A microwave-assisted highly practical chemoselective esterification and amidation of carboxylic acids

Abstract

The ubiquitousness of esters and amide functionalities makes their coupling reaction one of the most sought-after organic transformations. Herein, we have described an efficient microwave-assisted synthesis of esters and amides. Soluble triphenylphosphine, in conjugation with molecular iodine, gave the desired products without the requirement for a base/catalyst. In addition, a solid-phase synthetic route is incorporated for the said conversion, which has added advantages over solution-phase pathways, such as low moisture sensitivity, easy handling, isolation of the product by simple filtration, and reusability. In short, our method is simple, mild, green, and highly chemoselective in nature.

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Introduction

Esters and amides of carboxylic acids have been the subject of numerous accounts throughout the years, owing to the fundamental importance of this synthetic transformation.¹ Their importance is reflected by a plethora of methods reported in the literature.² The mildest of these include the use of hypervalent iodine,³ Me₂NSO₂Cl/DMAP,⁴ DCC/DMAP,⁵ DEAD/Ph₃P,⁶N-hydroxysuccinimide/DCC,⁷ Yamaguchi conditions (TCBC, DIPEA, DMAP),⁸ 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-ethylmorpholinium chloride/N-methylmorpholine,⁹ TsCl/pyridine,¹⁰ benzotriazol-1-yloxytris-(dimethylamino) phosphonium hexafluorophosphate,¹¹ DIC/DMAP,¹² triflic anhydride,¹³ Ph₃P/trichloroisocyanuric acid,¹⁴ Bu₂SnO,¹⁵ and 2-halo-1-methyl pyridinium salt.¹⁶ Recently, Kohn et al.¹⁷ reported an efficient method of esterification using a Ph₃PBr₂/K₂CO₃ reagent system. In 2011, Robles et al.¹⁸ demonstrated a mild method of esterification and amidation using Gregg-Samuelson-type conditions,¹⁹ and reported that their method was highly regioselective. Chemoselective transformation using triphenylphosphine, I₂, and a catalytic amount of Zn (OTf)₂ was also reported by Manna et al.²⁰ Even though these protocols look attractive, a significant drawback is the requirement for excess reagents, base, and catalyst. Interestingly, in order to overcome the problems encountered with the isolation of the byproduct, phosphine oxide, while working with triphenylphosphine, Nowrouzi et al.²¹ reported the use of chlorodiphenylphosphine (CDP) and molecular iodine under reflux conditions to perform efficient esterification reactions. It is worth mentioning that the resulting phosphorus byproduct, diphenylphosphinic acid, can be extracted with an aqueous basic solution in the workup processes. Ironically, the reaction did not proceed at all in the absence of a base catalyst. Therefore, despite numerous literature reports, the development of a simple, mild, and highly efficient method of esterification and amidation is still urgently needed.

In the past few decades, green chemistry has evolved as a major scientific discipline.²² The investigation and application of green chemistry principles has led to the development of environmentally benign and sustainable methods for the generation of target molecules. For many chemical transformations, a major adverse effect on the environment is the consumption of energy for heating. To overcome the problems associated with this, it is highly desirable to develop efficient methods that use alternative energy sources to facilitate faster chemical transformations. In this context, the use of microwave (MW) energy as a non-conventional energy source has gained momentum, thereby making Microwave-Assisted Organic Synthesis (MAOS) a rapidly growing area in the field of synthetic organic chemistry.²³ Microwave irradiation leads to a large reduction in reaction times, higher yields, enhancement in product purities by reducing unwanted side reactions, and selectivity, which are complementary to green chemistry.²⁴ Unlike these, convective heating through the walls of a reaction flask is less efficient.²⁵ The number of publications on MAOS, which is considered to be very attractive in the context of green chemistry, has increased significantly since the pioneering work of Gedye²⁶ in 1986.

Recently, solid-phase organic syntheses are finding enormous applications in the synthesis of large libraries of compounds via combinatorial chemistry.²⁷ The solid-phase synthesis carries many advantages over classical solution-phase synthetic methods: (i) the compound of interest (starting material or catalyst) can be anchored to a solid matrix, which can be filtered, (ii) easy handling, (iii) low moisture susceptibility, (iv) minimum side reactions, (v) the ability to be recycled for repeated use, and (vi) the final product can be released from the matrix and can be obtained in an almost pure form. Therefore, polymer-bound reagents have drawn huge attention from industry and academia for their easy handling, separation, and reusability.²⁸ The fact that the polymer can be recovered, and the reagent or catalyst can be regenerated in many cases, means that they can be used in an excess amount to drive the reaction faster. On these lines, polymer-bound triphenylphosphine (PB-TPP) is a good example,²⁹ as its use avoids many of the problems common to the use of soluble triphenylphosphine, such as the removal of excess triphenylphosphine, the formation of phosphine complexes, and the difficulty in removing byproduct triphenylphosphineoxide.³⁰ Moreover, for the reactions where polymer-bound triphenylphosphine acts as an oxygen-acceptor, recycling of triphenylphosphine oxide with trichlorosilane is easy and convenient.³¹

Results and discussion

In the esterification reaction, if other functional groups capable of competing with hydroxy or carboxy functions, such as additional hydroxy/carboxy or amino groups, are present in a molecule, the esterification reaction tends to yield mixtures of isomeric products of mono- and polyacylation.³² The problem of the chemoselectivity of esterification is particularly acute in multistep syntheses, where the possibility for accurate predictions of chemoselectivities in the advanced stages of a synthetic project is one of the most important requirements for the overall success. Despite high demand, only limited reports appear in the literature on the chemoselective esterification of phenolic acids with alcohols or carboxylic acids with amino alcohols,³³ which are often accompanied by certain drawbacks, like the use of excessive reagents, harsh reaction conditions, and low yields. In connection with our ongoing research interest in the application of triphenylphosphine and iodine³⁴ in useful organic transformations, herein, we described a facile method involving triphenylphosphine/polymer-bound triphenylphosphine (PB-TPP) and molecular iodine for the synthesis of esters and amides with high chemoselectivity (Scheme 1).

Scheme 1 Esterification using PB-TPP or PPh₃/I₂.

Literature reviews revealed that PPh₃/I₂ in combination with bases could act as a robust dehydrating reagent.^18,19,34,35 The role of the base is to increase reactivity, as well as neutralize the liberated hydroiodic acids.^19,21 However, Hajipour et al.³⁶ smoothly converted alcohols to iodides using PPh₃/I₂, with no requirement for base. Hence, we hypothesized that PPh₃/I₂ may also be sufficient for the esterification of carboxylic acids with alcohols. At the onset, we investigated the esterification of benzoic acid and 1-propanol as a pilot protocol in the solution phase. Treatment of benzoic acid (1 mmol) with 1-propanol (1 mmol) in the presence of triphenylphosphine (1.2 mmol) and iodine (1.2 mmol) in anhydrous acetonitrile at room temperature did not give an ester, even after stirring for 4 h. However, under microwave (MW) irradiation at 85 °C, the desired ester was obtained in 93% yield within 30 min (Table 1, entry 6). However, under conventional reflux heating in an oil bath, the reaction time was increased to 120 min (Table 1, entry 7). To optimize the reaction conditions, we first examined the effect of different ratios of RCOOH/ROH/PPh₃/I₂ under microwave irradiation in anhydrous acetonitrile for the conversion of benzyl alcohol to propyl benzoate. After varying different parameters, we have concluded that employing molar ratios of 1 : 1 : 1.2 : 1.2 gave the best result. The effects of various solvents were also investigated. Amongst them, anhydrous acetonitrile was found to be superior to other solvents and mixed solvents in terms of reaction time and yield (Table 1).

Table 1 The effect of solvent on the esterification using PPh₃/I₂^a

Entry	Solvent	Time^b (min)	Yield^c (%)
a Reaction conditions: benzoic acid (1 mmol), 1-propanol (1 mmol), PPh3 (1.2 mmol), I2 (1.2 mmol), 20 mL solvent, MW. b Reaction time is the period for which the reaction mixture is kept at the specified reaction temperature (‘fixed hold time’). c Isolated yields. d Reaction was performed under conventional reflux in an oil bath.
1	Hexane	45	9
2	THF	45	63
3	DCM	75	42
4	EtOAc	75	13
5	CHCl3	45	34
6	Acetonitrile	30	93
7	Acetonitrile	120	92^d

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To investigate the scope and limitations of the reaction, a variety of alcohols were examined under our optimal reaction conditions in the solution phase. Additionally, taking into account the numerous advantages of solid-phase over solution-phase synthesis; solid-phase synthetic pathways using polymer-bound triphenylphosphine (PB-TPP) in place of soluble PPh₃ are also incorporated in the report. Under our optimized conditions, the desired ester can be obtained in good to excellent yield, regardless of substitution patterns, as shown in Table 2. Both primary and secondary alcohols were smoothly converted into their corresponding esters. To determine the effect of substituents on acid, we then tried the esterification reaction using both electron-withdrawing and donating groups in benzoic acid. As expected, the presence of an electron-donating group in benzoic acid bolstered a faster reaction and gave a good yield of the product (Table 2, entries 5, 6, 7, 21, and 22). This is attributed to the increase in nucleophilicity of the substrates towards the in situ formed phosphonium complexes. However, the reaction had a longer completion time when performed in the solid phase using PB-TPP, but gave a better yield (Table 2, entry 7 vs. entry 21), while working with the same class of substrates. It was observed that carboxylic acids with electron-withdrawing substituents gave low yields of the desired esters, even after a prolonged reaction time (Table 2, entries 8–14). To our pleasure, the reactions of carboxylic acid with less nucleophilic phenolic alcohols were also successful and gave the desired esters in good to excellent yields (Table 2, entries 4, 9, and 23). Secondary alcohols, such as menthol, reacted with 2-ethyl hexanoic acid under reflux to give the menthol ester in 87% yield (Table 2, entry 20).

Table 2 Esterification between carboxylic acid and alcohol under optimized conditions^a

Entry	Acid	Alcohol	Product	Time (min)	Yield^b (%)
a Reaction conditions: carboxylic acid (1 mmol), alcohol (1 mmol), PPh3 (1.2 mmol), I2 (1.2 mmol), 20 mL acetonitrile, MW (85 °C). b Isolated yield. c PB-TPP was used instead of PPh3.
1		n-C3H7OH		30	93
2		n-C12H15OH		40	95^c
3				40	85
4		PhOH		56	89
5		n-C8H17OH		17	93
6		n-C12H25OH		19	91
7		n-C18H37OH		30	94^c
8		n-C8H17OH		45	79
9				53	81
10		n-C12H26OH		55	82
11				45	93^c
12		n-C18H37OH		45	91
13		n-C12H26OH		45	78
14				45	84
15		n-C8H17OH		37	82
16				20	92
17		n-C8H17OH		38	95^c
18		n-C12H26OH		18	90
19				40	93^c
20				45	87
21		n-C12H26OH		21	91
22				20	93
23				45	82

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The esterification of primary and secondary hydroxy groups in the presence of tertiary hydroxyls is trivial in most cases, since tertiary alcohols are about two orders of magnitude less reactive than secondary ones and more than 10³ times less reactive than primary alcohols.³⁷ In contrast, achieving highly selective mono-esterification of primary hydroxy groups in the presence of secondary ones is problematical, as the difference in acylation rates between primary and secondary alcohols is only about one order of magnitude, and the acylation of diols containing both types of hydroxyls provides substantial amounts of a corresponding secondary ester and/or diester.³⁸ As can be seen from Table 2, a secondary alcohol took a longer time to complete the esterification reaction as compared to primary ones, while working with same acid substrate (Table 2, entry 1 vs. 3). We, therefore, presumed that our method could be highly chemoselective in favour of the primary alcohols. With this in mind, the chemoselectivity was first established by the reaction of benzoic acid with 1,3-butanediol. Under our optimized reaction conditions, the reaction gave a 95% yield of an exclusively chemoselective product in favor of the primary moiety (Scheme 2).

Scheme 2 Chemoselective esterification.

Chemoselectivity in favour of aliphatic alcohols over aromatic ones was also achieved by reaction of hydroxybenzoic acid with aliphatic alcohol (Table 3, entries 3–4). This high chemoselectivity may be attributed to the generally lower basicity and nucleophilicity of the oxygen atom in phenols.

Table 3 Chemoselective esterification between carboxylic acid and alcohol under optimized conditions^a

Entry	Acid	Alcohol	Product	Time (min)	Yield^b (%)
a Reaction conditions: carboxylic acid (1 mmol), alcohol (1 mmol), PPh3 (1.2 mmol), I2 (1.2 mmol), 20 mL acetonitrile, MW (85 °C). b Isolated yield. c PB-TPP was used instead of PPh3.
1				30	95
2				17	94
3		n-C8H17OH		45	88
4		n-BuOH		51	84
5				45	87
6		n-BuOH		30	94
7		n-C12H26OH		70	95^c
8		n-PrOH		30	89^c
9				30	91
10		n-C8H17OH		38	91
11		HO(CH2)6OH		45	91^c
12		HO(CH2)6OH		50	82^c
13		HO(CH2)6OH		45	87^c
14		HO(CH2)8OH		45	85^c

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Interestingly, unsymmetrical dicarboxylic acids undergo esterification exclusively in the aliphatic moiety (Table 3, entry 6). Gratifyingly, non-conjugated carboxylic acids were selectively esterified in the presence of conjugated ones. Therefore, itaconic acid was selectively esterified at the allylic carboxylic group (Table 3, entries 8–9). Selective mono-esterification of symmetrical diols is useful in organic transformations in many instances³⁹ and has recently been addressed by our group using a PB-TPP/I₂/DMAP reagent system.^29b However, the base catalyst used in the reaction (i.e. DMAP) was discarded during aqueous extraction. In this report, mono-esterification of symmetrical diols was successfully achieved by employing a solid-phase synthesis technique, using a PB-TPP/I₂ reaction system without the requirement for base (Table 3, entries 11–14).

After successfully synthesizing diverse esters, we reasoned that changing the nucleophile from alcohols to amines would give the corresponding amide product via similar pathways, since amines are more nucleophilic as compared to their alcohol counterparts. Under our optimized conditions for the esterification, when benzoic acid was treated with benzyl amine, the corresponding amide 40 was formed in 92% yield within 30 min (Table 4, entry 1). The effects of electron-withdrawing and donating groups in the benzene ring of the carboxylic acid, and substitution patterns of the amines, are further investigated. These results are consistent with the esterification reactions described above. In all cases, the overall yields of these amides are generally good to excellent.

Table 4 Amidation between carboxylic acid and amine under optimized conditions^a

Entry	Acid	Amine	Product	Time (min)	Yield^b (%)
a Reaction conditions: carboxylic acid (1 mmol), amine (1 mmol), PPh3 (1.2 mmol), I2 (1.2 mmol), 20 mL acetonitrile, MW (85 °C). b Isolated yield. c PB-TPP was used instead of PPh3.
1				30	92
2				39	91
3				41	90
4		n-C4H9NH2		30	93
5				30	91
6				30	93^c
7				45	87
8				45	92
9		n-C4H9NH2		38	91^c
10				30	88
11		n-C4H9NH2		30	89^c
12				60	73

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Conclusion

In summary, we have demonstrated a safe, inexpensive and fast microwave-assisted method of esterification. The protocol is further extended for the synthesis of diverse amides, which have a potential utility in the synthesis of amide linkages in peptide chemistry. Mild conditions, operational simplicity, the ready availability and non-toxic nature of the reagent, general applicability, and high yields are considered as the advantageous features of this protocol. Compared to solution-phase methods, the solid-phase procedure using polymer-bound triphenylphosphine (PB-TPP) offers some advantages, as its use avoids many of the problems common to the use of soluble triphenylphosphine, such as the removal of excess triphenylphosphine, the formation of phosphine complexes, and the difficulty in removing byproduct triphenylphosphine oxide. Additionally, purification of the final products does not require chromatographic separation.

Experimental

All reactions were carried out with dry, freshly distilled solvents under anhydrous conditions, unless otherwise stated. Solvents were extra pure grade, purchased from Merck India, and were dried by using the reported procedure. Polymer-bound triphenylphosphine (∼3 mmol triphenylphosphine moiety per g) was purchased from Sigma-Aldrich. Other reagents were purchased at the highest available commercial quality and were used without further purification. The reaction progress was monitored by thin layer chromatography (TLC), carried out on glass plates coated with silica gel G, using the solvent system EtOAc/hexane. Milestones StartSYNTH microwave was used for all the reactions. The infrared spectra were recorded with an FT-IR-3000 Hyperion Microscope (Bruker, Germany) with a Vertex 80 FTIR system. The ¹H-NMR and ¹³C-NMR spectra were recorded on Bruker Avance III 300, 400, and 500 MHz FT NMR Spectrometers, using CDCl₃ as solvent and TMS as an internal reference.

General procedure for the solution-phase synthesis of ester and amide

A mixture of polymer-bound triphenylphosphine PB-TPP (1.2 mmol), iodine (1.2 mmol), and carboxylic acid (1 mmol) in 20 mL of anhydrous acetonitrile was placed in a 50 mL round-bottom flask. This flask was irradiated in a Milestone StartSYNTH microwave reactor, equipped with a condenser [a graphical image of all heating equipment used in these studies can be found in the ESI†], for 15 min, according to the following heating program: step (1) 25 °C to 85 °C (10 min (ramp time), max power 700 W), step (2) hold at 85 °C for 5 min. After 5 min holding, alcohol/amine was added and the reaction was allowed to run at this temperature for the time specified in the table (‘fixed hold time’). The hot reaction mixture was allowed to cool; at this point, ethylacetate was added and the mixture was washed with saturated sodium thiosulfate solution, followed by brine. The organic layer was dried over anhydrous sodium sulfate and concentrated under vacuum. The crude product was further purified by column chromatography using 10% ethylacetate/hexane mixture as eluent.

General procedure for the solid-phase synthesis of ester and amide

A mixture of polymer-bound triphenylphosphine PB-TPP (1.2 mmol), iodine (1.2 mmol), and carboxylic acid (1 mmol) in 20 mL of anhydrous acetonitrile was placed in a 50 mL round-bottom flask. This flask was irradiated in a Milestone StartSYNTH microwave reactor, equipped with a Vigreux column and condenser [a graphical image of all heating equipment used in these studies can be found in the ESI†], for 15 min according to the following heating program: step (1) 25 °C to 85 °C (10 min, max power 700 W), step (2) hold at 85 °C for 5 min. After 5 min holding, alcohol/amine was added and the reaction was allowed to run at this temperature for the time specified in the table. The hot reaction mixture was allowed to cool; the reaction mixture was then filtered and washed with DCM (50 mL). The filtrate was extracted, dried over anhydrous sodium sulfate and concentrated under vacuum to give the desired product in high purity.

Spectral data

Dodecylbenzoate, 2

0.275 g, 95% yield, colourless liquid, ¹H-NMR (400 MHz, CDCl₃): δ 7.95 (d, 1H, J = 7.2 Hz), 7.45 (t, 2H, J = 7.2 Hz), 7.33 (t, 2H, J = 7.6 Hz), 4.21 (t, 2H, J = 6.4 Hz), 1.66 (m, 2H), 1.34 (m, 2H), 1.23 (m, 2H), 1.16 (m, 14H), 0.78 (t, 3H, J = 7.2 Hz); ¹³C-NMR (100 MHz, CDCl₃): δ 132.76, 130.54, 129.52, 128.30, 65.14, 31.90, 29.63, 29.62, 29.57, 29.52, 29.39, 29.28, 28.72, 26.04, 22.68, 14.10; IR (KBr pellet, ν_max/cm⁻¹): 2960, 2861, 2355, 2340, 1722, 1580, 1450, 1108, 998, 706; elemental analysis for C₁₉H₃₀O₂, calculated: C 78.85, H 10.10; found: C 78.87, H 10.08.

Octyl 4-methoxybenzoate, 5

0.245 g, 93% yield, colourless liquid, ¹H-NMR (400 MHz, CDCl₃): δ 7.87 (d, 2H, J = 8 Hz), 6.78 (d, 2H, J = 8 Hz), 4.14 (t, 2H, J = 6 Hz), 3.71 (s, 3H), 1.61 (m, 2H), 1.29 (m, 2H), 1.16 (m, 8H), 0.74 (s, 3H); ¹³C-NMR (100 MHz, CDCl₃): δ 166.45, 163.21, 13.19, 131.21, 122.94, 113.52, 64.83, 55.37, 31.62, 29.70, 29.26, 28.76, 26.06, 22.65; IR (KBr pellet, ν_max/cm⁻¹): 3010, 2858, 2332, 1710, 1565, 1440, 1012, 850, 720; elemental analysis for C₁₆H₂₄O₃, calculated: C 72.69, H 9.15; found: C 72.68, H 9.17.

Dodecyl 4-methoxybenzoate, 6

0.291 g, 91% yield, colourless liquid, ¹H-NMR (400 MHz, CDCl₃): δ 7.93 (d, 2H, J = 8.8 Hz), 6.85 (d, 2H, J = 8.8 Hz), 4.20 (t, 2H, J = 6.8 Hz), 3.79 (s, 3H), 1.65 (m, 2H), 1.54 (m, 2H), 1.33 (m, 2H), 1.18 (m, 14H), 0.80 (t, 3H, J = 6.4 Hz); ¹³C-NMR (100 MHz, CDCl₃): δ 165.46, 162.21, 130.51, 12.97, 112.52, 63.84, 54.39, 30.89, 28.63, 28.62, 28.56, 28.52, 28.32, 28.28, 28.23, 27.75, 27.58, 25.04, 21.66, 13.09; IR (KBr pellet, ν_max/cm⁻¹): 2995, 2856, 2360, 1745, 1603, 1480, 996, 765; elemental analysis for C₂₀H₃₂O₃, calculated: C 74.96, H 10.07; found: C 74.97, H 10.05.

Octadecyl 4-methylbenzoate, 7

0.365 g, 94% yield, white solid, mp 36–38 °C, ¹H-NMR (400 MHz, CDCl₃): δ 7.94 (d, 2H, J = 7.6 Hz), 7.24 (d, 2H, J = 6.4 Hz), 4.29 (t, 2H, J = 6.4 Hz), 2.40 (s, 3H), 1.75 (m, 2H), 1.42 (m, 2H), 1.34 (m, 2H), 1.25 (m, 26H), 0.87 (t, 3H, J = 5.2 Hz); ¹³C-NMR (100 MHz, CDCl₃): δ 166.79, 143.4, 129.55, 129.02, 127.77, 64.97, 31.93, 29.71, 29.67, 29.60, 29.55, 29.38, 29.30, 28.73, 26.05, 22.71, 21.65, 14.14; IR (KBr pellet, ν_max/cm⁻¹): 3010, 2878, 2386, 1750, 1560, 1428, 1210, 1030, 835; elemental analysis for C₂₆H₄₄O₂, calculated: C 80.35, H 11.41; found: C 80.38, H 11.40.

Octyl 4-nitrobenzoate, 8

0.220 g, 79% yield, white solid, mp 63–65 °C, ¹H-NMR (300 MHz, CDCl₃): δ 8.39 (t, 1H, J = 1.8 Hz), 8.27 (t, 1H, J = 2.1 Hz), 8.22 (t, 1H, J = 2.1 Hz), 8.19 (t, 1H, J = 1.8 Hz), 4.37 (t, 2H, J = 6.9 Hz), 1.43 (m, 2H), 1.29 (m, 8H), 0.90 (t, 3H, J = 6 Hz); ¹³C-NMR (75 MHz, CDCl₃): δ 164.94, 150.67, 136.08, 130.85, 123.71, 66.32, 33.77, 31.96, 30.71, 29.40, 28.71, 26.17, 22.82, 14.28; IR (KBr pellet, ν_max/cm⁻¹): 2985, 2835, 2329, 1737, 1535, 1422, 1367, 1032, 853; elemental analysis for C₁₅H₂₁NO₄, calculated: C 64.50, H 7.58, O 22.91; found: C 64.51, H 7.56, O 22.90.

p-Tolyl 4-nitrobenzoate, 9

0.208 g, 81% yield, white solid, mp 75–77 °C, ¹H-NMR (500 MHz, CDCl₃): δ 8.381 (d, 2H, J = 2.5 Hz); 8.399 (d, 2H, J = 7 Hz), 7.28 (d, 2H, J = 1.5 Hz), 7.142 (d, 2H, J = 8.5 Hz), 2.44 (s, 3H); ¹³C-NMR (125 MHz, CDCl₃): δ 163.52, 150.88, 148.30, 136.15, 135.11, 135.11, 131.27, 130.17, 123.70, 121.06, 20.92; IR (KBr pellet, ν_max/cm⁻¹): 3107, 2920, 2854, 1728, 1523, 1398, 1349, 1282, 1028; elemental analysis for C₁₄H₁₁NO₄, calculated: C 65.37, H 4.31, O 24.88; found: C 65.38, H 4.29, O 24.88.

Dodecyl 3-nitrobenzoate, 10

0.275 g, 82% yield, Pale yellow oil, ¹H-NMR (400 MHz, CDCl₃): δ 8.01 (d, 1H, J = 8 Hz), 7.47 (d, 1H, J = 7.6 Hz), 7.39 (t, 1H, J = 7.6 Hz), 7.19 (s, 1H), 3.99 (t, 2H, J = 6.8 Hz), 2.99 (t, 2H, J = 7.6 Hz), 2.61 (t, 2H, J = 7.6 Hz), 1.53 (m, 2H), 1.18 (m, 18H), 0.80 (t, 3H, J = 6.8 Hz); ¹³C-NMR (100 MHz, CDCl₃): δ 172.25, 148.36, 14.54, 134.71, 129.36, 123.23, 121.50, 64.95, 35.24, 31.90, 30.43, 29.63, 29.61, 29.55, 29.49, 29.33, 29.21, 28.56, 25.86, 22.68, 14.11; IR (KBr pellet, ν_max/cm⁻¹): 3032, 2987, 2840, 2395, 1723, 1520, 1411, 1370, 1068, 793; elemental analysis for C₁₉H₂₉NO₄, calculated: C 68.03, H 8.71, O 19.08; found: C 68.02, H 8.73, O 19.05.

Phenethyl 4-flurobenzoate, 11

0.227 g, 93% yield, pale yellow oil, ¹H-NMR (300 MHz, CDCl₃): δ 8.03 (d, 2H, J = 2.1 Hz), 7.35 (t, 1H, J = 1.5 Hz), 7.32 (t, 1H, J = 1.5 Hz), 7.30 (d, 1H, J = 2.7 Hz), 7.27 (d, 1H, J = 2.7 Hz), 7.08 (d, 2H, J = 1.19 Hz), 7.06 (t, 1H, J = 2.7 Hz), 4.52 (t, 2H, J = 6.9 Hz), 3.07 (t, 2H, J = 6.9 Hz); ¹³C-NMR (75 MHz, CDCl₃): δ 167.63, 165.76, 138.01, 132.36, 129.15, 128.78, 128.54, 126.85, 126.70, 115.86, 115.56, 65.79, 35.42; IR (KBr pellet, ν_max/cm⁻¹): 3080, 3025, 2948, 2860, 1720, 1560, 1405, 1270, 1080, 760; elemental analysis for C₁₅H₁₃FO₂, calculated: C 73.76, H 5.36, O 13.10; found: C 73.76, H 5.35, O 13.09.

Octadecyl 4-fluorobenzoate, 12

0.357 g, 91% yield, white solid, mp 38–40 °C, ¹H-NMR (400 MHz, CDCl₃): δ 8.06 (dd, 2H, J = 2.4 Hz), 7.10 (dd, 2H, J = 8.8 Hz), 4.3 (t, 2H, J = 6.8 Hz), 1.75 (m, 2H), 1.59 (m, 2H), 1.41 (m, 4H), 1.35 (m, 24H), 0.87 (t, 3H, J = 6.8 Hz); ¹³C-NMR (100 MHz, CDCl₃): δ 165.74, 132.09, 132, 126.74, 126.71, 115.55, 115.32, 65.30, 31.93, 29.70, 29.65, 29.58, 29.53, 29.37, 29.29, 28.68, 26.02, 22.70, 14.14; IR (KBr pellet, ν_max/cm⁻¹): 3045, 2990, 2866, 2310, 1732, 1520, 1436, 1035, 826; elemental analysis for C₂₅H₄₁FO₂, calculated: C 76.48, H 10.53, O 8.15; found: C 76.50, H 10.53, O 8.14.

Phenethyl 4-chlorobenzoate, 14

0.281 g, 84% yield, pale yellow oil, ¹H-NMR (400 MHz, CDCl₃): δ 7.75 (d, 2H, J = 7.6 Hz), 7.42 (t, 2H, J = 7.6 Hz), 7.39 (d, 2H, J = 7.6 Hz), 7.32 (t, 1H, J = 7.2 Hz), 7.29 (d, 2H, J = 6.8 Hz), 4.55 (t, 2H, J = 7.2 Hz), 3.08 (t, 2H, J = 6.8 Hz); ¹³C-NMR (100 MHz, CDCl₃): δ 165.62, 137.65, 133.73, 132.54, 131.39, 131.08, 130.10, 128.99, 128.69, 128.57, 126.66, 126.56, 66.02, 35.06; IR (KBr pellet, ν_max/cm⁻¹): 3048, 2987, 2829, 2356, 1747, 1510, 1435, 1112, 1005, 860; elemental analysis for C₁₅H₁₃ClO₂, calculated: C 69.10, H 5.03, O 12.27; found: C 69.11, H 5.01, O 12.27.

Phenethyl propionate, 16

0.163 g, 92% yield, pale yellow oil, ¹H-NMR (400 MHz, CDCl₃): δ 7.21 (m, 4H), 7.13 (t, 1H, J = 7.2 Hz), 4.19 (t, 2H, J = 7.2 Hz), 2.84 (t, 2H, J = 6.8 Hz), 2.02 (m, 2H), 1.02 (t, 3H, J = 7.6 Hz); ¹³C-NMR (100 MHz, CDCl₃): δ 174.42, 128.94, 128.65, 128.49, 128.34, 126.87, 126.55, 64.96, 35.36, 27.58, 9.12; IR (KBr pellet, ν_max/cm⁻¹): 2930, 2882, 2345, 1747, 1426, 1020, 796; elemental analysis for C₁₁H₁₄O₂, calculated: C 74.13, H 7.92; found: C 74.13, H 7.91.

Octyl 2-ethylhexanoate, 17

0.243 g, 95% yield, colourless oil, ¹H-NMR (400 MHz, CDCl₃): δ 4.00 (t, 2H, J = 6.4), 2.19 (m, 1H), 1.55 (m, 2H), 1.53 (m, 2H), 1.38 (m, 2H), 1.34 (m, 2H), 1.32 (m, 2H), 1.23 (m, 10H), 0.81 (t, 9H, J = 6.8 Hz); ¹³C-NMR (100 MHz, CDCl₃): δ 176.57, 64.14, 47.42, 31.85, 31.76, 31.59, 29.63, 29.18, 29.17, 28.68, 25.93, 25.52, 22.62, 14.07, 13.93, 11.85; IR (KBr pellet, ν_max/cm⁻¹) 2950, 2930, 2875, 1740, 1510, 1435, 1025, 850; elemental analysis for C₁₆H₃₂O₂, calculated: C 74.94, H 12.58; found: C 74.95, H 12.56.

Dodecyl 2-ethylhexanoate, 18

0.281 g, 90% yield, pale yellow oil, ¹H-NMR (400 MHz, CDCl₃): δ 4.00 (t, 2H, J = 6.8), 2.18 (m, 1H), 1.61 (m, 2H), 1.58 (m, 2H), 1.54 (m, 2H), 1.50 (m, 2H), 1.41 (m, 2H), 1.36 (m, 2H), 1.88 (m, 16H), 0.81 (t, 9H, J = 2.4 Hz); ¹³C-NMR (100 MHz, CDCl₃): δ 176.57, 64.15, 47.43, 31.91, 31.85, 29.64, 29.63, 29.56, 29.53, 29.34, 29.22, 28.69, 25.94, 25.53, 22.68, 22.64, 14.11, 13.95, 11.86; IR (KBr pellet, ν_max/cm⁻¹) 2965, 2879, 1743, 1523, 1390, 1005, 790; elemental analysis for C₂₀H₄₀O₂, calculated: C 76.86, H 12.90; found: C 76.87, H 12.88.

sec-Butyl-2-ethylhexanoate, 19

0.186 g, 93% yield, colourless oil, ¹H-NMR (400 MHz, CDCl₃): δ 4.86 (m, 1H), 2.22 (m, 1H), 1.58 (m, 2H), 1.49 (m, 4H), 1.20 (m, 4H), 1.07 (d, 3H, J = 6.4 Hz), 0.89 (t, 9H, J = 3.6 Hz); ¹³C-NMR (100 MHz, CDCl₃): δ 176.17, 71.71, 47.67, 33.29, 31.87, 29.29, 25.66, 22.61, 19.63, 13.95, 11.86, 9.78; IR (KBr pellet, ν_max/cm⁻¹) 2915, 2894, 2315, 1712, 1487, 1075, 863; elemental analysis for C₁₂H₂₄O₂, calculated: C 71.95, H 12.08; found: C 71.97, H 12.05.

(1R,2S,5R)-2-Isopropyl-5-methylcyclohexyl 2-ethylhexanoate, 20

0.245 g, 87% yield, pale yellow oil, ¹H-NMR (400 MHz, CDCl₃): δ 1.98 (m, 1H), 1.91 (m, 2H), 1.69 (t, 1H, J = 3.2 Hz), 1.66 (m, 6H), 1.59 (m, 2H), 1.55 (m, 1H), 1.50 (m, 1H), 1.41 (m, 1H), 1.39 (m, 1H), 1.36 (m, 1H), 1.31 (m, 2H), 1.25 (m, 2H), 0.89 (t, 6H, J = 6.8 Hz), 0.82 (d, 3H, J = 4 Hz), 0.75 (d, 6H, J = 6.8 Hz); ¹³C-NMR (100 MHz, CDCl₃): δ 175.97, 73.76, 47.91, 46.91, 40.87, 34.29, 32.13, 31.36, 29.62, 25.94, 25.57, 24.60, 23.07, 22.60, 22.04, 20.81, 13.94, 11.89; elemental analysis for C₁₈H₃₄O₂, calculated: C 76.54, H 12.13; found: C 76.55, H 12.11.

Dodecyl-4-methylbenzoate, 21

0.277 g, 91% yield, colourless oil, ¹H-NMR (400 MHz, CDCl₃): δ 7.86 (d, 2H, J = 7.6 Hz), 7.15 (d, 2H, J = 7.59 Hz), 4.21 (t, 2H, J = 6.4 Hz), 2.314 (s, 3H), 1.670 (m, 2H), 1.349 (m, 18H), 0.797 (t, 3H, J = 6 Hz); ¹³C-NMR (100 MHz, CDCl₃): δ 166.79, 143.39, 134.63, 132.07, 129.16, 128.28, 127.18, 64.97, 31.93, 29.65, 29.60, 29.55, 29.37, 29.30, 28.73, 26.06, 22.7, 21.63, 14.13; IR (KBr pellet, ν_max/cm⁻¹) 3020, 2842, 2336, 1740, 1486, 1100, 910; elemental analysis for C₂₀H₃₂O₂, calculated: C 78.90, H 10.59; found: C 78.89, H 10.60.

Phenethyl 4-methylbenzoate, 22

0.197 g, 93% yield, pale yellow oil, ¹H-NMR (400 MHz, CDCl₃): δ 7.91 (d, 4H, J = 7.6 Hz), 7.33 (d, 2H, J = 7.6 Hz), 7.29 (t, 2H, J = 6.8 Hz), 7.23 (t, 1H, J = 6.4 Hz), 4.5 (t, 2H, J = 10.4 Hz), 3.07 (t, 2H, J = 6.8 Hz), 2.39 (s, 3H); ¹³C-NMR (100 MHz, CDCl₃): δ 166.63, 143.60, 137.99, 129.59, 129.08, 128.99, 128.53, 127.51, 126.56, 65.33, 35.26, 21.68; IR (KBr pellet, ν_max/cm⁻¹) 3036, 2994, 2826, 2354, 1723, 1540, 1462, 1025, 893; elemental analysis for C₁₆H₁₆O₂, calculated: C 79.97, H 6.71; found: C 79.96, H 6.73.

Octyl 4-hydroxybenzoate, 26

0.220 g, 88% yield, colourless liquid, ¹H-NMR (400 MHz, CDCl₃): δ 7.86 (d, 2H, J = 6.4 Hz), 7.18 (s, 1H), 6.81 (d, 2H, J = 6 Hz), 4.21 (t, 2H, J = 6.8 Hz), 1.67 (m, 2H), 1.34 (m, 2H), 1.2 (m, 8H), 0.80 (t, 3H, J = 6.4 Hz); ¹³C-NMR (100 MHz, CDCl₃): δ 167.36, 160.58, 132.19, 131.91, 122.26, 115.30, 65.26, 31.78, 29.19, 28.69, 26.03, 22.64, 14.09; IR (KBr pellet, ν_max/cm⁻¹) 3456, 3256, 2990, 2874, 2312, 1736, 1510, 1435, 1205, 1022, 924; elemental analysis for C₁₅H₂₂O₃, calculated: C 71.97, H 8.86; found: C 71.98, H 8.83.

6-Hydroxyhexyl benzoate, 36

0.202 g, 91% yield, colourless liquid, ¹H-NMR (500 MHz, CDCl₃): δ 8.04 (d, 2H, J = 5.5 Hz), 7.55 (t, 1H, J = 1.5 Hz), 7.43 (t, 2H, J = 6.5 Hz), 4.32 (t, 2H, J = 6.5 Hz), 3.65 (t, 2H, J = 6.5 Hz), 1.78 (m, 2H), 1.92 (s, 1H), 1.60 (m, 2H), 1.46 (m, 4H); ¹³C-NMR (125 MHz, CDCl₃): δ 166.75, 132.85, 130.43, 129.53, 129.53, 128.34, 128.34, 64.96, 62.76, 32.58, 28.70, 25.84, 25.42; IR (KBr pellet, ν_max/cm⁻¹): 3443, 2932, 2854, 1735, 1620, 1562, 1407, 1326, 1156; elemental analysis for C₁₃H₁₈O₃, calculated: C 70.24, H 8.16; found: C 70.23, H 8.18.

6-Hydroxyhexyl 4-nitrobenzoate, 37

0.219 g, 82% yield, colourless liquid, ¹H-NMR (500 MHz, CDCl₃): δ 8.28 (d, 2H, J = 5 Hz); 8.20 (d, 2H, J = 5 Hz), 4.38 (t, 2H, J = 7 Hz), 3.66 (t, 2H, J = 6.5 Hz), 1.82 (m, 2H), 1.61 (m, 2H), 1.48 (m, 4H); ¹³C-NMR (125 MHz, CDCl₃): δ 164.77, 150.48, 135.79, 130.65, 130.65, 123.52, 123.52, 65.94, 62.68, 32.52, 28.57, 25.78, 25.40; IR (KBr pellet, ν_max/cm⁻¹): 3412, 3247, 2943, 1726, 1514, 1510, 1434, 1387, 1330, 1272; elemental analysis for C₁₃H₁₇NO₅, calculated: C 58.42, H 6.41, O 29.93; found: C 58.44, H 6.40, O 29.93.

6-Hydroxyhexyl propionate, 38

0.151 g, 87% yield, colourless liquid, ¹H-NMR (500 MHz, CDCl₃): δ 4.07 (t, 2H, J = 7 Hz), 3.63 (t, 2H, J = 6.5 Hz), 2.23 (s, 1H), 2.33 (m, 2H), 1.67 (m, 2H), 1.57 (m, 2H), 1.39 (m, 4H), 1.13 (t, 3H, J = 7.5 Hz); ¹³C-NMR (125 MHz, CDCl₃): δ 174.73, 64.32, 62.63, 32.51, 28.57, 27.59, 25.68, 25.36, 9.11; IR (KBr pellet, ν_max/cm⁻¹): 3345, 2968, 2814, 1749, 1605, 1547, 1458, 1431, 1372, 1206; elemental analysis for C₉H₁₈O₃, calculated: C 62.04, H 10.41; found: C 62.03, H 10.43.

N-Benzylbenzamide, 40

0.194 g, 92% yield, white solid, mp 104–105 °C, ¹H-NMR (400 MHz, CDCl₃): δ 4.62 (d, 2H, J = 5.6 Hz), 6.54 (s, 1H), 7.25 (m, 1H), 7.3 (t, 2H, J = 4 Hz), 7.35 (d, 2H, J = 3.6 Hz), 7.41 (t, 2H, J = 7.6 Hz), 7.49 (t, 1H, J = 7.6 Hz), 7.79 (d, 2H, J = 7.6 Hz); ¹³C-NMR (100 MHz, CDCl₃): 167.41, 138.17, 134.34, 131.56, 129.75, 128.79, 128.59, 127.91, 127.61, 126.97, 124.82, 44.12; IR (KBr pellet, ν_max/cm⁻¹): 3242, 3015, 2872, 1656, 1605, 1575, 1527, 1453, 1412, 1328; elemental analysis for C₁₄H₁₃NO, calculated: C 79.59, H 6.20, O 7.57; found: C 79.57, H 6.21, O 7.58.

N-Cyclohexylbenzamide, 41

0.184 g, 91% yield, colourless solid, mp 122–124 °C, ¹H-NMR (400 MHz, CDCl₃): δ 1.12 (m, 2H), 1.41 (m, 2H), 1.65 (m, 2H), 1.76 (m, 2H), 2.02 (m, 2H), 3.98 (m, 1H), 7.26 (s, 1H), 7.4 (t, 2H, J = 7.2 Hz), 7.48 (t, 1H, J = 6.8 Hz), 7.74 (d, 2H, J = 7.2 Hz); ¹³C-NMR (100 MHz, CDCl₃): 166.64, 135.08, 131.24, 128.51, 126.80, 48.66, 33.23, 25.56, 24.90; IR (KBr pellet, ν_max/cm⁻¹): 3365, 3094, 2982, 1686, 1548, 1522, 1436, 1312, 1293; elemental analysis for C₁₃H₁₇NO, calculated: C 76.81, H 8.43, O 7.87; found: C 76.80, H 8.45, O 7.87.

N-Phenylbenzamide, 42

0.177 g, 90% yield, white solid, mp 84–86 °C, ¹H-NMR (400 MHz, CDCl₃): δ 7.14 (t, 1H, J = 7.2 Hz), 7.33 (t, 1H, J = 7.6 Hz), 7.41 (t, 1H, J = 7.6 Hz), 7.46 (d, 2H, J = 6 Hz), 7.54 (t, 1H, J = 7.2 Hz), 7.63 (t, 1H, J = 4 Hz), 7.68 (t, 1H, J = 8 Hz), 7.89 (d, 2H, J = 7.6 Hz), 8.47 (s, 1H); ¹³C-NMR (100 MHz, CDCl₃): 165.98, 138.23, 135.08, 132.90, 132.11, 131.99, 131.87, 131.66, 128.96, 128.58, 127.24, 124.36, 120.38; IR (KBr pellet, ν_max/cm⁻¹): 3372, 3265, 3020, 1636, 1624, 1563, 1435, 1345, 1207; elemental analysis for C₁₃H₁₁NO, calculated: C 79.17, H 5.62, O 8.11; found: C 79.18, H 5.60, O 8.12.

N-Cyclohexyl-4-nitrobenzamide, 46

0.215 g, 87% yield, pale yellow solid, mp 174–176 °C, ¹H-NMR (500 MHz, CDCl₃): δ 8.26 (d, 2H, J = 1.5 Hz), 7.90 (d, 2H, J = 7 Hz), 6.07 (1H, s), 3.985 (m, 1H), 2.04 (m, 2H), 1.77 (m, 2H), 1.69 (m, 2H), 1.44 (m, 2H), 1.25 (m, 2H); ¹³C-NMR (125 MHz, CDCl₃): δ 164.60, 149.47, 140.69, 128.05, 128.05, 123.76, 123.76, 49.25, 33.10, 33.10, 25.46, 24.86, 24.86; IR (KBr pellet, ν_max/cm⁻¹): 3072, 2959, 1642, 1613, 1527, 1474, 1336, 1279, 1211, 1094; elemental analysis for C₁₃H₁₁N₂O₃, calculated: C 62.89, H 6.50, O 19.33; found: C 62.87, H 6.50, O 19.34.

Acknowledgements

The authors gratefully acknowledge SERB, the Department of Science and Technology (DST), Govt. of India, New Delhi for financial support (Grant No. SB/FT/CS-103/2013 and SB/EMEQ-076/2014).

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Footnote

† Electronic supplementary information (ESI) available: ¹H and ¹³C-NMR spectra for compounds 2, 5–12, 14, 16–22, 26, 36–38, 40–42, 46. See DOI: 10.1039/c6ra22558f

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