a MnIII(Salen) catalyst immobilized onto a range of inorganic-organic hybrid materials,including zirconium oligo-polstyrenylphosphonate-phosphate (ZSPP) [ 13, 14, 15 ],zirconium poly(styrene-isopropenyl phosphonate)-phosphate (ZPS-IPPA) [ 16, 17 ],and ZPS-PVPA [ 18, 19 ]. Furthermore,the use of chiral MnIII(Salen) axially immobilized onto amorphous phenoxyl modified ZPS-PVPA as a catalyst resulted in a remarkable increase in the conversion and ee values for the asymmetric epoxidation of olefins when the reaction was conducted in the absence of expensive O-coordinating axial additive,which is contrary to the results previously published in the literature pertaining to both homogeneous and heterogeneous systems [ 20, 21, 22, 23, 24 ].
As well as paying special attention to the nature of support and catalyst,it is also important to consider the nature of the linker component connecting these two materials because the nature of the linker can have a profound effect on the activity of the catalyst. Li et al. [ 25 ] reported that the linker length was one of the most significant factors affecting the stereo-selectivity of the immobilized MnIII(Salen) catalysts and suggested that the use of longer linkers would effectively highlight any confinement effects and consequently enhance the enantioselectivity exhibited by the catalyst. The use of longer axial linkers,however,could lead to a decrease in the degree of electronic communication between the support material and the MnIII(Salen) catalyst,which would have a negative effect on any enhancement in the enantioselectivity resulting from the axial linker group. Furthermore,it has been reported that longer and flexible linkers lead to an improved outcome in the bimetallic Co-Salen hydrolytic kinetic resolution (HKR) reaction [ 26, 27 ]. In contrast,the heterogeneous MnIII(Salen) catalysts prepared by co-polymerization or grafting of long flexible linker units always provide lower ee values compared with homogeneous catalysts for the asymmetric epoxidation of olefins [ 28 ]. MnIII(Salen) complexes have recently been successfully immobilized onto MCM-41 nanopores,using a rigid phenoxyl [ 29 ] or phenylsulfide [ 30 ] group as the axial immobilization linker. It has been demonstrated that the immobilization of MnIII(Salen) through a phenoxyl group leads to a 3-fold increase in the enantioselectivity of the epoxidation of cis-β-ethylstyrene compared with that of its homogeneous counterpart. Furthermore,the replacement of the immobilization linker with a phenylsulfide group leads to 3.6-fold increase in the enantioselectivity of the same epoxidation reaction compared with the homogenous system. These results therefore indicate that there is a correlation between the nature of the linker group and the activity of the catalyst.
To the best of our knowledge,there have been no reports in the literature pertaining to the use of tunable rigid phenoxyl linkers bearing different substituents at the o-positions of the phenoxyl group to influence the activity of the MnIII(Salen) complexes towards epoxidation. Given that there is still a high level of academic interest in these materials and their potential commercial importance,there is an urgent need for the development of efficient and reusable chiral catalyst for the epoxidation of non-functionalized olefins. As part of our ongoing work towards the development of novel supported catalysts for use in organic synthesis,we have prepared several ZPS-PVPA supported MnIII(Salen) catalysts bearing a tunable phenoxyl linker group (Cat1-Cat3). We have also prepared a series of non-supported analogues (Cat4-Cat6) for comparison,and both of these series were evaluated as catalyst for the epoxidation of olefins. The influence of different substituents at the o-position of the tunable phenoxyl linker on the catalytic performance has also been evaluated both in the presence and in the absence of an axial additive. Furthermore,we have evaluated the recyclability of these catalytic systems and found that they continued to show superior levels of enantioselectivity and catalytic activity for at least five runs. We have also provided some discussion of the mechanistic pathway involved in the attack of the olefin by the Mn intermediate (i.e.,Mn =Ov center).
(1S,2S)-1,2-Diaminocyclohexane,2,4- dihydroxybenzaldehyde,α-methylstyrene,indene,n-nonane,N-methylmorpholine N-oxide (NMO),chloromethyl methyl ether (toxic compound) and m-chloroperbenzoic acid (m-CPBA) were supplied by Alfa Aesar (Tianjin,China) and used as received without further purification. All of the other reagents chemicals used in this study were purchased as the laboratory grade from local suppliers. Styrene was distilled before use. The chiral salen ligand and MnIII(Salen) were synthesized according to the literature procedures [ 2 ].
The synthesis and characterization of ZPS-PVPA have previously been reported by our group [ 18, 19 ].
A mixture of chloromethyl methyl ether (9.30 mL),anhydrous zinc chloride (1.92 g,14.2 mmol),and ZSP-PVPA (6.00 g,8.55 mmol) was stirred at 45 °C for 8 h. The mixture was then cooled to ambient temperature before being treated with water (20 mL) and methanol (20 mL). The mixture was then filtered and the filter-cake washed sequentially with methanol and acetone before being collected and dried in vacuo to give ZCMPS-PVPA in 90.3% yield. The amount of elemental chloride in the product was determined from the results of the rhombic experiments of chloromethylation. Under the optimal conditions,a chloride content of 0.5023% Cl per gram of ZCMPS-PVPA was obtained following the titration of this material with AgNO3.
The appropriate amounts of L1,L2,and L3 were mixed with ZCMPS-PVPA (2.63 g,3.45 mmol),K2CO3 (4.50 g,30.0 mmol),and ethanol (20.0 mL) (the molar ratio of diphenol to elemental chlorine in ZCMPS-PVPA was set at 5:1),respectively,and the resulting mixtures were held at 80 °C for 24 h under N2. The resulting resin like products were then filtered and washed sequentially with large volumes of deionized water and ethanol to remove any residual diphenol before being dried under vacuum at 80 °C to give the corresponding products in yields of 93.0%,86.0%,and 95.0%,respectively. Following their reaction with sodium phenoxide,the residual Cl contents of the supports were obtained by titration against AgNO3. The results of these titrations allowed for the number of -CH2Cl groups replaced by phenoxyl groups to be conveniently calculated.
A solution of chiral MnIII(Salen) (2.73 g,4.30 mmol),phenoxyl modified ZCMPS-PVPA (S1-S3),(0.50 g),and an appropriate amount of sodium hydroxide in tetrahydrofuran (50.0 mL) was stirred vigorously for 24 h under reflux. The reaction mixture was then cooled to ambient temperature and filtered to give a dark brown powder,which was washed sequentially with ethanol,CH2Cl2,and demonized water before being dried under vacuum at 80 °C. The CH2Cl2 filtrate was analyzed by UV-vis spectroscopy until no peaks could be detected (with CH2Cl2 as reference). The products (Cat1-Cat3) were isolated in yields of 92.0%,89.1%,and 84.8%,respectively. The loading of the MnIII(Salen) complex in the heterogenized catalyst,based on the amount of elemental Mn,was determined to be in the range of 0.41-0.52 mmol/g by AAS.
MnIII(Salen) (1.56 g,2.56 mmol) and the appropriate amount of the linker (L4-L6,1.2 equiv.) were added to ethanol (60.0 mL),and the resulting mixture was refluxed for 5 h at 80 °C (Scheme 2). The reaction was then cooled to room temperature and evaporated to dryness,the resulting residue was dissolved in CH2Cl2 and washed sequentially with deionized water and brine before being dried over anhydrous Na2SO4. The solvent was then removed under vacuum to gives the desired product as a brown-dark solid (Cat4-Cat6). Cat4: yield 80.0%; Anal. Calcd for Cat4 (C42H57MnN2O3): C 72.81%,H 8.28%,N 4.05%; Found: C 74.15%,H 8.42%,N 4.17%. Cat5: yield 76.3%, Anal. Calcd for Cat5 (C43H59MnN2O3): C 73.06%,H 8.41%,N 3.96%; Found: C 75.26%,H 8.63%,N 4.21%. Cat6: yield 62.7%; Anal. Calcd for Cat6 (C46H65MnN2O3): C 73.77%,H 8.75%,N 3.74%; Found: C 74.88%,H 8.98%,N 3.93%.
FT-IR spectra were recorded on a Bruker RFS100/S spectrophotometer (Karlsruhe,Germany) as KBr disks. Diffuse reflectance UV-vis spectra of the solid samples were recorded in the spectrophotometer with an integrating sphere using BaSO4 as a reference standard. Number- and weight-average molecular weights (Mn and Mw) and polydispersity values (Mn and Mw) were estimated using a Waters 1515 gel permeation chromatograph (GPC) system (Massachusetts,USA) against polystyrene standards using Tetrahydrofuran (THF) as an eluent (1.0 mL/min) at 35 °C. The Mn content of the catalyst was determined by TAS-986G (Beijing,China). Elemental analyses were conducted on an EATM1112 automatic elemental analyzer (Massachusetts,USA). X-ray photoelectron spectra (XPS) were recorded on an ESCALAB250 system (Massachusetts,USA). Scanning electron microscopy (SEM) analysis was performed on a KYKY EM3200 system (Beijing,China). Transmission electron microscopy (TEM) analysis was conducted on a Philips TECNAI10 system (Amsterdam,Holland). Atomic force microspcopy (AFM) analysis was conducted on a NanoScope Quadrex system (NewYork,USA).
The racemic epoxides were prepared by the epoxidation of the corresponding olefins with m-CPBA in CH2Cl2,and their structures were confirmed by NMR analysis on a Bruker AV-300 system. The GC was calibrated using n-nonane,as well as several olefins and the corresponding racemic epoxides. A solution of alkene (0.500 mmol),NMO (338 mg,2.50 mmol,if necessary),n-nonane (internal standard,56.0 mL,0.500 mmol),and the immobilized MnIII(Salen) complex (0.025 mmol,5%,based on the Mn amount of the catalyst) in CH2Cl2 (3 mL) was cooled to the desired temperature. Solid m-CPBA (138 mg,0.80 mmol) was added in four portions over 2 min,and the resulting mixture was stirred at ambient temperature. Upon completion of the reaction,as indicated by TLC,the mixture was washed sequentially with saturated sodium hydroxide and brine to remove any residual m-CPBA and the corresponding acid and dried over anhydrous Na2SO4. The conversion and ee values were determined by GC using nonane as an internal standard. After the first run of the epoxidation reaction,the catalyst was separated by centrifugation and washed thoroughly with dichloromethane before being dried,and subjected to another cycle with fresh reactants under similar epoxidation conditions. The conversion (with n-nonane as an internal standard) and the ee values were determined by gas chromatography (GC) using a Shimadzu GC2014 (Kyoto,Japan) instrument equipped with a chiral column (HP19091G-B213,30 m × 0.32 mm × 0.25 μm) and FID detector. The injector and detector temperatures were set at 230 °C. A column temperature in the range of 80-180 °C was used for α-methylstyrene and indene.
The immobilized catalysts,as well as the homogeneous MnIII(Salen) for comparison,were characterized by FT-IR spectroscopy (Fig. 1.). The FT-IR spectrum of MnIII(Salen) showed characteristic vibration bands around 1620,1535,and 569 cm−1,which were attributed to the stretching vibration modes of C=N,C-O,and Mn-O,respectively. These bands were still present in the FT-IR spectra of the immobilized catalysts,which suggested that the heterogeneous catalysts had similar structures to that of neat chiral MnIII(Salen) and the structure of the catalytic active site remained intact. The stretching vibration observed at 1030 cm−1,which was assigned to the characteristic vibrations of phosphonate and phosphate in the support,was clearly weakened as a consequence of changes in the electronic structure of the host-guest interaction. These observations suggested that the chiral MnIII(Salen) complex has been successfully immobilized onto the tunable phenoxyl group modified ZPS-PVPA.
The diffuse reflectance UV-vis spectra of the MnIII(Salen) complex and immobilized catalysts are shown in Fig. 2. These spectra provided further evidence in support of the successful immobilization of the MnIII(Salen) complex,based on the fact that the characteristic bands for MnIII(Salen) at 340,435,and 510 nm had been blue-shifted to 330,410,and 505 nm, respectively,for the immobilized catalysts,which indicated the occurrence of an interaction between the MnIII(Salen) complex and the support through the tunable phenoxyl linkers. The characteristic bands at 262 and 330 nm were attributed to the charge transfer transition of the Salen ligand,whereas the bands at 410 and 505 nm were attributed to the ligand-to-metal charge transfer transition and d-d transition of the MnIII(Salen) complex [ 31 ].
SEM,TEM,and AFM images of the immobilized MnIII(Salen) catalyst (Cat3) are shown in Fig. 3. The SEM image of the particles of the Cat3 catalyst indicated that they were submicron in diameter. The SEM image also showed that Cat3 was in a loose,amorphous state. Numerous cavities,holes,and pores of different sizes and shapes also existed on the surfaces of the particles. The TEM and AFM images revealed that the particles of the Cat3 catalyst consisted of much smaller particles. The images indicated that the surface properties of the catalyst would lead to an increase in the surface area of the catalyst and allow the substrates better access to the catalytically active sites.
XPS has been used extensively to obtain detailed information about the state of metal species immobilized on the surfaces of catalyst in terms of allowing for the electronic properties of these species to be effectively investigated. Fig. 4. shows the Mn 2P3/2 XPS spectra of the MnIII(Salen) complex and the immobilized catalyst (Cat3). The shift in the binding energy of the major peak from 642.1 to 642.4 eV was attributed to Mn 2p3/2 following the immobilization process,which is in agreement with the values previously reported for MnIII(Salen) and porphyinic ligands [ 32, 33 ]. The observed increase of 0.3 eV in the chemical shift for the immobilized salen complex comparing with MnIII(Salen) was attributed to differences in the coordination environment of Mn.
The catalytic activities and enantioselectivities of the catalysts (Cat1-Cat6) were evaluated for the asymmetric epoxidation of unfunctionalized olefins in CH2Cl2 using m-CPBA as an oxidant. All of the reactions proceeded smoothly with the heterogeneous catalysts showing higher ee values but slightly lower levels of conversions compared with the corresponding homogeneous Jacobsen’s catalyst under the same conditions. Table 1 shows that Jacobsen’s catalyst and the non-supported analogues (Cat4-Cat6) were active and enantioselective for the epoxidation of α-methylstyrene although the enantioselectivities were not very high (49.5%-52.0%). After anchoring the Jacobsen’s catalyst onto the ZPS-PVPA using phenoxyl linkers of varying steric hindrance,there was a clear reduction in the activities of these heterogeneous catalysts (Cat1-Cat3) for the epoxidation of α-methylstyrene under the same conditions. Interestingly,however,there was a notable difference in the ee value,which increased from 52% to 73.2% for Cat3 towards the epoxidation of α-methylstyrene. Li et al. [ 29 ] reported that the ee values increased from 56% to 72% for the epoxidation of α-methylstyrene following the immobilization of MnIII(Salen) onto the surface of MCM-41 via the oxygen atoms of the phenoxyl group although the conversion remained low at only 60%. This increase in the ee has been mainly attributed to the axial phenoxyl linkers and the support [ 34 ]. These slight decreases in conversion are most likely due to the slow diffusion of the reactant and the oxidant within the microporous channels and secondary channels of ZSP-PVPA. However,following the immobilization process,there was a reduction in the selectivity of the product,which was mainly attributed to the exothermic activity associated with the dissolution of the m-CPBA and the acidic conditions,which co uld have contributed to the ring-opening of the epoxides. The heterogenized catalysts also showed good catalytic activity and enantioselectivity towards bulkier olefins,such as indene (98.9% conversion; 80.1% ee),which indicated that the active sites in the microporous channels of ZPS-PVPA could also be readily accessed by bulkier reactants. When styrene was used as substrate,the reaction gave low ee values in the range of 12.9%-38.6%. However,when the reaction temperature was lowed from 0 to -78 °C,the enantioselectivity of the Cat3 catalyst towards the epoxidation of styrene improved significantly from 38.6% to 56.2% ee. A comparison of Cat3 and Jacobsen’s catalyst with a previously reported catalyst [ 19 ] for the epoxidation of styrene revealed that these systems provided increases in the reported ee values of 9.2% and 12.8%,respectively,under the same conditions.
Table 1 shows a comparison of the results obtained for the enantioselective epoxidation of α-methylstyrene in the presence/absence of axial bases of NMO using Jacobsen’s catalyst and the non-supported analogues (Cat4-Cat6). It is clear from the results reported previously in homogeneous systems that the addition of an axial additive such as NMO induces a conformational change in the skeleton of the MnIII(Salen) complex,resulting in an increase in the reaction rate and an improvement in the enantioselectivity [ 21, 22, 23 ]. Interestingly,slight increases were observed in the ee values in the current study for the epoxidations catalyzed by MnIII(Salen) bearing axially coordinated phenoxyl linkers of varying steric hindrance. For example,the ee value for the epoxides of α-methylstyrene increased from 51.1% to 58.5% without the addition of NMO (Table 1, entry 4 vs 5),and similar results have also been reported by List et al. [ 35 ] and Huang et al. [ 36 ],where MnIII(Salen) was axially coordinated to rigid phosphate and diphenolate groups,which replaced the Cl atoms in the MnIII(Salen) complex. The heterogeneous catalysts (Cat1-Cat3) also displayed high ee values and conversions in the absence of the NMO additive,which is commonly used to improve the catalytic activity. From a practical perspective,the addition of NMO to the reaction mixture did not lead to an improvement in asymmetric induction but resulted in a significant reduction in the ee values and reactivity. The ee value for the epoxidation of α-methylstyrene increased from 6.8% to 76.8% without the addition of NMO. This phenomenon has been reported previously by our group,where it was attributed to the organic polymer-inorganic hybrid support ZPS-PVPPA and the axially coordinating phenoxide group [ 19 ]. In the current study,this novel catalytic result was induced by another important factor,namely the steric hindrance effect d erived from the ortho substituents of the tunable phenoxyl axial linkers. It was envisaged that the use of tunable phenoxyl axial linkers with substituents bearing structures similar to those of the N-oxide ligand would behave effectively as axial ligands. Thus,the addition of an N-oxide ligand led to the occurrence of steric hindrance effects,which resulted in changes to the optimal geometric configuration of the reactive intermediate or transition states of salen Mnv =O. These changes then led to a decrease in the chiral recognition properties of the catalytic system and the observed lower enantioselectivity.
Table 2 shows a comparison of the enantioselective epoxidation of styrene using the Cat3 catalyst in various solvents. As expected,the use of different solvents had a pronounced effect on the catalytic activity and enantioselective. The results of this study revealed that dichloromethane was the optimal solvent for the reaction,providing a yield of 72.09% and an ee of 38.6%. In contrast,the use of hexane and alcohol resulted in yields of 3.71 and 4.30% and ee values of 9.6 and 13.5%,respectively. These differences among the solvents were attributed to the fact that Cat3 swelling to a much greater extent in dichloromethane than any other solvent. The enantioselectivity was lower in ethyl acetate,acetonitrile and acetone than it was in dichloromethane,indicating that solvents containing oxygen and nitrogen atoms bearing lone pairs of electrons could be coordinating to the metal center of the chiral MnIII(Salen) complex and suppressing the formation of the active oxygen transfer species (Mn-V-oxo) required for the epoxidation of styrene. The ee of the reaction in acetonitrile was particularly low at only 24.8%.
The steric hindrance effect of the tunable phenoxyl linkers on the epoxidation of unfunctionalized alkenes was also evaluated,and Table 1 shows a comparison of the supported and non-supported catalysts in terms of their epoxidation of unfunctionalized alkenes under similar conditions. When the α-methylstyrene was used as substrate,the ee values of the resulting epoxides were found to be in the range of 49.5%-51.1%,with conversions in the range of 97.2%-98.0%. These values were obtained after the Cl atoms were replaced with a series of different phenoxyl linker groups,which suggested that the steric hindrance effects of the linker were not having a discernible impact on the homogeneous reaction results. However,the use of the heterogeneous catalyst Cat3 led to much higher ee values (up to 76.8%),which were 9.5% and 3.6% higher than those achieved with Cat1 and Cat2,respectively. With regard to the conversions,there was a slightly decrease going from 98.8% to 90.7%. Similar results were also obtained for styrene and indene. When the phenoxyl group on the linker was replaced with an o-t-Bu group,the enantioselectivity of the epoxidation of α-methylstyrene increased from 6.8% to 76.8%,representing increases of 9.0% and 2.3% compared with the results for Cat1 and Cat2,respectively. These results demonstrated that the substituents at the o-positions of the tunable phenoxyl linkers have a critical impact on the catalytic activity,in that the o-t-Bu group provided higher levels of enantioselectivity than the o-H and o-CH3 groups of the phenoxyl linker. These results therefore indicated that pathway (a) (Scheme 3) provided the most accurate representation of the way in which the Mn=Ov center reacts with the olefin. In principle,there are five possible trajectories ((a)-(e) in Scheme 3) for the reaction of the olefin with the Mn=Ov center ,which have been reported by Jacobsen et al. [ 37 ] and Hosoya et al. [ 38 ]. None of the other possible reaction pathways could account the different catalytic results observed for the linkers bearing different substituents at their o-positions. Furthermore,it has been reported that good yields and ee values for the epoxides can be achieved by adjusting the properties of 3,3′-phenyl groups in Scheme 3. Hence,we believe that pathway (a) represents one of the best pathways for rationalizing the results observed in the current study. More detailed studies concerning the mechanistic behavior of these supported and non- supported analogues are currently in underway in our laboratory.
The recyclability of the immobilized catalyst Cat3 was evaluated using the epoxidation of α-methylstyrene as a model reaction. After the first run of the epoxidation reaction,the catalyst was separated by centrifugation. The separated catalyst was then collected and washed thoroughly with dichloromethane before being dried and subjected to another cycle with fresh reactants under similar epoxidation conditions (i.e.,the same ratio of the substrate-to-catalyst and solvent-to- catalyst). The recovery of the catalyst after each run was found to be in the range of 93%-98%. As shown in Table 3,there were no significant changes in the catalytic activity and enantioselectivity exhibited by the catalyst after five runs,and no leaching of Mn into the filtrate was observed by AAS or UV-vis,which indicated that the chiral MnIII(Salen) complex was strongly bonded to the tunable phenoxyl modified ZPS-PVPA support. Considering the gradual reduction in the conversion and ee values of the catalyst following several runs,the immobilized Cat3 catalyst was subjected to a Soxhlet-extraction procedure using dichloromethane after six reaction cycles. This extraction process led to an increase in the conversion (86.5% vs 82.5%) in the seventh run (entry 7),which indicated that the catalyst had remained intact and that the observed reduction in activity was related to the blockage of active sites during the previous runs. After the Cat3 catalyst had been recycled 10 times,the Mn content of the heterogeneous catalyst had decreased by 78.0% compared with the total amount of Mn (0.41 mmol/g). The observed decreases in the activity as the number of cycles increased was therefore attributed to the leaching of the metal complex from the support.
We have developed a series of chiral MnIII(Salen) catalysts that were axially supported onto zirconium poly(styrene- phenylvinylphosphonate)-phosphate (ZPS-PVPA) through phenoxyl linkers of varying steric hindrance,and evaluated their performance as catalysts for the epoxidation of a variety of unfunctionalized olefins. Significant increases were observed in the activities and enantioselectivities of these catalysts over the corresponding heterogeneous catalysts such as Jacobsen’s catalyst and their non-supported analogues. The reactions were conducted in the absence of NMO in CH2Cl2 using m-CPBA as an oxidant. Furthermore,the catalysts could be recycled up to five times without any significant reduction in their catalytic activity or enantioselectivity. The influence of the steric hindrance effect derived from the tunable phenoxyl linkers used in the homogeneous and heterogeneous epoxidation systems was explored at length,and the results demonstrated that variations in the steric hindrance imposed by the phenoxyl linker had a significant effect on the catalytic activity and enantioselectivity. Further studies aimed at comparing the effect of different linker groups on the catalytic activities of these systems towards different reactions would therefore pave the way for the rational design of highly active and selective supported catalysts.
2014, Vol. 35 Issue (9): 1446-1455
PDF (719 KB)
2014, Vol. 35 Issue (9): 1446-1455
PDF (719 KB)