催化学报  2014, Vol. 35 Issue (7): 1024-1029   PDF (355KB)    
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Arash Ghorbani-Choghamarani
Shima Rashidimoghadam
One-pot synthesis of 1-amidoalkyl-2-naphthols catalyzed by melamine-Br3 under solvent-free conditions
Arash Ghorbani-Choghamarani , Shima Rashidimoghadam    
Department of Chemistry, Faculty of Science, Ilam University, Ilam, Iran
Received: 2013-12-17. Accepted: 2014-1-9. Published: 2014-7-20.
Corresponding author. Tel/Fax: +98‐841 2227022; E‐mail: arashghch58@yahoo.com, a.ghorbani@mail.ilam.ac.ir
Foundation Item: This work was supported by Research Facilities of Ilam University, Ilam, Iran.
Abstract: A facile and efficient method has been developed for the synthesis of 1-amidoalkyl-2-naphthols via the one-pot multi-component condensation of 2-naphthol with aromatic aldehydes and acetamide or thioacetamide in the presence of melamine-Br3 under solvent-free conditions. There are several advantages to this reaction, including high yields, short reaction time, and high catalytic efficiency.
© 2014, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
Key words: Multi-component reaction     Amidoalkyl naphthol     Tribromo-melamine     Solvent-free     One-pot    

1. Introduction

Multi-component reactions (MCRs), where three or more reactants are combined into a one-pot process, represent an increasingly important and attractive area of research in organic synthesis because they provide high levels of efficiency via the combination of several operational steps, as well as allowing for operational steps involving the isolation of intermediates or changing of reaction conditions to be avoided. In terms of the advantages that they offer, MCRs are general efficient procedures that provide high levels of atom economy and significant cost savings. The discovery and development of novel and known MCRs have consequently become a popular area of research in organic chemistry.

Compounds containing 1,3-amino-oxygenated functional groups are frequently used as biologically active natural products, and this structural motif can also be found in a number of potent drugs such as nucleoside antibiotics and the HIV protease inhibitors [1, 2], such as ritonavir and liponavir [3, 4]. 1-Amidoalkyl-2-naphthol derivatives can be converted to 1-aminomethyl-2-naphthols via an amide hydrolysis reaction. These compounds are important synthetic building blocks and exhibit a range of biological activity, including depressive and bradycardia effects in humans [4, 5].

1-Amidoalkyl-2-naphthols can be prepared by the multi- component condensation of aryl aldehydes with 2-naphthol and acetamide or thioacetamide in the presence of an appropriate catalyst such as RuCl2(PPh3)3 [6], sulfamic acid [7, 8], Ce(SO4)2 [9], [FemSILP]-L-prolinate [10], Bi(NO3)3·5H2O [11], K5CoW12O40·3H2O [12], Hf(NPf2)4 [13], H3PW12O40 [14], Yb(OTf)3 [15], Fe(HSO4)3 [16], montmorillonite K10 clay [17], p-TSA [18], H4SiW12O40 [19], zeolite [20], 2,4,6-trichloro-1,3,5- trizine [21], iodine [22], PFPAT [23], and poly(4- vinylpyridinium butane sulfonic acid) hydrogen sulfate [24].

In most cases, however, the application of these methods is limited by their requirement for prolonged reaction time, ultrasonic or microwave irradiation, and the use of stoichiometric quantities of toxic and corrosive catalysts. Furthermore, the existing procedures generally provide low yields of the desired products. Therefore, the discovery of a new, inexpensive, and readily available catalyst with high catalytic activity and short reaction time for the preparation of amidoalkyl naphthols is strongly desired. Tribromo-melamine (melamine-Br3) is a homogeneous and non-hygroscopic solid catalyst that can be readily prepared by reaction of melamine with Br under alkali conditions (Scheme 1) [25].

Scheme 1. Preparation of melamine-Br3.

Melamine-Br3 is stable under a variety of different reaction conditions, including acidic and basic conditions. It is noteworthy that melamine-Br3 is produced via a facile and clean process that does not require a complicated work-up procedure. Melamine-Br3 has been used in a variety of different reactions, including the synthesis of 2-aryl thiazolines [25] and the trimethyl silylation of hydroxyl groups with 1,1,1,3,3,3- hexamethyldisilazane (HMDS) [26]. Herein, we report a melamine-Br3- catalyzed one-pot MCR for the synthesis of biologically interesting 1-amidoalkyl-2-naphthols.

2. Experimental
2.1. General procedure for the synthesis of 1-amidoalkyl-2-naphthols

All of the chemicals used here were purchased from Fluka, Merck, and Aldrich chemical companies. The products were characterized by comparison of their spectral (1H and 13C NMR) and physical data with those of authentic samples.

Melamine-Br3 (0.054 g, 0.15 mmol) was added to a mixture of aldehyde (1 mmol), 2-naphthol (1 mmol) and acetamide or thioacetamide (1.5 mmol), and the resulting mixture was stirred at 130 °C in an oil bath for the appropriate time. Upon completion of the reaction, as determined by TLC, the mixture was cooled to room temperature, and purified directly by column chromatography over silica gel using a mixture of acetone/n-hexane (3:7, v/v) as the eluent to give the desired product.

2.2. Condensation of 2-naphthol with terephthaldehyde and acetamide to bis-1-amidoalkyl-2-naphthol

A mixture of terephthaldehyde (0.134 g, 1 mmol), 2-naphthol (0.36 g, 2.5 mmol), acetamide (0.22 g, 3.75 mmol), and melamine-Br3 (0.054 g, 0.15 mmol) was stirred at 130 °Cfor the appropriate time. Upon completion ofthe reaction, as determined by TLC, the mixture was cooled to room temperature and purified directly by column chromatography over silica gel using a mixture of acetone/n-hexane (1:1, v/v) as the eluent to give the desired product as a X solid.

2.3. Spectral data for products

N-((2-Hydroxynaphthalen-1-yl)-(phenyl)methyl))acetamide (a). 1H NMR (400 MHz, DMSO-d6): δ = 1.98 (s, 3H), 7.12-7.38 (m, 6H), 7.76-7.84 (m, 3H), 8.45 (d, J = 8, 1H), 10.0 (s, 1H).

N-((4-Chloro-phenyl)-(2-hydroxynapthalen-1-yl)methyl)acetamide (b). 1H NMR (400 MHz, DMSO-d6): δ = 2.0 (s, 3H), 7.10 (d, J = 8.0 Hz, 1H), 7.17 (d, J = 8.4 Hz, 2H), 7.23 (d, J = 8.8 Hz, 1H), 7.26-7.33 (m, 3H), 7.39 (m, 1H), 7.78-7.83 (m, 3H), 8.48 (d, J = 8.0 Hz, 1H), 10.06 (s, 1H).

N-((4-Bromo-phenyl)-(2-hydroxynapthalen-1-yl)methyl) acetamide (c). 1H NMR (400 MHz, DMSO-d6): δ = 2.0 (s, 3H), 7.10 (m, 3H), 7.23-7.30 (m, 2H), 7.38 (d, J = 8.0 Hz, 1H), 7.45 (d, J = 8.4 Hz, 2H), 7.80 (m, 3H), 8.49 (d, J = 8.0 Hz, 1H), 10.07 (s, 1H); 1H NMR (400 MHz, DMSO-d6 + D2O): δ = 1.98 (s, 3H), 7.07 (m, 3H), 7.20(d, J = 8.8, 1H), 7.26 (m, 1H), 7.38 (m, 3H), 7.77 (m, 3H).

N-((4-Flouro-phenyl)-(2-hydroxynapthalen-1-yl)methyl) acetamide (d). 1H NMR (400 MHz, DMSO-d6): δ = 2.0 (s, 3H), 7.06-7.12 (m, 3H), 7.17-7.3 (m, 4H), 7.39 (m, 1H), 7.77-7.83 (m, 3H), 8.49 (d, J = 8.4, 1H), 10.05 (s, 1H).

N-((2-Hydroxynaphthalen-1-yl)-(2-nitrophenyl)methyl)) acetamide (e). 1H NMR (400 MHz, DMSO-d6):δ = 9.75 (s, 1H), 8.60 (d, J = 8 Hz, 1H), 7.66-7.87 (m, 4H), 7.02-7.48 (m, 6H), 1.95 (s, 3H); 13C NMR (100 MHz, DMSO-d6):δ = 169.0, 153.6, 148.5, 136.8, 133.0, 131.9, 129.8, 128.8, 128.3, 127.9, 127.3, 126.5, 123.8, 122.4, 122.3, 118.3, 116.0, 45.5, 22.1.

N-((3-Nitro-phenyl)-(2-hydroxynapthalen-1-yl)methyl) acetamide (f). 1H NMR (400 MHz, DMSO-d6): δ = 2.04 (s, 3H), 7.27 (m, 3H), 7.43 (t, J = 7.2 Hz, 1H), 7.57 (m, 2H), 7.86 (m, 3H), 8.03 (s, 1H) 8.06 (m, 1H), 8.65 (d, J = 8.0 Hz, 1H), 10.17 (s, 1H); 13C NMR (100 MHz, DMSO-d6): δ = 23, 48.1, 118.3, 118.9, 120.9, 121.7, 123.1, 123.3, 127.3, 128.9, 129.2, 130.1, 130.4, 132.6, 133.3, 145.9, 148.2, 153.8, 170.2.

N-((2-Hydroxynaphthalen-1-yl)-(3-hydroxyphenyl)methyl))acetamide (g). 1H NMR (400 MHz, DMSO-d6): δ = 1.97 (s, 3H), 7.01-7.36 (m, 6H), 7.75-7.81 (m, 4H), 8.39 (d, J = 8.0 Hz, 1H), 9.19 (s, 1H), 9.96 (s, 1H).

N-((2-Hydroxynaphthalen-1-yl)-(4-hydroxyphenyl)methyl))acetamide (h). 1H NMR (400 MHz, DMSO-d6): δ = 1.96 (s, 3H), 7.0-7.45 (m, 6H), 7.78-7.83 (m, 2H), 8.49 (d, J = 8.0 Hz, 1H), 9.19 (s, 1H), 10.11 (s, 1H).

N-((2-Hydroxynaphthalen-1-yl)-(4-ethoxyphenyl)methyl)) acetamide (i). 1H NMR (400 MHz, DMSO-d6): δ = 9.98 (s, 1H), 8.42 (d, J = 8.0 Hz, 1H), 7.74-7.81 (m, 2H), 6.78-7.37 (m,6H), 3.94 (q, 2H), 1.96 (s, 3H), 1.28 (t, 3H); 13C NMR (100 MHz, DMSO-d6): δ = 169.0, 156.8, 153.0, 134.2, 132.2, 129.0, 128.4, 128.4, 127.1, 126.1, 123.3, 122.3, 118.9, 118.4, 113.8, 62.8, 47.3, 22.6, 14.6.

N-((4-Methyl-phenyl)-(2-hydroxynaphtalen-1-yl)methyl) acetamide (j). 1H NMR (400 MHz, DMSO-d6): δ = 1.99 (s, 3H), 2.24 (s, 3H), 7.1 (s, 1H), 7.23 (d, 1H), 7.24 (d, 4H), 7.26 (m, 1H), 7.75 (m, 1H), 7.78 (d, 1H), 7.79 (d, 1H), 7.82 (d, 1H), 8.44 (s,1H), 9.98 (s,1H); 13C NMR (100 MHz, DMSO-d6): δ = 47.61, 118.46, 118.98, 122.32, 125.97, 126.21, 129.1, 128.51, 132.3, 134.9, 139.55, 153.07, 169.14.

N-((4-Chlorophenyl)-(2-hydroxynaphthalen-1-yl)methyl) ethanethioamide (k).1H NMR (400 MHz, DMSO-d6): δ = 2.06 (s, 3H), 7.14-7.40 (m, 8H), 7.79-7.83 (m, 3H), 8.53 (d, 1H), 10.13 (s, 1H); 13C NMR (100 MHz, DMSO-d6): δ = 23.0, 47.8, 118.7, 118.9, 122.9, 123.6, 126.9, 128.3, 128.4, 129.1, 130.0, 131.1, 132.6, 142.2, 153.5, 153.6, 169.9.

N-((4-Bromo-phenyl)-(2-hydroxy-napthalen-1-yl)-methyl)- thioacetamide (l). 1H NMR (400 MHz, DMSO-d6): δ = 2.0 (s, 3H), 7.10 (m, 3H), 7.23-7.30 (m, 2H), 7.38 (d, J = 7.6 Hz, 1H), 7.45-7.47 (m, 2H), 7.80 (m, 3H), 8.49 (d, J = 8.4 Hz, 1H), 10.07 (s, 1H); 1H NMR (400 MHz, DMSO-d6 + D2O): δ = 1.98 (s, 3H), 7.1-7.6 (m, 3H), 7.21 (m, 1H), 7.24-7.28 (m, 1H), 7.39-7.43 (m, 3H), 7.76-7.80 (m, 3H). 13C NMR (100 MHz, DMSO-d6): δ = 23.1, 47.9, 118.8, 118.9, 119.5, 123. 123.7, 127, 128.8, 129, 129.1, 130, 131.3, 132.7, 142.7, 153.7, 170.

N-((2-Hydroxynaphthalen-1-yl)-(p-tolyl)methyl)ethanethio-amide (m). 1H NMR (400 MHz, solvent): δ = 1.99 (s, 3H), 2.23 (s, 3H), 7.06 (s, 1H), 7.35-7.38 (m, 6H), 7.76-7.84 (m, 4H), 8.46 (s, 1H), 10.03 (s, 1H); 13C NMR (100 MHz, DMSO-d6): δ = 21.0, 23.0, 48.9, 118.8, 118.9, 119.3, 119.4, 122.8, 123.8, 126.4, 129.0, 129.6, 132.7, 135.5, 139.9, 153.4, 169.7.

N-((3-Nitro-phenyl)-(2-hydroxynapthalen-1-yl)methyl) thioacetamide (n). (Table 1, Entry 14) 1H NMR (400 MHz, DMSO-d6): δ = 2.04 (s, 3H), 7.19-7.25 (m, 2H), 7.31 (m, 1H), 7.43 (m, 1H), 7.55-7.60 (m, 2H), 7.82-7.86 (m, 4H), 8.03-8.08 (m, 2H), 8.65 (d, J = 8.0 Hz, 1H), 10.16 (s, 1H); 1H NMR (400 MHz, DMSO-d6 + D2O): δ = 2.01 (s, 3H), 7.15 (s, 1H), 7.2 (d, J = 8.8 Hz, 1H), 7.3 (m, 1H), 7.42 (m, 1H), 7.54 (m, 1H), 7.6 (d, J = 8.0 Hz, 1H), 7.79-7.87 (m, 2H), 7.9 (s, 1H), 8.02-8.04 (m, 1H); 13C NMR (100 MHz, DMSO-d6): δ = 23, 48, 118.3, 118.9, 120.9, 121.7, 123.1, 123.3, 127.2, 128.9, 129.2, 130.1, 130.4, 132.6, 133.3, 145.9, 148.2, 153.8, 170.2.

Table 1
The effect of catalyst amounts on the reaction of 2-naphthol with acetamide and benzaldehyde at 130 °C

N-((5-Bromo-2-hydroxy-phenyl)-(2-hydroxynapthalen-1-yl)methyl)thioacetamide (o). 1H NMR (400 MHz, DMSO-d6): δ = 1.92 (s, 3H), 6.66 (d, J = 8.8 Hz, 1H), 7.12-7.20 (m, 3H), 7.27 (m, J = 14.8 Hz, 1H), 7.44 (m, J = 14.8 Hz, 1H), 7.60 (s, 1H), 7.71 (d, J = 8.8 Hz, 1H), 7.78 ( d, J = 8.0 Hz, 1H), 8.20 (d, J = 8.8 Hz, 1H), 8.44 (d, J = 8.4 Hz, 1H), 9.70 (s, 1H), 9.91 (s, 1H); 1H NMR (400 MHz, DMSO-d6 + D2O): δ = 1.88 (s, 3H), 6.68 (d, J = 8.8 Hz, 1H), 7.10-7.18 (m, 3H), 7.27 (m, 1H), 7.39-7.46 (m, 2H), 7.71 (d, J = 8.8 Hz, 1H), 7.76 (d, J = 8.0 Hz, 1H), 8.10 (d, J = 8.8 Hz, 1H); 13C NMR (100 MHz, DMSO-d6): δ = 23.1, 44.9, 110, 117.4, 119, 119.1, 122.7, 123.7, 126.5, 128.6, 128.8, 129.3, 130.3, 131.7, 131.8, 133.1, 153.7, 154.3, 169.0.

N-((4-(Acetylamino(2-hydroxynaphthalen-1-yl)methyl) phenyl)-(2-hydroxynaphthalen-1-yl)methyl)acetamide (A). White solid; m.p.: 280-282 °C (277-279 °C [27]); 1H NMR (400 MHz, DMSO-d6): δ = 9.96 (s, 2H), 8.38 (d, J = 8.0 Hz, 2H), 7.73-7.81 (m, 4H), 7.04-7.34 (m, 12H), 1.92 (s, 6H); 13C NMR (100 MHz, DMSO-d6): δ = 140.3, 132.2, 129.1, 128.4, 128.3, 126.2, 125.7, 122.3, 118.7, 118.6, 118.3, 118.2, 118.2, 47.6, 22.5.

3. Results and discussion

In a continuation of our ongoing research towards the development of novel transformation in organic synthesis using halogenating agents [26, 28, 29], we have developed a process for the construction of 1-amidoalkyl-2-naphthols using melamine-Br3 as an efficient catalyst. The reaction of benzaldehyde with 2-naphthol and acetamide was initially selected as a model transformation (Scheme 2) to optimized the reaction conditions.

Scheme 2. Preparation of N-((2-hydroxynaphthalen-1-yl)-(phenyl)methyl)acetamide.

As shown in Table 1, when the reaction was initially conducted in the absence of the catalyst, only a small amount of the product was formed over a long reaction time. The addition of 0.05 mmol of catalyst led to a significant increase in the yield (70%) with the reaction time being reduced to 6.5 h. Increasing the catalyst amount from 0.05 to 0.15 mmol continously, the product yield was increased, and the reaction time was reducted. Further increasing the catalyst amount to 0.20 mmol, however, led to a reduction in the product yield and an increase in the reaction time. 0.15 mmol of catalyst therefore gave the best results.

To determine the effect of temperature on the reaction, we evaluated the condensation reaction of 2-naphthol with acetamide and benzaldehyde in the presence of melamine-Br3 at a variety of different temperatures. As shown in Fig. 1, when the reaction temperature was varied from room temperature to 130 °C, the product yield gradually increased, with the highest yield being obtained at 130 °C.

Fig. 1. The effect of temperature on the reaction of 2-naphthol with acetamide and benzaldehyde.

With the optimized conditions (i.e., 2-naphthol 1 mmol, arylaldehyde 1 mmol, amide 1.5 mmol, melamine-Br3 0.15 mmol, 130 °C), we proceeded to evaluate the scope of the reaction by preparing a variety of different amidoalkyl naphthols (Table 2).

Table 2
Synthesis of 1-amido or thio alkyl-2-naphthols catalyzed by melamine-Br3.

From Table 2, it is clear that aromatic aldehydes bearing either an electron-donating or electron-withdrawing group proceeded smoothly under the optimized conditions to give the desired products in good yields. Aromatic aldehydes bearing an electron-donating group, however, gave a slightly lower yield and required longer reaction time (Table 2, entries 7-10). The optimized method also proceeded successfully when thioacetamide was used instead of acetamide in the reaction (Table 2, entries 11-16).

The presence of an electron-withdrawing group on the benzaldehyde led to an increase in the rate of the 1,4- nucleophilic addition reaction of the o-quinone methide (o-QM) intermediates because the electron-withdrawing group led to a reduction in the energy of the lowest unoccupied molecular orbital of the alkene.

The nature of the substituent on the aromatic aldehyde did not appear to have a significant effect on the product yield, with electron-donating and electron-withdrawing substituents both providing good to excellent yields of the corresponding products, although sterically hindered aromatic aldehydes required longer reaction time.

Based on our results and related reports from the literature [25], we have proposed a mechanism for the transformation which is shown in Scheme 3.

Scheme 3. Proposed mechanism for the construction of 1-amido or thio alkyl-2-naphthols.

The reaction is believed to proceed via the formation of an o-QM intermediate. Nucleophilic conjugate addition of the amide to the o-QM intermediate would lead to the formation of the amidoalkylnaphthol product in excellent yields. In this reaction, the melamine-Br3 could act as a bifunctional catalyst [27, 30, 31], in that it would activate both the carbonyl oxygen in the aldehyde and the acidic hydrogen in 2-naphthol. Since melamine-Br3 contains Br atoms that are attached to N atoms, it is likely that Br+ would be released in situ, and that this species would act as a catalyst in the reaction medium; leading to a considerable increase in the electrophilicity of the aldehyde. The reaction would proceed via the formation of an o-QM intermediate, which would be formed via the nucleophilic addition of 2-naphthol to the aldehyde. The o-QM intermediate would then react with acetamide or thioacetamide to produce the desired 1-amidoalkyl-2-naphthol product.

The reaction was also studied using the bis-aldehyde terephthalaldehyde with 2-naphthol and acetamide (Scheme 4).

Scheme 4. Synthesis of N-((4-(acetylamino-(2-hydroxynaphthalen-1-yl)methyl)phenyl)-(2-hydroxynaphthalen-1-yl)methyl)acetamide.

When the 2-naphthol (2.5 mmol) and the aldehyde (1 mmol) were reacted with acetamide (3.75 mmol) in the presence of melamine-Br3 (0.15 mmol) at 130 °C for 2 h, bis-1- amidoalkyl-2-naphthol (A) was obtained in 95%.

The results of the current study were compared with those of several other reported procedures from the same transformation to demonstrate the effectiveness of our newly developed method (Table 3). It revealed that melamine-Br3 was superior to other catalysts in terms of the product yields and/or the reaction time.

Table 3
Comparison of different catalysts for the one-pot three-component reaction of aldehydes with 2-naphthol and acetamide.
4. Conclusions

A new catalytic procedure for the synthesis of 1-amidoalkyl- 2-naphthol derivatives via the one-pot multi-component condensation of 2-naphthol with aromatic aldehydes and acetamide/thioacetamide under solvent-free conditions was developed using melamine-Br3 as an efficient catalyst. There are several advantages to this procedure compared with the existing methodologies, including good yield and short reaction time. Furthermore, the catalytic system is environmentally benign and highly efficient, as well as being easy to prepare.

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