Direct, rapid and convenient synthesis of esters and thioesters using PPh3/N-chlorobenzotriazole system

Article abstract of DOI:10.5935/0103-5053.20130291

We have developed an efficient method for esterification and thioesterification of various carboxylic acids with different alcohols and thiols using PPh3/N-chlorobenzotriazole mixed reagent in CH2Cl2 at room temperature.

Full text of DOI:10.5935/0103-5053.20130291

http://dx.doi.org/10.5935/0103-5053.20130291
J. Braz. Chem. Soc., Vol. 25, No. 2, 253-263, 2014.
Printed in Brazil - ©2014 Sociedade Brasileira de Química
0103 - 5053 $6.00+0.00
Article
A
Direct, Rapid and Convenient Synthesis of Esters and Thioesters Using
PPh3/N-Chlorobenzotriazole System
Hamed Rouhi-Saadabad and Batool Akhlaghinia*
Department of Chemistry, Faculty of Sciences, Ferdowsi University of Mashhad,
Mashhad 9177948974, Iran
Desenvolvemos um método eficiente de esterificação e tioesterificação de uma série de ácidos
carboxílicos com diferentes álcoois e tióis usando o reagente misto PPh3/N-clorobenzotriazol em
CH2Cl2 a temperatura ambiente.
We have developed an efficient method for esterification and thioesterification of various
carboxylic acids with different alcohols and thiols using PPh3/N-chlorobenzotriazole mixed reagent
in CH2Cl2 at room temperature.
Keywords: PPh3, N-chlorobenzotriazole (NCBT), esters, thioesters
Introduction
azopyridines,6 [{Cl(C6F13C2H4)2SnOSn(C2H4C6F13)2Cl}2]
graphite bisulfate,7 functionalized acidic ionic liquids,8,9
TiO(acac)2,10 is another possible but uneconomical choice.
These reactions historically faced purification challenges and
often haunt the chemist in the isolation of the desired product.
Development a new, simple, efficient, and highly profitable
esterification method under mild reaction conditions and
without tedious and difficult purification steps, is highly
desirable and challenging.
Esterification is the fundamental and routinely used
functional group transformation in organic chemistry1 and
it is extensively employed for the protection and further
manipulation of the carboxylic acid functional group as
well as the synthesis of natural products. Traditionally,
the simple condensation between a carboxylic acid and an
alcohol is the most straightforward way to esterification.
The difficulty stems primarily from the equilibration of the
condensation reaction. The commonest approach to bias the
equilibrium in favor of the product side is either by using
the reactants in excess and/or continuously removing of the
water formed during the reaction. The former treatment is
not desirable in terms of “atom economy”2 since the excess
reactant remains to be separated from the reaction mixture.
On the other hand, azeotropy is most frequently invoked,
but a variety of dehydration methods have been put forth,
although 100% conversion and, hence, 100% yield are, in
general, not easy to achieve.Another problem emerges from
the base or acid catalysts which are inevitably employed
in this reaction. Under such conditions, the tolerance of a
wide spectrum of functional groups that is often required
in modern synthetic chemistry is not easy to achieve.
Activation of the carboxylic acid or alcohol components with
a stoichiometric amount of promoter such as carbodiimides,3
diethylazodicarboxylate,4 5,5´-dimethyl-3,3’-azoisoxazole,5
The N-halo reagents in combination with PPh3 have
found widespread use in synthetic organic chemistry.11
In the present study, esterification and thioesterification
of carboxylic acids were investigated by using
N-chlorobenzotriazole (NCBT, as an N-halo reagent)
PPh3 system. The reaction proceeds under mild, essentially
neutral conditions and has been well documented for a
variety of substrates.
Experimental
General
The products were purified by column chromatography.
Thepuritydeterminationsoftheproductswereaccomplished
by thin layer chromatography (TLC) on silica gel polygram
STL G/UV 254 plates. The melting points of products were
determined with an Electrothermal Type 9100 melting point
apparatus. The Fourier transform infrared (FTIR) spectra
were recorded on an Avatar 370 FT-IR Therma Nicolet
*e-mail: akhlaghinia@um.ac.ir

254
Direct, Rapid and Convenient Synthesis of Esters and Thioesters
J. Braz. Chem. Soc.
spectrometer. The nuclear magnetic resonance (NMR)
spectra were provided on Bruker Ultrashield Avance III
400 MHz instruments in CDCl3. Mass spectra were recorded
with a CH7A Varianmat Bremem instrument at 70 eV, in
m/z (rel%). NCBT was prepared and purified by the method
described in the literature.12 Preparation of benzyl benzoate
by using PPh3/5,5’-dimethyl-3,3’-azoisoxazole, PPh3/4,4’-
azopyridine, Ph2PCl/I2/imidazole and PPh3/[bis(acetoxy)
iodo]benzene/diethylazodicarboxylate (DEAD) mixed
reagents was performed according the methods reported
previously.5,6,13,14
continuous stirring. Benzoic acid (0.122 g, 1 mmol) was
then added and stirring was continued for 15 min. Benzyl
alcohol (0.270 g, 2.5 mmol) was added and the temperature
was raised up to room temperature. The pale yellow solution
was neutralized by triethylamine (0.175 mL). Stirring was
continued for 3 h at room temperature. The progress of the
reaction was followed by TLC. Upon completion of the
reaction, the concentrated residue was passed through a
short silica-gel column using n-hexane–ethyl acetate (8:1)
as eluent. Benzyl benzoate was obtained with 80% yield
after removing the solvent under reduced pressure.
Preparation of benzyl benzoate by using PPh3/
trichloroisocyanuric acid (TCCA)
Preparation of benzyl benzoate by using PPh3/NCBT
To a cold solution of PPh3 (0.327 g, 1.25 mmol)
in CH2Cl2 (3 mL), freshly prepared NCBT (0.194 g,
1.25 mmol) was added with continuous stirring. Benzoic
acid (0.122 g, 1 mmol) was then added and stirring was
continued for 15 min. Benzyl alcohol (0.270 g, 2.5 mmol)
was added and the temperature was raised up to room
temperature. The pale yellow solution was neutralized
by triethylamine (0.175 mL). Stirring was continued for
40 min at room temperature. The progress of the reaction
was followed by TLC. Upon completion of the reaction,
the concentrated residue was passed through a short
silica-gel column using n-hexane–ethyl acetate (8:1) as
eluent. Benzyl benzoate was obtained with 95% yield after
removing the solvent under reduced pressure.
To a cold solution of PPh3 (0.327 g, 1.25 mmol) in
CH2Cl2 (3 mL), TCCA (0.0974 g, 0.42 mmol) was added
with continuous stirring. Benzoic acid (0.122 g, 1 mmol) was
then added and stirring was continued for 15 min. Benzyl
alcohol (0.270 g, 2.5 mmol) was added and the temperature
was raised up to room temperature. The white suspension
was neutralized by triethylamine (0.175 mL). Stirring was
continued for 2.5 h at room temperature. The progress of
the reaction was followed by TLC. Upon completion of
the reaction, the concentrated residue was passed through a
short silica-gel column using n-hexane–ethyl acetate (8:1) as
eluent. Benzyl benzoate was obtained with 85% yield after
removing the solvent under reduced pressure.
Preparation of benzyl benzoate by using PPh3/
N‑bromosuccinimide (NBS)
Benzyl benzoate (Table 3, entry 1)
m.p. 20-21°C (Lit. 19-21 °C);15 IR (neat) υmax/cm−1
3423, 3088, 3064, 3033, 2949, 2892, 1716 (C=O), 1601,
1585, 1451, 1376, 1314, 1270 (C–O), 1175, 1109, 1069,
710, 697; 1H NMR (400 MHz, CDCl3, 25 °C, ppm) δ 8.14-
8.11 (m, 2H, ArH), 7.62-7.58 (m, 1H, ArH), 7.51-7.41 (m,
7H, ArH), 5.41 (s, 2H, PhCH2).
To a cold solution of PPh3 (0.327 g, 1.25 mmol) in
CH2Cl2 (3 mL), NBS (0.223 g, 1.25 mmol) was added with
continuous stirring. Benzoic acid (0.122 g, 1 mmol) was
then added and stirring was continued for 15 min. Benzyl
alcohol (0.270 g, 2.5 mmol) was added and the temperature
was raised up to room temperature. The red suspension
was neutralized by triethylamine (0.175 mL). Stirring was
continued for 4.5 h at room temperature. The progress of
the reaction was followed by TLC. Upon completion of
the reaction, the concentrated residue was passed through
a short silica-gel column using n-hexane–ethyl acetate (8:1)
as eluent. Benzyl benzoate was obtained with 40% yield
after removing the solvent under reduced pressure.
Benzyl 4‑methylbenzoate (Table 3, entry 2)
Solid; m.p. 45-46 °C (Lit. 45-46 °C);16 IR (KBr) υmax/cm−1
3391, 3088, 3031, 2962, 2896, 1706 (C=O), 1609, 1454,
1370, 1267 (C–O), 1175, 1100, 751, 700; MS (EI) m/z 226
(M+, 10%), 118 (M+–PhCH2O, 100%), 91 (PhCH2, 90%).
Benzyl 3,5‑dimethylbenzoate (Table 3, entry 3)
Solid;m.p. 65-66°C(Lit. 66-67°C);17 IR(KBr)υmax/cm−1
3423, 3063, 3033, 3009, 2951, 2918, 1717 (C=O), 1608,
1498, 1555, 1308, 1211 (C–O), 1115, 1010, 766, 754,
697; 1H NMR (400 MHz, CDCl3, 25 °C, ppm) δ 7.73 (s,
2H, ArH), 7.50-7.48 (d, 2H, J 6.8 Hz, ArH) 7.45-7.36 (m,
3H, ArH), 7.22 (s, 1H, ArH), 5.39 (s, 2H, PhCH2), 2.39
(s. 6H, 2CH3).
Preparation of benzyl benzoate by using PPh3/
N‑chlorosuccinimide (NCS)
To a cold solution of PPh3 (0.327 g, 1.25 mmol) in
CH2Cl2 (3 mL), NCS (0.166 g, 1.25 mmol) was added with

Vol. 25, No. 2, 2014
Rouhi-Saadabad and Akhlaghinia
255
Benzyl 4‑methoxybenzoate (Table 3, entry 4)
1H NMR (400 MHz, CDCl3, 25 °C, ppm) δ 8.91 (s, 1H,
ArH), 8.45-8.41 (m, 2H, ArH), 7.68 (t, 1H, J 8 Hz, ArH),
7.51-7.29 (m, 5H, ArH), 5.45 (s. 2H, PhCH2).
m.p. 24-26 °C (Lit. 25-27 °C);18 IR (neat) υmax/cm−1
3415, 3068, 2962, 2937, 2839, 1712(C=O), 1606, 1581,
1511, 1456, 1376, 1316, 1270, 1256 (C–O), 1167, 1099,
1029, 769, 696; 1H NMR (400 MHz, CDCl3, 25 °C, ppm) δ
8.91 (td, 2H, J 9.2, 2.4 Hz, ArH), 7.49-7.46 (m, 2H, ArH),
7.44-7.29 (m, 3H, ArH), 6.97-6.93 (m, 2H, ArH), 5.37 (s,
2H, PhCH2), 3.89 (s, 3H, OCH3).
Benzyl cinnamate (Table 3, entry 11)
Solid; m.p. 31-32 °C (Lit. 32-33 °C);21 IR (KBr)
υ
max/cm−1 3064, 3027, 2966, 2896, 1711 (C=O), 1636,
1310, 1162 (C–O), 980, 767, 697; MS (EI) m/z 238 (M+,
10%), 130 (M+–PhCH2O, 100%), 103 (PhCH2O, 90%), 91
(PhCH2, 90%).
Benzyl 2‑chlorobenzoate (Table 3, entry 5)
m.p. 18-20 °C; IR (neat) υmax/cm−1 3064, 3027, 2925,
2847, 1722 (Lit. 1729, C=O),19 1589, 1484, 1451, 1378,
1290 (C–O), 1131, 1046, 936, 751, 740.
(E)‑Benzyl 3‑(4‑chlorophenyl)acrylate (Table 3, entry 12)
Solid; m.p. 122-124 °C; IR (KBr) υmax/cm−1 3064, 3027,
2953, 1708 (Lit. 1709, C=O),22 1637, 1488, 1309, 1166
(C–O), 988, 820, 695; 1H NMR (400 MHz, CDCl3, 25 °C,
ppm) δ 7.70 (d, 1H, J 16 Hz, PhCH=CH–), 7.49-7.29 (m,
9H, ArH), 6.49 (d, 1H, J 16 Hz, PhCH=CH–), 5.29 (s, 2H,
PhCH2); MS (EI) m/z 274 (M+2, 10%) 272 (M+, 35%), 164
(M+–PhCH2O), 91 (M+–PhCH2, 100%).
Benzyl 4‑chlorobenzoate (Table 3, entry 6)
m.p. 25-26 °C (Lit. 25-26 °C);16 IR (neat) υmax/cm−1
3431, 3064, 3035, 2949, 1721 (C=O), 1594, 1487, 1400,
1270 (C–O), 1114, 1092, 1014, 758, 696.
Benzyl 4‑bromobenzoate (Table 3, entry 7)
Solid; m.p. 51-52 °C (Lit. 52-53 °C);16 IR (KBr)
(E)‑Benzyl 3‑(3‑nitrophenyl)acrylate (Table 3, entry 13)
υ
max/cm−1 3415, 3072, 3039, 2974, 2892, 1715 (C=O),
Solid; m.p. 148-149 °C (Lit. 147-149 °C);22 IR (KBr)
1588, 1455, 1396, 1269 (C–O), 1169, 1089, 1008, 759,
698; 1H NMR (400 MHz, CDCl3, 25 °C, ppm) δ 7.97 (d,
2H, J 8.4 Hz, ArH), 7.61 (d, 2H, J 8.4 Hz, ArH), 7.49-7.38
(m, 5H, ArH), 5.40 (s, 2H, PhCH2); MS (EI) m/z 291 (M+,
5%), 182 (M+–PhCH2O, 87%), 91 (PhCH2, 87%).
υ
max/cm−1 3072, 2925, 1711 (C=O), 1641, 1526, 1351, 1176
(C–O), 1008, 730; MS (EI) m/z 282 (M+, 10%), 175 (M+–
PhCH2O, 81%), 103 (PhCH2O, 80%) 91 (PhCH2, 100%).
Benzyl 2‑phenylacetate (Table 3, entry 14)
Solid; m.p. 51-52 °C (Lit. 52 °C);23 IR (KBr)
Benzyl 3,4‑dichlorobenzoate (Table 3, entry 8)
υ
max/cm−1 3084, 3060, 3031, 2953, 1737 (C=O), 1496, 1454,
Solid; m.p. 57-58 °C (Lit. 58-60 °C);16 IR (KBr)
1380, 1260 (C–O), 1145, 749, 695; 1H NMR (400 MHz,
CDCl3, 25 °C, ppm) δ 7.41-7.29 (m, 10H, ArH), 5.17 (s,
2H, PhCH2–O), 3.71 (s, 2H, PhCH2–CO).
υ
max/cm−1 3092, 3072, 3039, 2953, 2892, 1724 (C=O), 1585,
1564, 1458, 1379, 1273 (C–O), 1236, 1106, 1032, 757,
696; 1H NMR (400 MHz, CDCl3, 25 °C, ppm) δ 8.16 (d,
1H, J 2 Hz, ArH), 7.91 (dd,1H, J 8.4, 2 Hz, ArH), 7.53 (d,
1H, J 8.4 Hz, ArH), 7.48-7.39 (m, 5H, ArH), 5.39 (s, 2H,
PhCH2); MS (EI) m/z 284 (M+4, 5%), 282 (M+2, 26%), 280
(M+, 32%), 245 (M+–Cl, 38%), 173 (M+–PhCH2O, 100%)
145 (M+–PhCl2, 35%), 91(PhCH2, 100%).
Benzyl 2,2‑diphenylacetate (Table 3, entry 15)
m.p. 34-35 °C (Lit. 35 °C);24 IR (neat) υmax/cm−1
3084, 3063, 3030, 2953, 1736 (C=O), 1600, 1496, 1453,
1184, 1144 (C–O), 1004, 975, 744, 696; MS (EI) m/z 193
(M+‑PhCH2O, 30%), 166 ((Ph)2CH, 100%), 91 (PhCH2,
80%).
Benzyl 4‑nitrobenzoate (Table 3, entry 9)
Solid; m.p. 82-83 °C (Lit. 82-83°C);16 IR (KBr)
Benzyl 2‑(4‑methoxyphenyl)acetate (Table 3, entry 16)
υ
max/cm−1 3112, 3051, 1712 (C=O), 1604, 1522, 1347, 1277
Solid; m.p. 142-144 °C (Lit. 141-144 °C);25 IR (KBr)
1
(C–O), 1121, 1104, 744, 715.695; H NMR (400 MHz,
CDCl3, 25 °C, ppm) δ 8.32-8.25 (m, 4H, ArH), 8.50-8.40
(m, 5H, ArH), 5.44 (s, 2H, PhCH2); MS (EI) m/z 257 (M+,
10%), 150 (M+–PhCH2O, 100%), 91 (M+–PhCH2, 100%).
υ
max/cm−1 3063, 3035, 2957, 2839, 1713 (C=O), 1606, 1511,
1455, 1315, 1257 (C–O), 1167, 1100, 1028, 768, 750, 796;
MS (EI) m/z 256 (M+, 5%), 164 (M+–PhCH2, 20%), 149
(M+–PhCH2O, 80%), 91 (PhCH2, 80%).
Benzyl 3‑nitrobenzoate (Table 3, entry 10)
Benzyl stearate (Table 3, entry 17)
Solid; m.p. 48-49 °C (Lit. 48-49 °C);20 IR (KBr)
Solid; m.p. 44-45°C (Lit. 44-45 °C);26 IR (KBr)
υ
max/cm−1 3436, 3084, 3039, 2962, 2872, 1727 (C=O), 1613,
υ
max/cm−1 3092, 2955, 2917, 2849, 1743 (C=O), 1471, 1393,
1531, 1350, 1293, 1258 (C–O), 1130, 1070, 717, 697;
1286 (C–O), 961.

256
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J. Braz. Chem. Soc.
Benzyl thiophene‑3‑carboxylate (Table 3, entry 18)
m/z 168 (M+–cyclohexyl, 45%), 149 (4-NO2PhCO, 60%),
104 (PhCO, 100%), 82 (cyclohexyl, 90%).
Solid; m.p. 139-141°C (Lit. 140-142 °C);27 IR (KBr)
υ
max/cm−1 3111, 3035, 1716 (C=O), 1522, 1407, 1261
(C–O), 1187, 1100, 747; 1H NMR (400 MHz, CDCl3, 25 °C,
ppm) δ 8.18-8.17 (m, 1H, ArH), 7.60-7.58 (m, 1H, ArH),
7.48-7.33 (m, 6H ArH), 5.36 (s, 2H, PhCH2).
Phenyl 4‑nitrobenzoate (Table 3, entry 26)
Solid; m.p. 130-132 °C (Lit. 129-132 °C);34 IR (KBr)
υ
max/cm−1 3113, 1741 (C=O), 1609, 1520, 1484, 1348, 1269
(C–O), 1183, 1079, 1017, 847; 1H NMR (400 MHz, CDCl3,
25 °C, ppm) δ 8.41 (dd, 2H, J 6.4, 2.8 Hz, ArH), 8.38 (dd,
2H, J 6.4, 2.8 Hz, ArH), 7.50-7.46 (m, 2HArH), 7.36-7.33
(m, 1H, ArH), 7.28-7.24 (m, 2H, ArH).
Phenethyl 4‑nitrobenzoate (Table 3, entry 19)
Solid; m.p. 60-61 °C (Lit. 59-61 °C);28 IR (KBr)
υ
max/cm−1 3068, 1710 (C=O), 1597, 1486, 1450, 1379,
1362, 1287 (C–O), 1051, 940, 750, 695; MS (EI) m/z 164
(M+–PhCH2CH2, 17%), 149 (4-NO2PhCO, 80%), 104
(PhCH2CH2, 100%), 91 (PhCH2, 80%).
m-Tolyl 4‑nitrobenzoate (Table 3, entry 27)
Solid; m.p. 86-87 °C (Lit. 87 °C);35 IR (KBr) υmax/cm−1
3109, 3080, 2985, 2921, 2850, 1736 (C=O), 1607, 1529,
1487, 1352, 1273 (C–O), 1236, 715; MS (EI) m/z 257 (M+,
5%), 149 (M+–(m-MePhO)), 103 (m-MePhO, 62%).
3‑Phenylpropyl 4‑nitrobenzoate (Table 3, entry 20):
Solid; m.p. 46-47 °C (Lit. 47-48 °C);29 IR (KBr)
υ
max/cm−1 3120, 2958, 1716 (C=O), 1602, 1523, 1352,
1286 (C–O), 1103, 870, 746, 717, 700; MS (EI) m/z 284
(M+, 5%), 149 (4-NO2PhCO, 60%), 118 (PhCH2CH2CH2,
100%), 91 (PhCH2, 90%).
4-Chlorophenyl 4‑methoxybenzoate (Table 3, entry 28)
Solid; m.p. 97-99 °C (Lit. 97-99 °C);36 IR (KBr)
υ
max/cm−1 3015, 2982, 2847, 1727 (C=O), 1610, 1515, 1489,
1267 (C–O), 1204, 1167, 1072, 1021, 842, 761; 1H NMR
(400 MHz, CDCl3, 25 °C, ppm) δ 8.16 (dd, 2H, J 6.8, 2 Hz,
ArH), 7.41(dd, 2H, J6.8, 2Hz,ArH), 7.17(dd, 2H, J6.8, 2Hz,
ArH), 1.48 (dd, 2H, J 7.2, 2 Hz, ArH), 3.92 (s, 3H, OCH3).
Butyl 4‑nitrobenzoate (Table 3, entry 21)
m.p. 33-34 °C (Lit. 34-35 °C);30 IR (neat) υmax/cm−1
3117, 3080, 3060, 2963, 2938, 2868, 1717 (C=O), 1606,
1526, 1352, 1278 (C–O), 1103, 872, 846, 786, 714;
1H NMR (400 MHz, CDCl3, 25 °C, ppm) δ 8.30 (d, 2H,
J 8.8 Hz, ArH), 8.22 (d, 2H, J 8.8 Hz, ArH), 4.39 (t, 2H,
J 6.8 Hz, OCH2CH2), 1.83-1.76 (m, 2H, OCH2CH2), 1.55-
1.46 (m, 2H, CH2CH3), 1.01 (t, 3H, J 7.2 Hz, OCH2CH3).
Phenyl stearate (Table 3, entry 29)
Solid; m.p. 48-49 °C (Lit. 49-50 °C);37 IR (KBr)
υ
max/cm−1 2954, 2917, 2849, 1743 (C=O), 1741, 1393
(C–O), 961, 754.
1‑Phenylethyl 4‑nitrobenzoate (Table 3, entry 22)
S‑Cyclohexyl 3,5‑dimethylbenzothioate (Table 3, entry 30)
Solid; m.p. 44-45 °C (Lit. 44 °C);31 IR (KBr)
Solid; m.p. 57-58 °C (Lit. 56-58 °C);38 IR (KBr)
υ
max/cm−1 3113, 3039, 2978, 2933, 1723 (C=O), 1607, 1528,
υ
max/cm−1 2929, 2853, 1659 (O=C–S), 1606, 1448, 1292,
1
1454, 1351, 1271 (C–O), 1102, 1060, 1014, 873, 841, 719;
1H NMR (400 MHz, CDCl3, 25 °C, ppm) δ 8.32-8.29 (m, 2H,
ArH),8.27-8.24(m,2H,ArH),7.49-7.34(m,5H,ArH),6.18(q,
1H, J6.4Hz, OCH(Ph)CH3), 1.74(d, 3H, J6.4Hz, OCHCH3).
1149, 1034, 697, 861, 786, 703; H NMR (400 MHz,
CDCl3, 25 °C, ppm) δ 7.58 (s, 2H,ArH), 7.20 (s, 1H,ArH),
3.73 (t, 1H, J 4 Hz, SCH–), 2.37 (s, 6H, 2CH3), 2.05-1.27
(m, 10H, CH2 cyclohexyl ring).
Benzhydryl 4‑nitrobenzoate (Table 3, entry 23)
S‑Octyl 4‑methoxybenzothioate (Table 3, entry 31)
m.p. 24-26 °C (Lit. 25-27 °C);38 IR (neat) υmax/cm−1 3011,
2955, 2926, 2854, 1655 (O=C–S), 1602, 1578, 1508, 1462,
Solid; m.p. 131-132 °C (Lit. 132 °C);32 IR (KBr)
υ
max/cm−1 3109, 3051, 2859, 1721 (C=O), 1609, 1525, 1446,
1
1345, 1280 (C–O), 1261, 1116, 1103, 967, 763, 719, 699;
1H NMR (400 MHz, CDCl3, 25 °C, ppm) δ 8.33 (s, 4H,
ArH), 8.47-8.35 (m, 10H, ArH), 7.17 (s, 1H, Ph2CH); MS
(EI) m/z 333 (M+, 10%), 182 (M+–2Ph, 88%) 165 (M+–
NO2PhCO2, 100%) 151 (M+–Ph2CHO, 72%).
1315, 1259, 1213, 1167, 1031, 913; H NMR (400 MHz,
CDCl3, 25 °C, ppm) δ 7.97 (d, 2H, J 8.8 Hz, ArH), 6.93 (d,
2H, J 8.8 Hz, ArH), 3.06 (t, 2H, J 7.6 Hz, S–CH2), 1.71-1.60
(m, 2H, CH2), 1.43 (q, 2H, J 6.8 Hz, R‑CH2CH3), 1.32-1.29
(m, 8H, 4CH2), 0.89 (t, 3H, J 6.8 Hz, R–CH2CH3).
Cyclohexyl 4‑nitrobenzoate (Table 3, entry 24)
(E)‑S‑Cyclohexyl 3‑(3‑nitrophenyl)prop‑2‑enethioate (Table
3, entry 32)
Solid; m.p. 141-143 °C (Lit. 142-145 °C);39 IR (KBr)
υ
max/cm−1 2930, 2852, 1681 (O=C–S), 1530, 1448, 1350,
Solid; m.p. 50-51 °C (Lit. 51-52 °C);33 IR (KBr)
υ
max/cm−1 2117, 2938, 2860, 1720 (C=O), 1609, 1528, 1454,
1348, 1319, 1278 (C–O), 1115, 1013, 835, 719; MS (EI)

Vol. 25, No. 2, 2014
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257
1050, 997, 736, 702; MS (EI) m/z 291 (M+, 5%), 175
(M+–cyclohexyl–S, 100%), 115 (cyclohexyl–S, 80%), 82
(cyclohexyl, 100%).
S‑p‑Tolyl 2,2‑diphenylethanethioate (Table 3, entry 38)
Solid; m.p. 96-98 °C (Lit. 98 °C);43 IR (KBr) υmax/cm−1
3084, 3059, 3027, 2917, 2843, 1674 (O=C–S), 1482, 1451,
981, 741, 698; MS (EI) m/z 316 (M+, 3%), 193 (M+–4-
MePhS, 60%), 166 ((Ph)2CH, 100%), 90 (PhCH2, 80%).
S‑Octyl 4‑nitrobenzothioate (Table 3, entry 33)
m.p. 27-29 °C (Lit. 28-30 °C);39 IR (neat) υmax/cm−1
3105, 2953, 2927, 2855, 1666 (O=C–S), 1605, 1528, 1349,
1202, 923, 848.
Results and Discussion
In continuation of our study to extend the scope
of N-halo reagents in conjunction with PPh3,11,44 we
investigated the applicability of PPh3/trichloroisocyanuric
acid (TCCA), PPh3/N-bromosuccinimide (NBS),
PPh3/N-chlorosuccinimide (NCS) and PPh3/(NCBT)
systems in direct esterification reaction of benzoic acid
with benzyl alcohol (Table 1, entries 1-3 and 8). Recently,
direct esterification reaction was also reported by using
PPh3 and an electron deficient reagent such as PPh3/5,5’-
dimethyl-3,3’-azoisoxazole,5 PPh3/4,4’-azopyridine,6
Ph2PCl/I2/imidazole13 and PPh3/[bis(acetoxy)iodo]benzene/
diethylazodicarboxylate (DEAD)14 (Table 1, entries 4-7).
As is apparent from Table 1, PPh3/(NCBT) mixed reagent
is the most efficient mixed-reagent system, for conversion
of benzoic acid to benzyl benzoate. Replacement of NCBT
by every above-mentioned mixed reagent systems produces
benzyl benzoate in longer reaction time.
S‑Benzyl 4‑nitrobenzothioate (Table 3, entry 34)
Solid; m.p. 85-86 °C (Lit. 85.4-86.5 °C);40 IR (KBr)
υ
max/cm−1 3113, 1643 (O=C–S), 1601 1521, 1349, 1318,
1
1203, 1193, 930, 850, 711, 691; H NMR (400 MHz,
CDCl3, 25 °C, ppm) δ 8.32 (dd, 2H, J 7.2, 2 Hz, ArH),
8.14 (dd, 2H, J 7.2, 2 Hz, ArH), 7.42-7.28 (m, 5H, ArH),
4.39 (s, 2H, PhCH2).
S‑Cyclohexyl 4‑nitrobenzothioate (Table 3, entry 35)
Solid; m.p. 139-140 °C; IR (KBr) υmax/cm−1 3109, 2942,
2928, 2851, 1655 (O=C–S), 1604, 1523, 1349, 1318, 1195,
1
6
1174, 1109, 922, 885, 848, 690; H NMR (400 MHz,
CDCl3, 25 °C, ppm) δ 8.32-8.29 (m, 2H, ArH), 8.13-8.10
(m, 2H, ArH), 3.82-3.76 (m, 1H, SCH–), 2.06-1.48 (m,
10H, CH2 cyclohexyl ring).
S‑Benzyl 2,2‑diphenylethanethioate (Table 3, entry 36)
According the data from Table 1, PPh3/NCBT system
is the best choice for direct esterification of benzoic acid
(Scheme 1).
Solid; m.p. 61-63 °C (Lit. 62-64 °C);41 IR (KBr)
υ
max/cm−1 3333, 3088, 3064, 3027, 2917, 2843, 1680
(O=C–S), 1494, 1453, 1011, 994, 741, 698; MS (EI) m/z
317 (M+, 5%), 196 (M+–PhCH2S, 10%), 166 ((Ph)2CH,
100%), 91 (PhCH2, 90%).
To achieve high reaction efficiency, the reaction of
benzoic acid with benzyl alcohol was chosen as model
reaction to investigate the applicability of PPh3/NCBT
system in direct esterification and thioesterification
reactions of carboxylic acids. The effects of different molar
ratios of PPh3/NCBT/RCO2H/ROH in various solvents were
examined on the model reaction.
S‑p‑Tolyl benzothioate (Table 3, entry 37)
Solid;m.p.75-77°C(Lit.76.5-77°C);42 IR(KBr) υ /cm−1
max
3047, 2917, 2847, 1668 (O=C–S), 1482, 1450, 1274, 1203,
1169, 897, 808, 773, 689; MS (EI) m/z 228 (M+, 10%), 122
(M+–p-MePhS, 80%), 105 (PhCO, 100%), 91 (PhCH2, 90%).
Treating a solution of PPh3 (1 equiv.) and NCBT
(1 equiv.) in CH3CN at room temperature with different
Table 1. Esterification of benzoic acid with benzyl alcohol by using different mixed reagent systema
entry
1
Mixed reagents
Reaction condition
CH2Cl2/r.t.b
time / h
Isolated yield / %
PPh3/trichloroisocyanuric acid (TCCA)
PPh3/N-bromosuccinimide (NBS)
PPh3/N-chlorosuccinimide (NCS)
PPh3/5,5’-dimethyl-3,3’-azoisoxazole
PPh3/4,4’-azopyridine
2.5
4.5
3
85
40
80
89
86
91
76
95
2
CH2Cl2/r.t.b
3
CH2Cl2/r.t.b
45
CH3CN/reflux
CH3CN/reflux
CH3CN/reflux
THF/r.t.b
6.5
3
56
613
714,c
8
Ph2PCl, I2, imidazole
4
PPh3/[bis(acetoxy)iodo]benzene/diethylazodicarboxylate (DEAD)
PPh3/N-chlorobenzotriazole (NCBT)
16
CH2Cl2/r.t.b
40 min
aThe experimental details are shown in experimental section; br.t.: room temperature; cthe result corresponds to the esterification reaction of p-nitrobenzoic
acid with benzyl alcohol.

258
Direct, Rapid and Convenient Synthesis of Esters and Thioesters
J. Braz. Chem. Soc.
Scheme 1.
Table 2. Conversion of benzoic acid to benzyl benzoate with PPh3/NCBT/
benzyl alcohol system under different reaction conditions
molar ratios of benzoic acid and benzyl alcohol afforded
benzyl benzoate in high yield over 2-5 h (Table 2, entries
1-4). Increasing the molar ratios of PPh3/NCBT and benzyl
alcohol in CH3CN gave 100% conversion of benzoic acid
to benzyl benzoate in 40 min (Table 2, entries 5-6). As
the applying 1.25/1.25/1/2.5 molar ratios of PPh3/NCBT/
RCO2H/ROH in CH3CN gave 100% conversion of benzoic
acid to benzyl benzoate in 40 min, esterification reaction was
examined in CH2Cl2 at the same conditions. Surprisingly,
there is no difference between the rate of esterification
reaction in CH3CN and CH2Cl2 (Table 2, compare entries
5 and 7).At the same conditions performing the reaction in
other solvents such as THF, CHCl3, 1,4-dioxane, acetone,
toluene and hexane produced the desired product with lower
yield and in longer reaction time (Table 2, entries 8-13).
The best result was obtained by applying 1.25/1.25/1/2.5
molar ratios of PPh3/NCBT/ RCO2H/ROH in CH3CN and
CH2Cl2. Because of economic consideration CH2Cl2 was
chosen for further experiments. To investigate the chemical
activities of PPh3 and NCBT in the esterification reaction,
the model reaction was carried out in the absence of PPh3
and NCBT respectively. As summarized in Table 2, no
desired product was detected in the absence of PPh3 and
NCBT (Table 2, entries 14-15).
To explore the generality and scope of the esterification
and thioesterification reaction by using PPh3/NCBT mixed
reagent, the optimized reaction conditions 1.25/1.25/1/2.5
molar ratio of PPh3/NCBT/RCO2H/ROH or RSH in
CH2Cl2 were used for the synthesis of a series of esters
and thioesters (Table 3). According to the results obtained
(Table 3) esters and thioesters were prepared from the
reaction of aromatic and aliphatic carboxylic acids with
primary and secondary aliphatic and benzylic alcohols,
phenols and aliphatic and aromatic thiols by using
PPh3/NCBT system in high isolated yields.
Molar Ratio
Isolated
entry
Solvent
PPh3/NCBT/
RCO2H/ROH
time / min
yield / %
1
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH2Cl2
THF
1/1/1/1
1/1/1/1.5
5 h
3 h
2 h
2 h
40
80
85
90
92
95
95
95
80
90
65
75
72
55
0
2
3
1/1/1/2
4
1/1/1/2.5
5
1.25/1.25/1/2.5
1.25/1.25/1/3.125
1.25/1.25/1/2.5
1.25/1.25/1/2.5
1.25/1.25/1/2.5
1.25/1.25/1/2.5
1.25/1.25/1/2.5
1.25/1.25/1/2.5
1.25/1.25/1/2.5
0/1.25/1/2.5
6
40
7
40
8
100
80
9
CHCl3
10
11
12
13
14
15
1,4-dioxane
acetone
toluene
hexane
CH2Cl2
CH2Cl2
90
70
60
90
90
1.25/0/1/2.5
90
0
The aromatic carboxylic acids with electron-
withdrawing substituents were rapidly reacted with benzyl
alcohol and converted into their corresponding esters in a
very short reaction time (20-35 min) with 100% conversion
(Table 3, entries 6-10). In spite of inductive effect of
chlorine which caused o-chlorobenzoic acid (pKa = 2.89)
stronger acid than p-chlorobenzoic acid (pKa = 4.03),
o-chlorobenzoic acid was converted to the corresponding
ester in longer reaction time than p-chlorobenzoic acid
(e.g., compare entry 5 with 6). Difference in reactivity
between o-chlorobenzoic acid and p-chlorobenzoic acid
can be rationalized by the steric effect of chlorine in

Vol. 25, No. 2, 2014
Rouhi-Saadabad and Akhlaghinia
259
ortho position of aromatic ring. The reaction of aromatic
carboxylic acids bearing electron-donating substituents,
with benzyl alcohol was completed in longer reaction
time (55-70 min) than the above-mentioned acids (e.g.,
compare entries 2-4 with 6-10). By now, we can conclude
that the electron deficiency in carbonyl group plays an
important role in the reaction rate of esterification. This
effect has been observed in the esterification reaction of
cinnanic acid and substituted cinnamic acids (e.g., compare
entries 11 with 12-13). PPh3/NCBT system converted
aliphatic carboxylic acids to the corresponding esters in
a more longer reaction time (Table 3, entries 14-17). In
comparison, primary aliphatic alcohols have low reactivity
than primary benzylic ones towards p-nitro benzoic acid
(Table 3, entries 19-21). Also, secondary alcohols have
little reactivity than primary alcohols in the presence of
PPh3/NCBT system (Table 3, entries 22-24). As is to be
expected, tertiary alcohols because of steric hindrance
were resistant to react with carboxylic acids by using the
above-mentioned mixed reagent (Table 3, entry 25). As far
as we know, none of the reported methods on esterification
reaction by using PPh3/5,5’-dimethyl-3,3’-azoisoxazole,5
PPh3/4,4’-azopyridine,6 Ph2PCl/I2/imidazole13 and
PPh3/[bis(acetoxy)iodo]benzene/diethylazodicarboxylate
(DEAD)14 have shown any reactivity from tertiary alcohols
towards benzoic acids. In order to gain more insight into
the general applicability of this method, we also studied the
possibility of applying PPh3/NCBT system to the reaction
of carboxylic acids with phenols. On the basis of the results
obtained from Table 3, aromatic and aliphatic carboxylic
acids react smoothly with phenols, and the corresponding
esters are produced with high yields (Table 3, entries 26-29).
This mixed reagent system also converts aliphatic and
aromatic carboxylic acids to the corresponding thioesters
with primary and secondary aliphatic and aromatic thiols
(Table 3, entries 30-38).
Table 3. Conversion of different carboxylic acids to different esters and thioesters by using PPh3/NCBT/alcohol (or phenol or thiol) system
entry
1
Carboxylic acid
Alcohol/phenol/ thiol
benzyl alcohol
Producta
time / min
40
Yield / %
95
2
3
benzyl alcohol
benzyl alcohol
55
60
90
95
4
5
6
7
benzyl alcohol
benzyl alcohol
benzyl alcohol
benzyl alcohol
70
60
35
35
89
80
85
85
8
benzyl alcohol
25
90

260
Direct, Rapid and Convenient Synthesis of Esters and Thioesters
J. Braz. Chem. Soc.
Table 3. continuation
entry
Carboxylic acid
Alcohol/phenol/ thiol
benzyl alcohol
Producta
time / min
Yield / %
98
9
20
10
11
benzyl alcohol
benzyl alcohol
20
90
88
120
12
13
14
benzyl alcohol
benzyl alcohol
benzyl alcohol
100
80
85
92
85
100
15
16
benzyl alcohol
benzyl alcohol
110
130
80
85
17
18
benzyl alcohol
benzyl alcohol
120
180
90
87
19
2-phenyl ethanol
50
90
20
21
3-phenyl-1-propanol
1-butanol
90
90
85
110

Vol. 25, No. 2, 2014
Rouhi-Saadabad and Akhlaghinia
261
Table 3. continuation
entry
Carboxylic acid
Alcohol/phenol/ thiol
1-phenyl ethanol
Producta
time / min
130
Yield / %
22
23
85
benzhydrol
180
70
24
25
26
27
cyclohexanol
1-adamantanol
phenol
100
24 h
90
92
trace
90
m-cresol
95
90
28
29
p-chlorophenol
100
150
95
87
phenol
30
cyclohexanthiol
90
85
31
32
33
1-octanthiol
cyclohexanthiol
1-octanthiol
120
120
120
90
90
80

262
Direct, Rapid and Convenient Synthesis of Esters and Thioesters
J. Braz. Chem. Soc.
Table 3. continuation
entry
Carboxylic acid
Alcohol/phenol/ thiol
benzyl mercaptan
Producta
time / min
Yield / %
95
34
35
40
65
cyclohexanthiol
benzyl mercaptan
p-methyl thiophenol
p-methyl thiophenol
92
80
85
80
36
37
38
120
90
200
aAll the products were identified by comparing their spectral data with those of an authentic sample.
In our experiments, the completion of the reaction
was confirmed by the disappearance of the carboxylic
acids on TLC followed by the disappearance of acidic OH
stretching frequency at 3400-2400 cm-1 in FTIR spectra.
Also, absorption bands at 1743-1706 and 1393-1144 cm-1
due to carbonyl and C–O group of esters in FTIR spectra
confirmed the ester formation. Formation of thioesters
was also confirmed by appearance of an absorption bands
at 1681-1643 cm-1 due to carbonyl group (O=C–S) of
thioesters.All of the products were known compounds and
characterized by the IR and comparison of their melting
points with known compounds. The structure of selected
products was further confirmed by 1H NMR spectroscopy
and mass spectrometry.
thioesters; (iv) in contrast to the previously reported systems,
which proceeded by dehydration reaction between carboxylic
acids and alcohols, in the present method, esters are produced
in a short reaction time. (v) PPh3 and NCBT system could
be considered as an attractive and useful contribution to
the present organic synthesis for direct esterification and
thioesterification of different carboxylic acids.
Supplementary Information
Supplementary information is available free of charge
at http://jbcs.sbq.org.br as PDF file.
Acknowledgement
Conclusion
The authors gratefully acknowledge the partial support
of this study by Ferdowsi University of Mashhad Research
Council (Grant no. p/3/19312).
In this study, we introduced the application of NCBT (as
an N-halo reagent) in conjunction with PPh3 for esterification
and thioesterification reactions. In comparison with the
previously reported methods, the present protocol offers
several advantages: (i) the reaction proceeds smoothly with
a wide range of carboxylic acids (aromatic and aliphatic)
and alcohols /or phenols and thiols. (ii) the reagents (PPh3
and NCBT) offers easy handling and simple work-up; (iii)
this method has satisfactory yields of a variety of esters and
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Submitted: September 6, 2013
Published online: December 10, 2013