Solvent-free conversion of thioamides to thioesters

Article abstract of DOI:10.1080/00397911.2010.503001

Diverse thioesters were efficiently prepared via the solvent-free reaction of thioamide derivatives with alkyl halides in the presence of catalytic amounts of 1,4-diazabicyclo[2.2.2]octane (DABCO) in small quantities of water and in good to excellent yiel

Full text of DOI:10.1080/00397911.2010.503001

Synthetic Communications1, 41: 2421–2429, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2010.503001
SOLVENT-FREE CONVERSION OF THIOAMIDES TO
THIOESTERS
Hassan Zali Boeini1 and Abbas Zali2
1Department of Chemistry, University of Isfahan, Isfahan, Iran
2Department of Chemistry, Malek-Ashtar University of Technology,
Shahin-shahr, Iran
GRAPHICAL ABSTRACT
Abstract Diverse thioesters were efficiently prepared via the solvent-free reaction of thioa-
mide derivatives with alkyl halides in the presence of catalytic amounts of 1,4-diazabicy-
clo[2.2.2]octane (DABCO) in small quantities of water and in good to excellent yields.
Keywords Alkyl halides; S-alkylation; thioamides; thioesters; thiols
INTRODUCTION
Recently, thioesters have attracted considerable interest in organic synthesis
because of their distinctive chemical properties compared to oxoesters,[1] and their
enhanced reactivity has been employed successfully in a wide range of synthetic
organic transformations.[2] Thioesters of coenzyme A (CoA) are ubiquitous in all
living organisms and play central roles in their metabolism.[3] Also, they exhibit
a wide range of biological activities and have considerable applications in drug
development[4–8] and industry.[9–11]
Hence, developing a simple and versatile method for the preparation of thioe-
sters is still an urgent need. The most known approach to the synthesis of thioesters
is the direct reaction of the corresponding thiols with a suitable acid chloride[12] or
acid anhydride.[13–15] Moreover, thioesters have been synthesized by direct reaction
of carboxylic acids with thiols and in various reaction conditions and catalysts.[16–18]
Thioesters have also been prepared by the reaction of esters[19] or N-acylbenzotria-
zoles[20] with thiols. Recently, tert-butyl protected thiols have been utilized for the
Received May 12, 2010.
Address correspondence to Hassan Zali Boeini, Department of Chemistry, University of Isfahan,
81746-73441, Isfahan, Iran. E-mail: h.zali@chem.ui.ac.ir
2421

2422
H. Z. BOEINI AND A. ZALI
preparation of thioesters.[21] More recently, the N-S acyl shift mediated by attaching
a thiol auxiliary residue to the peptide backbone has been applied for peptide thioe-
ster syntheses.[22] This method employs thiols in long reaction times, and the overall
yields are not satisfactory (16–31%). Dess–Martin periodinane (DMP)–mediated
reaction of thiols and aldehydes is another method for the synthesis of thioesters.[23]
Although most of these approaches provide efficient access to thioesters, they
suffer from the use of corrosive reagents, harsh reaction conditions, expensive cata-
lysts or reagents, and unfriendly organic solvents. Nevertheless, the greatest disad-
vantage of the previously mentioned methods is application of thiols as starting
materials, which are very unpleasant and noxious compounds. However, despite
the efficiency of the latter protocols, the development of less expensive and environ-
mentally benign reactions is a major goal for organic synthesis. Therefore, designing
a single-step and quite ecofriendly method for the synthesis of diverse thioesters
without using thiols was the main goal of the present study.
It is well known that thioamides are very prominent building blocks in organic
synthesis and particularly in construction of heterocyclic compounds.[24] Recently,
we studied the use of tertiary thioamides for thioester synthesis in various
solvents.[25] Herein, an efficient and fast method for the preparation of miscellaneous
thioesters using tertiary thioamides in solvent-free condition is reported.
The study was started by examining the reaction of thioamide 1a as a test
thioamide starting material, prepared by one of the known methods[26] with 4-nitro-
benzyl bromide 2a and in various reaction conditions to produce the corresponding
S-4-nitrobenzyl benzothioate 3a (Scheme 1).
At the outset of our study, a test temperature of 80 ꢀC was chosen for the
reaction course. Therefore, the starting thioamide and equimolar amounts of
4-nitrobenzyl bromide were heated to 80 ꢀC for 75 min. Afterward, catalytic amounts
of 1, 4-diazabicyclo[2.2.2]octane (DABCO) (5 mol %) in small amounts of water
were added to the reaction mixture, and heating was continued for a further
15 min and at the same temperature to produce thioester 3a in good isolated
yield (69%).
Various temperatures were also examined to obtain the best reaction condition,
and the screening results indicate that a remarkable yield was achieved in the tem-
perature 110 ꢀC (Table 1). It is also worthwhile to note that increasing the reaction
times and temperatures does not lead to an increase in the yield of thioester, and
the investigations revealed that in these conditions the reaction mixture was
Scheme 1.

CONVERSION OF THIOAMIDES TO THIOESTERS
2423
Table 1. Temperature screening in synthesis of thioesters
Entry
Temperature (ꢀC)
Product
Yielda (%)
1
2
3
4
80
95
3a
3a
3a
3a
69
83
93
85b
110
130
aYields refer to isolated products.
bFormation of some colored impurities was observed.
contaminated with some colored or tarry materials. A variety of substituted phenyl
groups were tolerated on thioamides and reacted with various alkyl halides, leading
to different thioesters.
The generality of the method has been demonstrated by the successful synthesis
of several thioesters in rather good to excellent yields (63–94%) (Scheme 2), and
Table 2 summarizes our results.
These experiments obviously revealed that all kind of alkyl halides (chloride,
bromide, and iodide) could be successfully applied in the course of reaction, but bet-
ter results were obtained with bromomethyl aromatics.
A mechanism is also proposed and shown in Scheme 3. Thioamide 1 first
undergoes an S-alkylation with the alkyl halide 2 to form thioformamidinium
salt 4, and subsequent hydrolysis catalyzed by DABCO affords the corresponding
thioesters 3.
To assess the feasibility of applying this method on a preparative scale, we car-
ried out the reaction of thiobenzamide 1a with 4-nitrobenzyl bromide 2a on a
20-mmol scale. As expected, the reaction proceeded gently, similar to the case on
a smaller scale (Table 2, entry 1), and the desired S-4-nitrobenzyl benzothioate 3a
was obtained in 90% isolated yield.
In addition to the simplicity of the method, good yields, and easy workup, the
salient futures of this methodology lie in the fact that the reactions are carried out in
solvent-free conditions and in short times. Furthermore, purification of thioester
products is achieved with simple recrystallization in MeOH. Moreover, the method
is compatible with many substituents such as halogen, alkoxy, dialkylamino, and
Scheme 2.

2424
H. Z. BOEINI AND A. ZALI
Table 2. Efficient synthesis of thioesters in solvent-free conditionsa
Entry
1
Ar
2
Product 3b
Yieldc
1
2
a
b
c
d
d
d
e
e
f
Ph
4-Tolyl
4-Cl-Ph
2a
2a
2a
2e
2f
3a
3b
3c
3d
3e
3f
93
93
3
90
4
4-Me2N-Ph
4-Me2N-Ph
4-Me2N-Ph
4-biphenyl
4-biphenyl
2-naphthyl
2-naphthyl
2-naphthyl
4-is-Pr-Ph
3,4-di-MeO-Ph
2-quinolyl
2-quinolyl
2-quinolyl
66d,e
63d
94
5
6
2b
2b
2a
2b
2a
2c
2a
2a
2e
2d
2b
7
3g
3h
3i
89
91
8
9
90
91
10
11
12
13
14
15
16
f
3j
f
3k
3l
72
92
g
h
i
3m
3n
3o
3p
88
64d,e
67
i
i
89
aThioamide derivative (1 mmol), alkyl halide (1 mmol), DABCO (0.05 mmol), temperature ¼ 110 ꢀC,
time ¼ 70 min.
bAll products were characterized by IR, 1H NMR, 13C NMR, and elemental analysis.
cAll yields refer to pure isolated products.
dColorless to pale yellow liquids, which were crystallized very slowly (12–24 h), isolated by flash chro-
matography (silica gel, EtOAc–Hexane, 1:8).
eAn excess amount of CH3I was added (2 mmol).
nitro in the thioamide substrates and works efficiently with both types of alkyl and
aralkyl halides.
In conclusion, the methodology reported herein is expected to be a genuinely
general route for the efficient synthesis of a wide range of thioesters in solvent-free
Scheme 3.

CONVERSION OF THIOAMIDES TO THIOESTERS
2425
conditions. Apart from being an environmentally benign reaction, the method prof-
its from the use of cheap and safe starting materials and avoids from the use of very
unpleasant and noxious thiols as well as corrosive acid chlorides in the course of
reaction.
EXPERIMENTAL
General Experimental Information
Chemicals were purchased from Fluka, Merck, and Aldrich chemical compa-
nies. Starting thioamides were readily obtained by the Willgerodt–Kindler reaction
of the corresponding benzaldehydes.[26] Thin-layer chromatography (TLC) was per-
formed on ultraviolet (UV)–active aluminum-backed plates of silica gel (TLC silica
˚
gel 60 F254). Flash chromatography was performed using silica gel (60 A, 230–400
mesh) with reagent-grade solvents. NMR spectra were recorded at 400 MHz
(1H NMR) and 100 MHz (13C NMR) referenced to an internal standard [tetra-
methylsilane (TMS)] or residual solvent protons and signals are reported in ppm
(d). Low-resolution mass spectra (LRMS) were recorded on Bell and Howell
21–490 spectrometers. Melting points were obtained on a Fisher-Johns melting point
apparatus and are uncorrected. Fourier transform (FT)–IR spectra were obtained on
a Perkin-Elmer Spectrum 1000 with samples loaded as KBr discs.
Typical Experimental Procedure
In a 5-ml round-bottom flask, thioamide 1 (2 mmol) and alkyl halide 2
(2 mmol) were added. The reaction mixture was heated at 110 ꢀC for 75 min. There-
after, DABCO (0.1 mmol, 11.2 mg) in water (0.5 ml) was added, and heating was
continued for a further 15 min at the same temperature. After cooling to room tem-
perature, an oily residue was left, which slowly solidified. Then, the solid was filtered
and washed with water (2 ꢁ 10 ml). Finally, the solid compound was recrystallized
from MeOH (in some cases with MeOH-CHCl3) to afford pure thioesters as white
or pale yellow needles.
Spectroscopic Data for Compounds 3a–3p
1
Compound 3a. Mp (MeOH): 93–94 ꢀC; H NMR (400 MHz, CDCl3): d 8.19
(d, J ¼ 8.8 Hz, 2H), 7.97 (d, d, J ¼ 8.5 Hz, J ¼ 1.3 Hz, 2H), 7.61 (t, J ¼ 7.5 Hz, 1H),
7.58 (d, J ¼ 8.8, 2H), 7.48 (t, J ¼ 7.5, 2H), 4.38 (s, 2H); 13C NMR (100 MHz, CDCl3):
d 190.5, 147.1, 145.6, 136.3, 133.9, 129.9, 128.8, 127.4, 124.0, 32.6. Anal. calcd. for
C14H11NO3S: C, 61.52; H, 4.06; N, 5.12; S, 11.73. Found: C, 61.48; H, 3.95; N,
5.23; 11.81.
1
Compound 3b. Mp (MeOH): 102–103 ꢀC; H NMR (400 MHz, CDCl3): d
8.21 (d, J ¼ 8.9 Hz, 2H), 7.86 (d, J ¼ 8.2 Hz, 2H), 7.56 (d, J ¼ 8.9 Hz, 2H), 7.27 (d,
J ¼ 8.2 Hz, 2H), 4.36 (s, 2H), 2.42 (s, 3H); 13C NMR (100 MHz, CDCl3): d 190.0,
147.1, 145.8, 144.9, 133.7, 130.1, 129.2, 127.5, 123.8, 32.4; Anal. calcd. for
C15H13NO3S: C, 62.70; H, 4.56; N, 4.87; S, 11.16. Found: C, 62.53; H, 4.43; N,
5.01; S, 11.28.

2426
H. Z. BOEINI AND A. ZALI
1
Compound 3c. Mp (MeOH): 118–119 ꢀC; H NMR (400 MHz, CDCl3): d
8.19 (d, J ¼ 8.7 Hz, 2H), 7.91 (d, J ¼ 8.6 Hz, 2H), 7.57 (d, J ¼ 8.7 Hz, 2H), 7.45 (d,
J ¼ 8.6 Hz, 2H), 4.38 (s, 2H); 13C NMR (100 MHz, CDCl3): d 189.3, 147.2, 145.2,
140.3, 134.6, 129.9, 129.1, 128.7, 123.9, 32.6. Anal. calcd. for C14H10ClNO3S: C,
54.64; H, 3.28; N, 4.55; S, 10.42. Found: C, 54.45; H, 3.19; N, 4.71; S, 10.61.
Compound 3d1. Mp (MeOH): 54–56 ꢀC; 1H NMR (400 MHz, CDCl3): d 7.90
(d, J ¼ 9.0 Hz, 2H), 6.67 (d, J ¼ 9.0 Hz, 2H), 3.06 (s, 6H), 2.45 (s, 3H).
Compound 3e. Mp (MeOH-CHCl3): 106–108 ꢀC; 1H NMR (400 MHz,
CDCl3): d 7.91 (d, J ¼ 9.0 Hz, 2H), 7.41 (d, J ¼ 7.1 Hz, 2H), 7.33 (t, J ¼ 7.3 Hz,
2H), 7.26 (t, J ¼ 7.3 Hz, 1H), 6.71 (d, J ¼ 9.0 Hz, 2H), 4.32 (s, 2H), 3.09 (s, 6H);
13C NMR (100 MHz, CDCl3): d 189.0, 153.6, 138.3, 129.4, 129.0, 128.6, 127.1,
126.4, 110.9, 40.2, 32.9. Anal. calcd. for C16H17NOS: C, 70.81; H, 6.31; N, 5.16;
S, 11.82. Found: C, 70.64; H, 6.12; N, 5.21; S, 11.94.
1
Compound 3f. Mp (MeOH): 74–76 ꢀC; H NMR (400 MHz, CDCl3): d 8.09
(d, J ¼ 8.3 Hz, 2H), 7.71 (d, J ¼ 8.3 Hz, 2H), 7.66 (d, J ¼ 7.3 Hz, 2H), 7.51 (d,
J ¼ 7.4 Hz, 2H), 7.43–7.46 (m, 3H), 7.38 (t, J ¼ 7.4 Hz, 2H), 7.31 (t, J ¼ 7.2 Hz,
1H), 4.40 (s, 2H); 13C NMR (100 MHz, CDCl3): d 190.9, 146.2, 139.8, 137.6,
135.5, 129.1, 129.0, 128.9, 128.7, 128.4, 127.9, 127.4, 127.3, 33.4. Anal. calcd. for
C20H16OS: C, 78.91; H, 5.30; S, 10.53. Found: C, 78.77; H, 5.11; S, 10.76.
1
Compound 3g. Mp (MeOH): 150–151 ꢀC; H NMR (400 MHz, CDCl3): d
8.20 (d, J ¼ 8.7 Hz, 2.H), 8.05 (d, J ¼ 8.5 Hz, 2H), 7.71 (d, 8.5 Hz, 2H), 7.64 (d,
J ¼ 5.2 Hz, 2H), 7.59 (d, J ¼ 8.7 Hz, 2H), 7.50 (t, J ¼ 6.8, 2H), 7.44 (t, J ¼ 7.3, 1H),
4.41 (s, 2H); 13C NMR (100 MHz, CDCl3): d 190.0, 147.1, 146.6, 145.6, 139.6,
134.9, 129.9, 129.0, 128.5, 128.0, 127.6, 127.3, 123.9, 32.6. Anal. calcd. for
C20H15NO3S: C, 68.75; H, 4.33; N, 4.01; S, 9.18. Found: C, 68.61; H, 4.14; N,
4.11; S, 9.30.
Compound 3h. Mp (MeOH): 75–77 ꢀC; 1H NMR (400 MHz, CDCl3): d 8.58
(s, 1H), 8.04 (d, d, J ¼ 8.6 Hz, J ¼ 1.5 Hz, 1H), 7.98 (d, J ¼ 8.0 Hz, 1H), 7.90 (t,
J ¼ 8.3 Hz, 2H), 7.56–7.64 (m, 2H), 7.46 (d, J ¼ 7.3 Hz, 2H), 7.38 (t, J ¼ 7.6 Hz,
2H), 7.31 (t, J ¼ 7.1 Hz, 1H), 4.42 (s, 2H); 13C NMR (100 MHz, CDCl3): d 191.3,
137.6, 135.8, 134.1, 132.5, 129.6, 129.1, 128.8, 128.7, 128.6, 127.8, 127.4, 127.0,
125.9, 123.2, 33.5. Anal. calcd. for C18H14OS: C, 77.66; H, 5.07; S, 11.52. Found:
C, 77.55; H, 5.01; S, 11.65.
1
Compound 3i. Mp (MeOH): 127–128 ꢀC; H NMR (400 MHz, CDCl3): d
8.54 (s, 1H), 8.20 (d, J ¼ 8.6 Hz, 2H), 7.99 (t, J ¼ 9.6 Hz, 2H), 7.90 (t, J ¼ 8.3 Hz,
2H), 7.57–7.65 (m, 4H), 4.43 (s 2H); 13C NMR (100 MHz, CDCl3): d 190.3, 147.2,
145.6, 136.0, 133.6, 132.4, 129.9, 129.6, 129.1, 128.8, 128.7, 127.9, 127.1, 123.9,
123.0, 32.7. Anal. calcd. for C18H13NO3S: C, 66.86; H, 4.05; N, 4.33; S, 9.92 Found:
C, 66.61; H, 3.98; N, 4.45; S, 10.02.
1
Compound 3j. Mp (MeOH): 62–64 ꢀC; H NMR (400 MHz, CDCl3): d 8.56
(s, 1H) 8.02 (d, d, J ¼ 8.6 Hz, J ¼ 1.7 Hz, 1H), 8.97 (d, J ¼ 8.0 Hz, 1H), 7.90 (t,
J ¼ 8.3 Hz, 2H), 7.56–7.64 (m, 2H), 7.40 (t, J ¼ 4.6 Hz, 2H), 7.04 (t, J ¼ 8.7 Hz,

CONVERSION OF THIOAMIDES TO THIOESTERS
2427
2H), 4.37 (s, 2H); 13C NMR (100 MHz, CDCl3): d 191.1, 163.3, 160.9, 135.8, 134.0,
130.7, 130.6, 129.6, 128.8, 128.6, 127.8, 127.0, 123.1, 115.7, 115.4, 32.7.
1
Compound 3k. Mp (MeOH): 100–102 ꢀC; H NMR (400 MHz, CDCl3): d
8.18 (d, J ¼ 9.3 Hz, 2H), 7.91 (d, J ¼ 8.3 Hz, 2H), 7.57 (d, J ¼ 8.7 Hz, 2H), 7.33 (d,
J ¼ 8.3 Hz, 2H), 4.37 (s, 2H), 2.97 (sep, J ¼ 6.9 Hz, 1H), 1.28 (d, J ¼ 7.0 Hz, 6H);
13C NMR (100 MHz, CDCl3): d 190.0, 155.6, 147.1, 145.8, 134.1, 129.8, 127.6,
126.9, 123.8, 34.3, 32.4, 32.6. Anal. calcd. for C17H17NO3S: C, 64.74; H, 5.43; N,
4.44; S, 10.17. Found: C, 64.62; H, 5.32; N, 4.60; S, 10.29.
1
Compound 3l. Mp (MeOH): 104–106 ꢀC; H NMR (400 MHz, CDCl3): d
8.15 (d, J ¼ 8.6 Hz, 2H), 7.63 (d, d, J ¼ 8.5 Hz, J ¼ 1.9 Hz, 1H), 7.54 (d, J ¼ 8.6 Hz,
2H), 7.44 (d, J ¼ 1.9 Hz, 1H), 6.89 (d, J ¼ 8.5 Hz, 1H), 4.34 (s, 2H), 3.94 (s, 3H),
3.92 (s, 3H); 13C NMR (100 MHz, CDCl3): d 189.1, 153.8, 149.0, 147.1, 145.8,
130.0, 129.2, 123.8, 122.0, 110.3, 109.4, 56.2, 56.0, 32.5. Anal. calcd. for
C16H15NO5S: C, 57.65; H, 4.54; N, 4.20; S, 9.62. Found: C, 57.57; H, 4.41; N,
4.42; S, 9.75.
1
Compound 3m2. Mp (MeOH): 60–61 ꢀC; H NMR (400 MHz, CDCl3): d
8.27 (d, J ¼ 8.6 Hz, 1H), 8.24 (d, J ¼ 8.6 Hz, 1H), 8.04 (d, J ¼ 8.4 Hz, 1H), 7.85 (d,
J ¼ 8.1 Hz, 1H), 7.78 (t, J ¼ 7.1 Hz, 1H), 7.63 (t, J ¼ 7.1 Hz, 1H), 2.49 (s, 3H).
1
Compound 3n. Mp (MeOH): 140–142 ꢀC; H NMR (400 MHz, CDCl3): d
8.32 (d, J ¼ 8.5 Hz, 1H), 8.23 (d, J ¼ 8.5 Hz, 1H), 8.08 (d, J ¼ 8.5 Hz, 1H), 7.87 (d,
J ¼ 10.3 Hz, 1H), 7.81 (t, d, J ¼ 8.4 Hz, J ¼ 1.3 Hz, 1H), 7.67 (t, J ¼ 7.9 Hz, 1H),
7.42 (t, d, J ¼ 5.4 Hz, J ¼ 3.2 Hz, 2H), 7.02 (t, J ¼ 8.7 Hz, 2H), 4.31 (s, 2H); 13C
NMR (100 MHz, CDCl3): d 193.4, 163.2, 160.8, 151.3, 147.0, 137.6, 130.8, 130.7,
130.4, 128.8, 127.7, 117.1, 115.6, 115.3, 32.6. Anal. calcd. for C17H12FNOS: C,
68.67; H, 4.07; N, 4.71; S, 10.78. Found: C, 68.55; H, 4.14; N, 4.84; S, 10.89.
1
Compound 3o. Mp (MeOH): 114–115 ꢀC; H NMR (400 MHz, CDCl3): d
8.31 (d, J ¼ 8.5 Hz, 1H), 8.22 (d, J ¼ 8.5 Hz, 1H), 8.08 (d, J ¼ 8.5 Hz, 1H), 7.89 (d,
J ¼ 8.0, 1H), 7.80 (t, d, J ¼ 8.4 Hz, 1.4 Hz, 1H), 7.66 (t, d, J ¼ 7.0 Hz, J ¼ 1.1 Hz,
1H), 7.45 (d, J ¼ 7.0 Hz, 2H), 7.34 (t, J ¼ 7.4 Hz, 2H), 7.27 (t, J ¼ 7.0 Hz, 1H), 4.35
(s, 2H); 13C NMR (100 MHz, CDCl3): d 193.5, 151.5, 147.1, 137.6, 137.5, 130.4,
130.2, 129.6, 129.1, 128.8, 128.6, 127.7, 127.2, 117.1, 33.4. Anal. calcd. for
C17H13NOS: C, 73.09; H, 4.69; N, 5.01; S, 11.48. Found: C, 72.91; H, 4.56; N,
5.09; S, 11.57.
Compound 3p. Mp (MeOH): 84–86 ꢀC; 1H NMR (400 MHz, CDCl3): d 7.88
(d, J ¼ 8.8 Hz, 2H), 6.64 (d, J ¼ 8.8 Hz, 2H), 3.06 (s, 6H), 3.05 (t, J ¼ 8.0 Hz, 2H),
1.73 (sep, J ¼ 6.4 Hz, 1H), 1.54–1.59 (m, 2H), 0.95 (d, J ¼ 6.4 Hz); 13C NMR
(100 MHz, CDCl3): d 189.9, 153.6, 129.2, 124.9, 110.6, 40.0, 38.8, 27.7, 26.7, 22.3.
Anal. calcd. for C14H21NOS: C, 66.89; H, 8.42; N, 5.57; S, 12.76. Found: C,
68.65; H, 8.56; N, 5.79; S, 12.83.
ACKNOWLEDGMENT
The University of Isfahan, which provided facilities for this research, is
gratefully acknowledged.

2428
H. Z. BOEINI AND A. ZALI
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