Organic Chemistry: Current Research

Organic Chemistry: Current Research
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Research Article - (2013) Volume 2, Issue 2

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Tetra-N-Butyl Ammonium Hydroxide as Highly Efficient for the Acylation of Alcohols, Phenols and Thiols

Mosstafa Kazemi* and Mohammad Soleiman-Beigi
Department of Chemistry, Ilam University, Ilam, Iran
*Corresponding Author: Mosstafa Kazemi, Department of Chemistry, Ilam University, PO Box 69315-516, Ilam, Iran Email:

Abstract

Aqueous tetra-n-butyl ammonium hydroxide solution (TBAOH) is an efficient catalyst for the acylation of alcohols, phenols and thiols. This procedure is convenient, simple and suitable for the synthesis of esters and thioesters in high yields.

Introduction

One of the most important priorities within organic chemistry research is to find methods and processes more compatible and more economical compared to preceding methods. To accomplish this, we use aqueous solutions and ionic liquids as solvent in large scale. It should be noted reduction of reaction’s time and the number of processes stage is to be taken into account.

Acylation of alcohols, phenols and thiols is one of very valuable and widely used transformations in organic synthesis because of their important role in the fields of biological, industry, synthetic and medicine chemistry [1-4].

In the past decades, many methods were described (reported) for the acylation of alcohols, phenols and thiols. Generally, acylation of alcohols and phenols is performed by means of acid anhydrides or acid chlorides in the presence of tertiary amines such as triethylamine, pyridine [1]. In addition to, other catalysts such as montmorillonite [5], ionic liquids [6], triflates [7-9], tributylphosphine [10], distannoxane [11], magnesium bromide [12], indium trihalides [13] and CsF–Celite have also been utilized to achieve the acylation of alcohols, phenols, thiols [14,15]. Some of the above described methods require the use of harshreaction conditions, hazardous materials, excess acylating agent, long reaction time, high temperature and low yields.However, still it is of great importance to find new useful and environmentally friendly methods with use of base catalysts for the acylation of alcohols, phenols and thiols.

Tetra-n-butyl ammonium hydroxide is a strong organic base, which also acts as a phase transfer reagent and a surfactant. It has been used as a base or additive in Aldol [16], Ullmann [17], non-Sonogashira [18] types and Knoevenagel [19] reactions, elimination [20], addition [21] reactions, as well as hydrolysis of esters and amides [22], alkylation [23], titration [24] and synthesis of nanoparticles [25] and titanium silicate [26].

In continuation of our interest in exploring new application of tetran- butyl ammonium hydroxide in organic synthetic methodologies [27-34] and attempts to develop previous methods, we wish to report acylation of alcohols, phenols under neat conditions in the presence of an aqueous solution of TBAOH at mild conditions.

Results And Discussion

In order to optimize of the reaction conditions in terms of the amount of TBAOH (20% in water), time and temperature, the reaction of benzyl alcohol (1.5 mmol) with acetic anhydride (1.0 mmol) and benzyl mercaptan (1.0 mmol) with acetic anhydride (1.2 mmol) were studied without the presence of solvent (Scheme 1) as models reactions. As can be seen from Table 1, the rate and efficiency of reactions depend on the amount of TBAOH and temperature. The best results were obtained in the presence of 2 mL of TBAOH (Table 1, entry 4) at 50°C under air (Scheme 1 and Table 1).

organic-chemistry-alcohol

Scheme 1: The reaction of benzyl alcohol (1.5 mmol) with acetic anhydride (1.0 mmol) and benzyl mercaptan (1.0 mmol) with acetic anhydride (1.2 mmol).

Entry Temp. (°C) TBAOH (mL) Thioether Ether
  Time (min) Yield (%)* Time (min) Yield (%)b
1 50 0.5 400 70 380 72
2 50 1.0 240 77 260 75
3 50 1.5 110 85 100 81
4 50 2.0 70 90 80 92
5 50 2.5 70 90 80 91
6 25 2.0 400 30 400 25
7 40 2.0 400 74 400 65

aModel reaction conditions: molar ratio of benzyl alcohol (benzyl mercaptan)/acetic anhydride(acetic anhydride) was 1.5 (1.0)/1.0 (1.5). The reactions run in the presence of TBAOH (20% in water) without any extra solvent under an air atmosphere conditions
bYield refer to an isolated yield by preparative chromatography

Table 1: Optimization of TBAOH amount and temperature of reactiona.

Therefore, a diverserange of dialkyl (symmetric and unsymmetric) and aryl alkyl esters and thioesters was synthesized in good to excellent yields (80-92%) in the presence of 2 mL of aqueous solution of TBAOH (Scheme 2) in optimal reaction condition. The method is very general, which aliphatic alcohols as well as phenols react easily with acyl halides andacid anhydrides was converted to corresponding esters and thioesters exclusively (Scheme 2, Table 2 and 3).

organic-chemistry-phenols

Scheme 2:The reaction of phenols with acyl halides and acid anhydrides forming thioesters.

Entry Substrate Reagent Product Time (min) Yieldb (%)
1 image (CH3CO)2O image 80 92
2 image 4- NO2PhCOCl image 95 85
3 image (CH3CO)2O image 80 83
4 image (CH3CO)2O image 90 86
5 (CH3)2CHOH 4- NO2PhCOCl image 95 87
6 image (CH3CO)2O image 85 85
7 image (CH3CO)2O image 80 88
8 (CH3)2CHOH 4- CH3PhCOCl image 90 87
9 image PhCOCl image 100 84
10 (CH3)3CH2OH PhCOCl image 105 85
11 image (CH3CO)2O image 95 87
12 CH3CH2OH PhCOCl PhCO2CH2CH3 80 82
13 (CH3)4CH2OH PhCOCl image 95 80
14 PhCH2OH PhCOCl PhCH2O2CPh 90 83
15 image (CH3CO)2O image 110 83
16 image (CH3CO)2O image 95 89
17 image (CH3CO)2O image 105 84
18 image (CH3CO)2O image 100 81
19 image (CH3CO)2O image 90 83
20 image (CH3CO)2O image 95 80
21 image PhCOCl image 80 83
22 image (CH3CO)2O image 100 89
23 image PhCOCl image 110 86
24 image (CH3CO)2O image 95 81
25 image PhCOCl image 105 82
26 image 4-OMePhCOCl image 110 83
27 image 4-OMePhCOCl image 95 89
aAll the products are known compounds and were characterized by comparison of their IR and NMR spectral data and physical properties with those reported in the literature [11,24-30]. bIsolated yield

Table 2: Acylation of alcohols and phenols in the presence of TBAOH (20% in water)a.

Entry Substrate Reagent Product Time (min) Yieldb (%)
1 image (CH3CO)2O image 70 90
2 image (CH3CO)2O image 85 84
3 image (CH3CO)2O image 80 86
4 image 4-OMePhCOCl image 95 87
5 image PhCOCl image 80 89
6 image (CH3CO)2O image 95 85
7 image PhCOCl image 90 83
8 image PhCOCl image 95 83
9 image (CH3CO)2O image 90 88
10 image (CH3CO)2O image 100 86
11 image (CH3CO)2O image 110 84
12 (CH3)3CH2SH PhCOCl image 105 88
aAll the products are known compounds and were characterized by comparison of their NMR spectral data and physical properties with those reported in the literature [11,27,30]. bIsolated yield

Table 3: Acylation of thiols in the presence of TBAOH (20% in water)a.

Products data

• Phenyl acetate (C8H8O2) (Table 2, entry 15). Yield: 83%; colorless liquid;

1H NMR (CDCl3, 400 MHz): δ 2.08 (3H, s, COCH3), 7.24-7.54 (5H, m, ArH). 13C NMR (100 MHz, CDCl3): δ = 20.1, 113.8, 129.3, 130.7, 159.2, 170.5 ppm.

• Benzyl acetate (C9H10O2) (Table 2, entry 1). Yield: 92%; colorless liquid;

1H NMR (CDCl3, 400 MHz): δ 2.17 (3H, s, COCH3), 5.2 (2H, s, PhCH2), 7.25-7.35 (5H, m, ArH). 13C NMR (100 MHz, CDCl3): δ = 22, 65.4, 126.9, 128.5, 128.9, 138.7, 170.4 ppm.

• 4-Bromo Phenyl acetate (C8H7BrO2) (Table 2, entry 18). Yield: 81%; colorless liquid;

1HNMR (CDCl3, 400 MHz): δ 2.45 (3H, s, COCH3), 6.9 (2H, d, J=8.8, ArH), 7.29 (2H, d, J=8.8, ArH). 13C NMR (100 MHz, CDCl3): δ = 21.1, 121.6, 125.9, 129.5, 156.8, 170.4 ppm.

• Isopropyl 4-methylbenzoate (C11H14O2) (Table 2, entry 8). Yield: 87%; colorless liquid;

1H NMR (CDCl3, 400 MHz): δ 1.03 (6H, d, J=7.2, (CH3)2 CH), 2.05 (3H, s, COCH3), 4.2 (1H, m, J=7.2, ArH), 7.47 (2H, d, J=8.8, ArH).7.56 (2H, d, J=8.8, ArH). 13C NMR (100 MHz, CDCl3): δ = 20.3, 59.1, 126.8, 128.8, 132.9, 135.5, 159.1 ppm.

• 2-Naphthyl acetate (C12H10O2) (Table 2, entry 22). Yield: 89%; colorless liquid;

1H NMR (CDCl3, 400 MHz): δ 2.40(s, 3H, COCH3), 7.27-7.30 (m, 1H, ArH), 7.49-7.5(m, 2H, ArH), 7.61 (d, 1H, ArH), 7.84-7.91 (m , 3H, ArH).13C NMR (100 MHz, CDCl3): δ = 21.2, 118.7, 121.3, 125.8, 126.7, 127.8, 127.9, 129.5, 131.6, 133.9, 148.5, 169.8 ppm.

IR (KBr) cm-1: 1758(C=O)

• 4-nitrobenzyl acetate (C9H9NO4) (Table 2, entry 3). Yield: 83%; colorless liquid;

1H NMR (CDCl3, 400 MHz): δ 2.17(s, 3H, COCH3), 5.22(s, 2H, PhCH2), 7.53 (2H, d, J=8.8Hz, ArH), 8.23(2H, d, J=8.8Hz, ArH) ppm.

IR (KBr) cm-1:1235(C-O), 1738(C=O)

• 3-Methoxybenzyl acetate (C10H12O3) (Table 2, entry 11). Yield: 87%; colorless liquid;

1H NMR (CDCl3, 400 MHz): δ 2.13 (s, 3H, COCH3), 3.83(s, 3H, OCH2), 5.11(s, 2H, PhCH2), 6.88-6.97(m, 3H, ArH), 7.30(t, 1H,ArH) ppm.

IR (KBr) cm-1:1227(C-O), 1742(C=O)

• 2-phenyl propyl acetate (C11H14O2) (Table 2, entry 6). Yield: 85%; colorless liquid;

1H NMR (CDCl3, 400 MHz): δ 1.34 (d, J=6.8Hz, 3H, CH3), 2.05 (3H, J=6.8Hz, s, CH3, COCH3), 3.09-3.18(m, 1H, PhCH2), 4.14- 4.25(2H, m, J=6.8Hz, CH2CO), 7.25-7.29 (m, 3H,), 7.33-7.37 (m, 2H). 13CNMR(10MHz, DMSO)=18.1, 20.9, 38.9, 69.4, 126.7, 127.3, 128.5, 143.2, 171.1 ppm.

IR (KBr) cm-1:1233(C-O), 1741(C=O)

• phenylthioacetate (C8H8OS) (Table 3, entry 9). Yield: 88%; colorless liquid;

1H NMR (CDCl3, 400 MHz): δ 3.51 (3H, s, COCH3), 7.29-7.44 (5H, m, ArH).13C NMR (100 MHz, CDCl3): δ = 29.8, 127.7, 127.8, 129.4, 137.2, 196.1 ppm.

• n-ButylThiobenzoate (C11H14OS) (Table 3, entry 12). Yield: 88%; colorless liquid;

1H NMR (CDCl3, 400 MHz): δ 0.93 (3H, t, J=7.2, CH3), 1.42 (2H, sex, J=7.2, CH2CH3), 1.58 (2H, quint, J=7.2, CH2CH2), 2.45 (2H, t, J=7.2, CH2S).7.26 (1H, quint, J=7.2, ArH) 7.34 (2H, t, J=7.2, ArH).7.53 (2H, d, J=7.2, ArH). 13C NMR (100 MHz, CDCl3): δ = 13.7, 22.1, 31.1, 31.3, 127.7, 127.8, 128.5, 132.2, 137.4, 196.7 ppm.

• benzylthioacetate (C9H10OS) (Table 3, entry 1). Yield: 90%; colorless liquid;

1H NMR (CDCl3, 400 MHz): δ 2.17 (3H, s, COCH3), 5.19 (2H, s, CH2S), 7.3 (1H, t, J=7.2, ArH) 7.4 (2H, t, J=7.2, ArH). 7.53 (2H, quint, J=7.2, ArH). 13C NMR (100 MHz, CDCl3): δ = 21.2, 43.8, 121.6, 127.6, 127.8, 128.5, 189.5 ppm.

• 4-methoxybenzyl thioacetate (C10H12O2S) (Table 3, entry 2). Yield: 84%; colorless liquid;

1H NMR (CDCl3, 400 MHz): δ 2.02 (3H, s, COCH3), 3.4 (2H, s, OCH3), 4.71 (2H, s, CH2S), 7.1 (2H, d, J=8, ArH), 7.4 (2H, d, J=8, ArH). 13C NMR (100 MHz, CDCl3): δ = 15.3, 55.3, 65.4, 113.8, 129.3, 130.7, 159.1, 196, 196.7 ppm.

Conclusions

In conclusion, we developed the utility of TBAOH as an efficient, versatile, commercially available, environmentally and economically friendly organo-basic catalyst for the preparation esters and thioesters. This method is applicable for the acylation of alcohols, phenols and thiols with acyl halides and acid anhydrides. In this way, we described a new, mild, simple and highly efficient method for the synthesis of esters and thioesters in excellent yields (80-92%) and short reaction times (70-110 min) under neat aqueous condition without using phase transfer reagent and organic solvent.

Experimental

Chemicals were purchased from commercial suppliers and used without further purification. Yields refer to isolated products. Melting points were determined by an Electrothermal 9100 apparatus and are uncorrected. The IR spectra were obtained on a FT-IR Hartman- Bomen spectrophotometer as KBr disks, or neat. The 1H NMR (400 MHz) and 13C NMR (100 MHz) spectra were recorded on a Bruker Avance NMR spectrometer in CDCl3 solution. The progress of the reaction was monitored by TLC using silica-gel SILG/UV 254 plates. All products are known and were characterized by comparing their physical and spectral data with those of the authentic samples.

Typical procedure: benzyl acetate synthesis (Table 2, entry 1)

A mixture of TBAOH (2.0 mL, 20% in water) and benzyl alcohol (1.5 mmol, 162 mg) was vigorously stirred at room temperature for 15 min. acetic anhydride (1.0 mmol, 102 mg) was then added to the mixture and stirring continued at 50°C for the appropriate times (Table 2) under air. The progress of reaction was monitored by TLC. After completion of the reaction, CH2Cl2 (15 mL) was added, and the mixture washed with H2O (3×10 mL). The organic layer was dried over anhydrous Na2SO4. The solvent was evaporated in vacuo to give benzyl acetate which was purified by preparative TLC (silica gel, eluent n-hexane: EtOAc = 4:1) to obtain 118 mg of the pure benzyl acetate (92%).

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Citation: Kazemi M, Soleiman-Beigi M (2013) Tetra-N-Butyl Ammonium Hydroxide as Highly Efficient for the Acylation of Alcohols, Phenols and Thiols. Organic Chem Curr Res 2: 119.

Copyright: © 2013 Kazemi M, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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