Appel Reaction of Carboxylic Acids with Tribromoisocyanuric Acid/Triphenylphosphine: a Mild and Acid-Free Preparation of Esters and Amides

Abstract

A facile and efficient method for esterification and amidation of carboxylic acids under neutral conditions has been developed. Esters and amides can be prepared by reacting a carboxylic acid (1 mmol) with tribromoisocyanuric acid (0.37 mmol) and triphenylphosphine (1 mmol) in dichloromethane at room temperature, followed by addition of an alcohol or an amine, respectively.

Keywords:
amides; esters; tribromoisocyanuric acid; acylation; triphenylphosphine

Introduction

Esters and amides are important intermediates in the chemical and pharmaceutical industries. Besides, some of them are bioactive compounds.¹ In general, their preparations from the corresponding carboxylic acids are well-known transformations and numerous methods have been reported.² However, the coupling reactions of carboxylic acids with nucleophiles require activation of the carboxyl group by its conversion into the most reactive acyl halide.²

On the other hand, acid halides are traditionally prepared from carboxylic acids and several common reagents such as thionyl halides, oxalyl halides or phosphorus (oxy)halides.³ However, despite their success, there are still some remaining problems using these reagents as they cannot be applied to acid-sensitive substrates due to the high temperature conditions required and the formation of strong harmful corrosive acids by-produced during the process. Furthermore, if the amount of the reagent used is insufficient, acid anhydrides are obtained instead of acyl halides.⁴ Therefore, convenient protocols for the preparation of acid halides under mild conditions is being subject of intensive research.⁵ In this context, the Appel reaction⁶ of carboxylic acids with carbon tetrachloride,⁷ trichloroacetonitrile⁸ or trichloroacetamide⁹ (among others) in the presence of triphenylphosphine (PPh₃) have been efficiently used for the preparation of acyl chlorides with high efficiency. Despite being the driving force in Appel-type reactions, the formation of the triphenylphosphine oxide by-product can result in an inconvenient purification process; therefore the development of new methods that avoid or minimize the triphenylphosphine oxide by-product in Appel reactions is subject of intensive research.¹⁰

Trihaloisocyanuric acids [1,3,5-trihalo-1,3,5-triazine-2,4,6-(1H,3H,5H)-triones] (Figure 1) are safe, stable and easily handled solids used as electrophilic halogenating reagents.¹¹ Although diverse trihaloisocyanuric acids are known,¹² the most widely used is the inexpensive and commercially available trichloroisocyanuric acid (TCCA; Figure 1, X = Cl). While not yet commercially available, tribromoisocyanuric acid (TBCA; Figure 1, X = Br) can be easily prepared from inexpensive materials (cyanuric acid, KBr and oxone).¹³

Figure 1
Trihaloisocyanuric acids.

In a pioneering paper, Hiegel et al.¹⁴ reported the conversion of phenylacetic acid into the acyl chloride using trichloroisocyanuric acid along with triphenylphosphine and subsequent addition of dry methanol produced methyl phenylacetate in 95% yield. Later, the trichloroisocyanuric acid/triphenylphosphine system was used to convert several carboxylic acids into amides, esters and acyl azides, by reactions with amines, alcohols¹⁵ and sodium azide,¹⁶ respectively.

Recently, we showed an efficient Appel conversion of alcohols into the corresponding alkyl bromides using tribromoisocyanuric acid/triphenylphosphine system,¹⁷ being a bromophosphonium salt (1), formed by reaction of triphenylphosphine with TBCA, the key reactive species for such transformation (Scheme 1).^17,18

Scheme 1

In continuation of our studies on the application of tribromoisocyanuric acid in organic synthesis,¹⁹ herein we report the preparation of esters and amides by reaction of carboxylic acids with alcohols and amines, respectively, in the presence of the tribromoisocyanuric acid/triphenylphosphine system.

Results and Discussion

The reactions were carried out at room temperature by stirring together the carboxylic acid (1 mmol), tribromoisocyanuric acid (0.37 mmol) and triphenylphosphine (1 mmol) in dichloromethane. The progress of the reaction was followed by gas chromatography with mass spectrometer (GC-MS) and after complete substrate conversion to the acyl bromide (confirmed by MS), an alcohol (1 mmol) or an amine (excess) was added to the reaction media. The reactions gave, respectively, esters and amides (along with triphenylphosphine oxide and cyanuric acid), that were isolated by column chromatography and characterized by their melting points (amides) and spectroscopic techniques. In general, the reactions proceeded smoothly and gave products in moderate to excellent yields, showing that the method is quite general and suitable for the conversion of carboxylic acids into their corresponding esters or amides. Tables 1 and 2 show the results.

Table 1
Esterification of carboxylic acids promoted by tribromoisocyanuric acid (TBCA)/triphenylphosphine

Table 2
Amidation of carboxylic acids promoted by tribromoisocyanuric acid (TBCA)/triphenylphosphine

Interestingly, although tribromoisocyanuric acid is known to be a powerful alcohol oxidant²⁰ and electrophilic brominating reagent for alkenes²¹ and arenes,²² no brominated or oxidation products were detected by the analytical techniques employed in the crude reaction, indicating a fast formation of bromotriphenylphosphonium salt from the reaction of TBCA with triphenylphosphine.

Based on the above results, a plausible scheme for this transformation (Scheme 2) proceeds through the bromophosphonium salt (1), formed by bromination of triphenylphosphine by TBCA, that is further converted into the oxyphosphonium intermediate (2). Bromide anion attacks on the intermediate 2 gives the acyl bromide and triphenylphosphine oxide. Simple nucleophilic addition of the alcohol or the amine to the acyl bromide led to the ester or amide along with isocyanuric acid.

Scheme 2

A comparison between our methodology and similar ones previously published (trichloroacetamide/PPh₃,⁹ trichloroacetonitrile/PPh₃,²³ ethyl tribromoacetate/PPh₃²⁴ or hexabromoacetone/PPh₃)²⁵ for the preparation of N-cyclohexylbenzamide indicated similar yields (Table 3). However, the advantages of TBCA compared to other reagents used in such transformations is its stability, easy manipulation and preparation and high atom economy,²⁶ i.e., percentage of mass transferred to triphenylphosphine to generate the halophosphonium salt (1), the reactive species. Besides, the cyanuric acid by-product formed can be reused to produce more TBCA through a green process (Scheme 3).²¹

Table 3
Preparation of N-cyclohexylbenzamide

Scheme 3

Conclusions

In summary, we have developed a new and convenient route for the preparation of esters and amides from carboxylic acids under neutral conditions. The reaction is easily reproducible, the conditions are mild and the reagents are easily available.

Experimental

General information

All chemicals and solvents were used as received. Tribromoisocyanuric acid was prepared as previously described.¹³ Nuclear magnetic resonance (NMR) spectra were recorded on a Bruker AC-200 or AC-300 spectrometers (Bruker, Billerica, MA, USA) at 200 or 300 MHz (¹H) and 50 or 75 (¹³C) using tetramethylsilane (TMS) as internal standard. Gas chromatography were performed on a HP 5890 Series II gas chromatograph (Hewlett-Packard, Palo Alto, CA, USA) with flame ionization detector (FID) using a 30 m (length), 0.25 mm (inner diameter), and 25 µm (phase thickness) RTX-5 capillary column (Restek Corporation, Bellefonte, PA, USA) and H₂ (flow rate 50 cm s^-1) as carrier gas (split 1:10). GC-MS analyses were performed on a Shimadzu GCMS-QP2010S gas chromatograph (Shimadzu, Kyoto, Japan) with electron impact (70 eV) by using a 30 m DB-5 silica capillary column with 0.25 mm inner diameter and 0.25 mm phase thickness. Melting points were determined on a Laboratory Device Mel-Temp II (Laboratory Devices, USA) and are corrected.

General procedure for esterification of carboxylic acids promoted by TBCA/triphenylphosphine

To a stirred solution of TBCA (0.37 mmol) and triphenylphosphine (1 mmol) in CH₂Cl₂ (6 cm³) was added the carboxylic acid (1 mmol) at room temperature. The progress of the reaction was monitored by GC-MS (time₁ in Table 1) and, then, alcohol (1 mmol) was added. After time₂ (Table 1), cyanuric acid was filtered off and the liquid was evaporated on a rotatory evaporator under reduced pressure. The producs were purified by column chromatography (silica gel 70-230 mesh) using diethyl ether/pentane as eluent.

Isopropyl benzoate²⁷

Colorless liquid; ¹H NMR (200 MHz, CDCl₃) δ 1.35 (d, 6H, J 6.3), 5.16-5.36 (hept, 1H, J 6.2), 7.38-7.52 (m, 3H), 8.05 (m, 2H); ¹³C NMR (50 MHz, CDCl₃) δ 22.0, 68.4, 128.3, 129.6, 131.0, 132.8, 166.2; MS (70 eV) m/z 164 (M⁺), 123, 105 (100%), 77, 59, 51, 43.

Benzyl benzoate²⁸

Colorless liquid; ¹H NMR (300 MHz, CDCl₃) δ 5.40 (s, 2H), 7.37-7.50 (m, 7H), 7.56-7.61 (m, 1H), 8.10-8.14 (m, 2H); ¹³C NMR (75 MHz, CDCl₃) δ 66.9, 128.3, 128.4, 128.6, 128.8, 129.9, 130.3, 133.2, 136.2, 166.6; MS (70 eV) m/z 212 (M⁺), 194, 105 (100%), 91, 77, 65, 51.

Isopropyl 4-methoxybenzoate²⁹

Yellowish liquid; ¹H RMN (300 MHz, CDCl₃) δ 1.35 (d, 6H, J 6.2), 3.83 (s, 3H), 5.16-5.28 (hept, 1H, J 6.2), 6.90 (d, 2H, J 8.9), 7.98 (d, 2H, J 9.0); ¹³C NMR (75 MHz, CDCl₃) δ 22.2, 55.6, 68.1, 113.7, 123.6, 131.7, 163.4, 166.1; MS (70 eV) m/z 194 (M⁺), 179, 152, 135 (100%), 92, 77, 64, 43.

Menthyl 4-methoxybenzoate³⁰

Yellowish oil; ¹H NMR (300 MHz, CDCl₃) δ 0.80 (d, 3H, J 6.9), 0.91-0.94 (m, 7H), 1,03-1.16 (m, 2H), 1.49-1.59 (m, 2H), 1.70-1.75 (m, 2H), 1.94-2.01 (m, 1H), 2.10-2,14 (m, 1H), 3.86 (s, 3H), 4.86-4.95 (m, 1 H), 6.92 (d, 2H, J 9.0), 8.01 (d, 2H, J 8.8); ¹³C NMR (75 MHz, CDCl₃) δ 16.8, 21.0, 22.2, 23.9, 26.7, 31.6, 34.6, 41.3, 47.5, 55.6, 74.7, 113.7, 123.5, 131.74, 163.4, 166.1; MS (70 eV) m/z 290 (M⁺), 152, 135 (100%), 123, 95, 81, 55, 41.

Cyclohexyl 4-methoxybenzoate³¹

Yellowish oil; ¹H NMR (300 MHz, CDCl₃) δ 1.31-1.63 (m, 6H), 1.76-1.83 (m, 2H), 1.91-1,95 (m, 2H), 3.87 (s, 3H), 4.96-5.04 (m, 1H), 6.91 (d, 2H, J 9.0), 7.99 (d, 2H, J 9.0); ¹³C NMR (75 MHz, CDCl₃) δ 23.9, 25.7, 31.9, 55.6, 72.9, 113.7, 123.7, 131.7, 163.4, 166.0; MS (70 eV) m/z 234 (M⁺), 152, 135 (100%), 107, 92, 77, 67, 55, 41.

Isopropyl cinnamate³²

Yellowish liquid; ¹H NMR (300 MHz, CDCl₃) δ 1.32 (d, 6H, J 6.7), 5.09-5.22 (hept, 1H, J 6.2), 6.43 (d, 1H, J 16.0), 7.35-7.39 (m, 3H), 7.49-7.54 (m, 2H), 7.68 (d, 1H, J 16.0); ¹³C NMR (75 MHz, CDCl₃) δ 22.1, 67.9, 119.0, 128.1, 129.0, 130.3, 134.7, 144.4, 166.6; MS (70 eV) m/z 190 (M⁺), 147, 131 (100%), 103, 77, 51, 43.

Pentyl 4-nitrobenzoate³³

Colorless liquid; ¹H NMR (300 MHz, CD₃CN) δ 0.92-0.97 (m, 3H), 1.37-1.48 (m, 4H), 1.74-1.83 (m, 2H), 4.35 (t, 2H, J 6.6), 8.19 (d, 2H, J 9.0), 8.29 (d, 2H, J 9.0); ¹³C NMR (75 MHz, CD₃CN) δ 12.8, 21.6, 27.4, 27.5, 65.3, 123.1, 130.0, 136.5, 150.1, 164.2; MS (70 eV) m/z 238 (M⁺), 168, 150, 120, 104, 70 (100%), 55, 42.

Pentyl octanoate³⁴

Colorless liquid;¹H NMR (300 MHz, CD₃CN) δ 0.88-0.95 (m, 6H), 1.31-1.37 (m, 12H), 1.57-1.64 ppm (m, 4H), 2.28 (t, 2H, t, J 7.3), 4.04 (t, 2H, J 6.6 Hz); ¹³C NMR (75 MHz, CD₃CN) δ 14.4, 14.5, 23.2, 23.5, 26.0, 29.0, 29.3, 29,9, 29.9, 32.6, 35.0, 65.0, 174.5; MS (70 eV) m/z 214 (M⁺), 145, 127, 70 (100%), 57, 55, 43.

General procedure for amidation of carboxylic acids promoted by TBCA/triphenylphosphine

To a stirred solution of TBCA (0.37 mmol) and triphenylphosphine (1 mmol) in CH₂Cl₂ (6 cm³) was added the carboxylic acid (1 mmol) at room temperature. The progress of the reaction was monitored by GC-MS (time₁ in Table 2) and, then, the amine (25-75 mmol) was added. After the time₂ (Table 2), cyanuric acid was filtered off, and the liquid was evaporated on a rotatory evaporator under reduced pressure. The producs were purified by column chromatography (silica gel 70-230 mesh) using diethyl ether/pentane as eluent.

N-Isopropyl-benzamide

Yellowish solid; m.p. 94-96 ºC (lit. 95 ºC);^{35) 1}H NMR (200 MHz, CDCl₃) δ 1.24 (d, 6H, J 6.5), 4.18-4.35 (sext, 1H, J 6.6), 6.27 (s, 1 H), 7.37-7.45 (m, 3 H), 7.74-7.77 (m, 2 H); ¹³C NMR (50 MHz, CDCl₃) δ 22.9, 42.0, 127.0, 128.6, 131.3, 135.1, 166.9; MS (70 eV) m/z 163 (M⁺), 148, 105 (100%), 77, 51.

N-Cyclohexyl-benzamide

White solid; m.p. 138-140 ºC (lit. 139-141 ºC);^{36) 1}H NMR (300 MHz, CDCl₃) δ 1.18-2.05 (m, 10H), 3.92-4.04 (m, 1H), 6.06 (s, 1H), 7.38-7.49 (m, 3H), 7.74-7.78 (m, 2H); ¹³C NMR (75 MHz, CDCl₃) δ 25.1, 25.8, 33.4, 48.9, 127.0, 128.7, 131.4, 135.3, 166.8.; MS (70 eV) m/z 203 (M⁺), 123, 122, 105 (100%), 77, 51.

N-Cyclohexyl-4-methoxybenzamide

White solid; m.p. 153-155 ºC (lit. 153-154 ºC);^{37) 1}H NMR (300 MHz, CDCl₃) δ 1.16-1.29 (m, 3H), 1.35-1.50 (m, 2H), 1.62-1.79 (m, 3H), 2.02-2.06 (m, 2H), 3.84 (s, 1H), 3.91-4.03 (m, 1H), 5.93 (s, 1H), 6.91 (d, 2H, J 8.9), 7.72 (d, 2H, J 8.8); ¹³C NMR (75 MHz, CDCl₃) δ 25.1, 25.8, 33.5, 48.8, 55.6, 113.9, 127.6, 128.8, 162.2, 166.3; MS (70 eV) m/z 233 (M⁺), 232 (2,49), 152, 151, 135 (100%), 107, 92, 79, 77.

N-Isopropyl-4-methoxybenzamide

White solid; m.p. 112-114 ºC (lit. 123ºC);^{35) 1}H NMR (300 MHz, CDCl₃) δ 1.25 (d, 6H, J 6.3), 3.83 (s, 3H), 4.21-4.33 (m, 1H), 6.00 (s, 1H), 6.89 (d, 2H, J 8.9), 7.73 (d, 2H, J 8.9); ¹³C NMR (75 MHz, CDCl₃) δ 23.0, 41.9, 55.5, 113.8, 127.3, 128.8, 162.1, 166.4; MS (70 eV) m/z 193 (M⁺), 135 (100%), 107, 104, 92, 77.

N-Benzyl-4-methoxybenzamide

White solid; m.p. 120-122 ºC (lit. 124-126 ºC);^{38) 1}H NMR (300 MHz, CD₃CN) δ 2.42 (s, 1H), 3.83 (s, 3H), 4.53 (d, 2H, J 6.1), 6.97 (d, 2H, J 8.9), 7.23-7.35 (m, 5H), 7.49 (s, 1H), 7.81 (d, 2H, J 8.9); ¹³C NMR (75 MHz, CD₃CN) δ 44.3, 56.5, 115.0, 128.3, 128.7, 129.8, 130.3, 141.2, 163.6, 167.9; MS (70 eV) m/z 241 (M⁺), 135 (100%), 107, 92, 77, 64, 51.

N-Isopropyl-cinnamamide

Yellowish solid; m.p. 94-96 ºC (lit. 101-103 ºC);^{39) 1}H NMR (300 MHz, CDCl₃) δ 1.22 (d, 6H, J 6.6), 4.24 (sext, 1H, J 6.5), 5.94 (s, 1H), 6.43 (d, 1H, J 15.6), 7.31-7.33 (m, 3H), 7.46-7.49 (m, 2H), 7.62 (d, 1H, J 15.3); ¹³C NMR (75 MHz, CDCl₃) δ 23.0, 41.8, 121.4, 128.0, 129.0, 129.7, 135.2, 140.9, 165.4; MS (70 eV) m/z 189 (M⁺), 174, 161, 146, 131 (100%), 118, 112, 103, 91, 87, 77, 63, 58, 51, 43.

N-(Cyclohexylcarbonyl)-morpholine⁴⁰

Yellowish oil; ¹H NMR (300 MHz, CDCl₃) δ 1.21-1.82 (m, 10 H), 2.37-2.47 (m, 1 H), 3.55-3.68 (m, 8 H); ¹³C NMR (75 MHz, CDCl₃) δ 26.0, 29.5, 40.4, 42.1, 46.1, 67.1, 174.9; MS (70 eV) m/z 197 (M⁺), 142, 129, 83 (100%), 70, 55, 41.

N-Butyl-4-nitrobenzamide

Yellowish solid; m.p. 102-104 ºC (lit. 104-104.5 ºC);^{41) 1}H NMR (300 MHz, CDCl₃) δ 0.92-0.97 (t, 3H, J 7.3), 1.40 (sext, 2H, J 7.2), 1.61 (quint, 2H, J 7.5), 3.45 (q, 2H, J 7.2), 6.60 (s, 1 H), 7.92 (d, 2H, J 9.0), 8.24 (d, 2H, J 9.0); ¹³C NMR (75 MHz, CDCl₃) δ 13.2, 19.6, 31.1, 39.7, 123.2, 127.6, 140.0, 149.0, 165.1; MS (70 eV) m/z 222 (M⁺), 193, 180, 150 (100%), 120, 104, 92, 76, 50, 41.

N-Butyl-octanamide⁴²

¹H NMR (300 MHz, CDCl₃) δ 0.83-0.92 (m, 6H), 1.26-1.36 (m, 10 H), 1.41-1.49 (m, 2 H), 1.57-1.62 (m, 2H), 2.14 (t, 2H, J 7.4), 3.22 (q, 2H, J 7.2), 5.72 (s, 1H); ¹³C NMR (75 MHz, CDCl₃) δ 13.2, 13.5, 19.6, 22.1, 25.4, 28.5, 28.8, 31.2, 36.4, 38.7, 172.9; MS (70 eV) m/z 199 (M⁺), 170, 156, 142, 128, 115 (100%), 100, 86, 73, 57, 44, 41.

Supplementary Information

¹H NMR, ¹³C NMR and MS spectra of synthesized compounds are available free of charge at http://jbcs.sbq.org.br.

https://minio.scielo.br/documentstore/1678-4790/x9b9zVFgZBwkcR9TQjR6qWt/0a4d5983584afb186980c3030153913a6a69a981.pdf

Acknowledgments

We thank CNPq and FAPERJ for the finacial support.

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Publication Dates

• Publication in this collection
  June 2016

History

• Received
  21 Oct 2015
• Accepted
  15 Jan 2016