One-pot odourless synthesis of thioesters via in situ generation of thiobenzoic acids using benzoic anhydrides and thiourea

Article abstract of DOI:10.3762/bjoc.11.141

An efficient and odourless procedure for a one-pot synthesis of thioesters by the reaction of benzoic anhydrides, thiourea and various organic halides (primary, allylic, and benzylic) or structurally diverse, electron-deficient alkenes (ketones, esters, and nitriles) in the presence of Et3 N has been developed. In this method, thiobenzoic acids were in situ generated from the reaction of thiourea with benzoic anhydrides, which were subjected to conjugate addition with electron-deficient alkenes or a nucleophilic displacement reaction with alkyl halides.

Full text of DOI:10.3762/bjoc.11.141

One-pot odourless synthesis of thioesters via in situ
generation of thiobenzoic acids using
benzoic anhydrides and thiourea
Mohammad Abbasi*1 and Reza Khalifeh2
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2015, 11, 1265–1273.
1Chemistry Department, Faculty of Sciences, Persian Gulf University,
Bushehr 75169, Iran and 2Department of Chemistry, Shiraz University
of Technology, Shiraz, Iran
doi:10.3762/bjoc.11.141
Received: 12 March 2015
Accepted: 16 July 2015
Published: 28 July 2015
Email:
Mohammad Abbasi* - abbassi@pgu.ac.ir
Associate Editor: R. Sarpong
* Corresponding author
© 2015 Abbasi and Khalifeh; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
benzoic anhydride; Michael addition; nucleophilic displacement;
thioester; thiourea
Abstract
An efficient and odourless procedure for a one-pot synthesis of thioesters by the reaction of benzoic anhydrides, thiourea and
various organic halides (primary, allylic, and benzylic) or structurally diverse, electron-deficient alkenes (ketones, esters, and
nitriles) in the presence of Et3N has been developed. In this method, thiobenzoic acids were in situ generated from the reaction of
thiourea with benzoic anhydrides, which were subjected to conjugate addition with electron-deficient alkenes or a nucleophilic dis-
placement reaction with alkyl halides.
Introduction
Thioesters have many uses in organic synthesis as intermedi- less, the reactions of thioacids have not been widely studied
ates, mild acyl transfer agents and thiol sources [1-19]. because they are not sufficiently available. On the other hand,
Thioesters can be synthesized in the laboratory by means of the usual methods for preparing thioacids involve the action of
different methods using diverse reagents and substrates [20-26]. toxic and unpleasant smelling, gaseous hydrogen sulfide on
The reaction of active carboxylic acid derivatives with thiols carboxylic acid derivatives [46,47]. Also, thioacids as thiols
[27-41] and the coupling of thiols with carboxylic acids using have a strong and repulsive smell. The in situ generation of
activating agents [42-45] have been mainly used for the syn- thioacids using odourless, easy to handle and inoffensive
thesis of thioesters in organic synthesis. Thiol-based reactions substrates is an appropriate solution to these problems. In this
have a foul smell, making them unpleasant. Thioesters can also regard, the synthesis of S-functionalized thioesters using
be prepared through the conjugate addition and nucleophilic thioaroylate ions in situ generated from acyloxyphosphonium
displacement reactions using thioacid nucleophiles. Neverthe- salts and tetrathiomolybdate has been reported [48-50].
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Beilstein J. Org. Chem. 2015, 11, 1265–1273.
Results and Discussion
preliminary tests were accomplished using thiourea and
The preparation of alkane thiols using thiourea and alkyl halides carboxylic acid derivatives such as esters, acyl halides and
is a well-known reaction in organic synthesis. In addition, two anhydrides under various conditions. First, acetyl and benzoyl
methods for the synthesis of thioacids from acyl chlorides using chlorides were separately treated with an equivalent amount of
N,N-dimethylthioformamide [51] or thioacetamide [52] have thiourea by using various solvents and temperatures. The results
been reported. As far as we know, a similar reaction using thio- of these experiments were not satisfactory and after work-up
urea and carboxylic acid derivatives to prepare thioacids has not with basic aqueous solutions, a complex mixture of unidenti-
yet been introduced. Hence, this work is focused on the devel- fied products was obtained in all experiments. The similar
opment of new reaction sequences for the synthesis of thioacid experiments with ethyl acetate and ethyl benzoate did not lead
derivatives via in situ generation of thioacids using thiourea as to the formation of any product and the substrates were intact
an inexpensive and easy to handle sulfur surrogate.
under all experimental conditions tested. Next, a possible reac-
tion between thiourea and carboxylic anhydrides was investi-
Recently, a one-pot procedure for the preparation of thioesters gated. As a model reaction, a mixture of well-powdered benzoic
via the reaction of in situ generated thiols (as suggested by other anhydride (1 mmol) and thiourea (1.1 mmol) in Et3N (0.5 mL)
authors) with benzoyl chlorides was reported (Scheme 1, path a) was stirred at 40 °C. The starting benzoic anhydride was gradu-
[53]. In this protocol, an alkyl halide was treated with thiourea ally consumed within 0.5 h and a two-phase system containing
and a benzoyl chloride in an aqueous Triton X-100 micelle [53]. Et3N and a thick brick-red liquid was formed. Next, H2O
Alternatively, the thioester could be synthesized from the reac- (1 mL) and H2O2 (1.2 mmol) were added to the reaction mix-
tion of the alkyl halide with the thioacid in situ generated from ture and the resulting solution was stirred for 1 h at room
the reaction of thiourea and benzoyl chlorides (Scheme 1, temperature. Extractive work-up with EtOAc followed by silica
path b).
gel column chromatography afforded benzoyl disulfide in 91%
yield.
In order to explore this possibility, the alkyl halide was
removed from the procedure and butyl acrylate was treated with We then focused our attention on developing a practical method
benzoyl chloride and thiourea (Scheme 2). Thiobenzoic acid or to prepare thia-Michael adducts of thioacids. In this regard, a
its corresponding thia-Michael adduct were not formed after mixture of benzoic anhydride and thiourea in Et3N was stirred
24 h. The butyl acrylate was intact and the starting benzoyl at 40 °C for 0.5 h. Then, water and n-butyl acrylate were added
chloride was mainly converted to potassium benzoate.
to the mixture and stirring was continued for another 1 h at
room temperature. The best results were obtained using
This result confirms that the thioester product cannot be 1.3 equiv of thiourea and 1.2 equiv of benzoic anhydride per
produced through generation of the thioacid from thiourea and 1 equiv of butyl acrylate. The desired thia-Michael product was
benzoyl chloride.
obtained in 86% yield (based on the starting butyl acrylate) and
72% (based on the starting benzoic anhydride) after extractive
To establish a protocol for the one-pot synthesis of thioacid work-up with EtOAc followed by silica gel column chromatog-
derivatives via in situ generation of thioacids, considerable raphy.
Scheme 1: One-pot procedure for the preparation of thioesters.
Scheme 2: Reaction of benzoyl chloride, thiourea and butyl acrylate.
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Beilstein J. Org. Chem. 2015, 11, 1265–1273.
The optimized reaction conditions were then applied to further thioacids. The reaction of aliphatic anhydrides such as acetic,
reactions using structurally diverse benzoic anhydrides and phenylacetic and hexanoic anhydrides with thiourea under the
different electron-deficient alkenes. The results are summarized reaction conditions did not lead to the desired thioacids after
in Table 1.
24 h. In another project, the optimized reaction conditions were
applied to further reactions using structurally diverse benzoic
It can be seen that benzoic anhydrides have readily produced anhydrides and different alkyl halides. The results are shown in
the corresponding thioesters through the in situ generation of Table 2.
Table 1: One-pot synthesis of thioesters using benzoic anhydrides, thiourea and electron-deficient alkenesa.
Entry
1
R1
H
EWG
t (h)
Product
Isolated yield (%)b
COCH3
0.5
80
1 [48]
2
3
4
5
6
7
8
H
CN
0.5
0.5
0.5
5
88
90
86
80
84
88
90
2 [20]
H
CO2Et
CO2n-Bu
COCH3
CN
3
4
5
6
7
8
H
CH3
CH3
CH3
CH3
5
CO2Et
CO2n-Bu
5
5
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Beilstein J. Org. Chem. 2015, 11, 1265–1273.
Table 1: One-pot synthesis of thioesters using benzoic anhydrides, thiourea and electron-deficient alkenesa. (continued)
9
Cl
Cl
COCH3
2
2
2
2
2
2
83
85
88
88
83
86
9
10
CN
10
11
12
13
11
Cl
CO2Et
CO2n-Bu
COCH3
CN
12
Cl
13c
14c
CH3O
CH3O
14
15
16
15c
16c
CH3O
CH3O
CO2Et
2
2
88
90
CO2n-Bu
aFirst-step reaction conditions: anhydride (1.2 mmol), thiourea (1.3 mmol), Et3N (0.5 mL), 40 °C; Second-step reaction conditions: Micheal acceptor
(1 mmol), H2O (1 mL), rt. bThe yields were calculated based on Michael acceptors as limiting reagents. To achieve yields based on the starting anhy-
drides or thiourea, they should be divided by 1.2 and 1.3, respectively. cThe first step reaction was conducted at 80 °C.
As evident from the results presented in Table 2, by this with N,N-dimethylthioformamide [51] or thioacetamide [52], a
method, primary, benzylic, allylic and propargylic halides have benzoic anhydride undergoes a nucleophilic acyl substitution
been easily converted to the related thioesters within appro- reaction with thiourea to produce the corresponding
priate reaction times in good to excellent yields (Table 2, entries S-benzoylisothiouronium salt as an intermediate. After addition
1–20).
of H2O to the reaction mixture, the salt is hydrolyzed by the
hydroxide anion to generate the thiobenzoate anion. Finally, the
A general pathway for the reaction has been proposed as thiobenzoate reacts with a Michael acceptor or an alkyl halide
presented in Scheme 3. Similar to reactions of acyl chlorides to give the thioester product.
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Beilstein J. Org. Chem. 2015, 11, 1265–1273.
Table 2: One-pot synthesis of thioesters using benzoic anhydrides, thiourea and alkyl halidesa.
Entry
1
R1
H
R2X
t (h)
Product
Isolated yield (%)b
n-C10H21I
0.5
85
17 [53]
2
3
H
H
0.5
0.5
0.5
0.5
0.5
5
90
81
92
87
90
91
80
84
90
1269
18
19 [54]
4
H
CH3I
20
5
H
n-C8H17Br
21 [53]
22 [55]
23 [56]
24
6
H
7
CH3
CH3
CH3
CH3
8
5
9
n-C8H17Br
5
25
10
CH3I
5
26 [20]

Beilstein J. Org. Chem. 2015, 11, 1265–1273.
Table 2: One-pot synthesis of thioesters using benzoic anhydrides, thiourea and alkyl halidesa. (continued)
11
CH3
5
90
27
28 [57]
29 [20]
30
12
13
Cl
Cl
n-C10H21I
2
2
2
2
2
86
91
87
84
88
CH3I
14
Cl
15
Cl
31
16c
CH3O
32
33
17c
18c
19c
20c
CH3O
CH3O
CH3O
CH3O
2
2
2
2
81
87
88
91
34
n-C8H17Br
35 [58]
CH3I
36
aFirst-step reaction conditions: anhydride (1.2 mmol), thiourea (1.3 mmol), Et3N (0.5 mL), 40 °C; Second-step reaction conditions: R2X (1 mmol), H2O
(1 mL), rt. bThe yields have been calculated based on alkyl halides as limiting reagents. To achieve yields based on the starting anhydrides or thio-
urea, they should be divided by 1.2 and 1.3, respectively. cThe first step reaction was conducted at 80 °C.
1270

Beilstein J. Org. Chem. 2015, 11, 1265–1273.
Scheme 3: A proposed reaction pathway.
Conclusion
192.0, 137.3, 133.1, 128.9, 127.4, 31.9, 29.6, 29.6, 29.5, 29.3,
In conclusion, an efficient, versatile, and odourless protocol for 29.2, 29.0, 29.0, 22.7, 14.1; IR (neat) ν (cm−1): 1666 (C=O
a one-pot preparation of thioesters from non-thiolic precursors thioester); anal. calcd for (C17H26OS): C, 73.33; H, 9.41; S,
under mild conditions has been developed. In this protocol, a 11.52; found: C, 73.31; H, 9.48; S, 11.46.
mixture of a benzoic anhydride, thiourea and an alkyl halide
(primary, allylic or benzylic), or a conjugated olefin (ketones, S-Prop-2-yn-1-yl benzothioate (19)
esters, nitriles), Et3N and H2O produced the related thioesters in 1H NMR (250 MHz, CDCl3) δ 7.88–7.84 (m, 2H), 7.53–7.46
good to excellent yields.
(m, 1H), 7.40–7.33 (m, 2H), 3.75 (d, J = 2.7 Hz, 2H), 2.15 (t, J
= 2.7 Hz, 1H); 13C NMR (62.5 MHz, CDCl3) δ 190.1, 136.2,
133.8, 128.8, 127.3, 78.9, 71.1, 17.5; IR (neat) ν (cm−1): 3294,
2125, 1670; anal. calcd for (C10H8OS): C, 68.15; H, 4.58; S,
Experimental
S-3-Oxobutyl benzothioate (1)
Colorless oil; 1H NMR (250 MHz, CDCl3) δ 7.95–7.91 (m, 18.19; found: C, 68.20; H, 4.55; S, 18.26.
2H), 7.58–7.52 (m, 1H), 7.45–7.39 (m, 2H), 3.24 (t, J = 6.7 Hz,
2H), 2.86 (t, J = 6.7 Hz, 2H), 2.16 (s, 3H); 13C NMR (62.5 S-(2-Methylallyl) 4-methylbenzothioate (27)
MHz, CDCl3) δ 206.5, 191.9, 136.8, 133.5, 128.6, 127.2, 43.4, 1H NMR (250 MHz, CDCl3) δ 7.79 (d, J = 8.1 Hz, 2H), 7.14
29.9, 22.7; IR (neat) ν (cm−1): 1717 (C=O ketone), 1655 (C=O (d, J = 8.1 Hz, 2H), 4.95 (s, 1H), 4.80–4.79 (m, 1H), 3.62
thioester); anal. calcd for (C11H12O2S): C, 63.43; H, 5.81; S, (s, 2H), 2.31 (s, 3H), 1.73 (s, 3H); 13C NMR (62.5 MHz,
15.40; found: C, 63.51; H, 5.77; S, 15.42.
CDCl3) δ 191.0, 144.2, 140.9, 134.4, 129.3, 127.3, 114.2, 35.7,
21.7, 21.3; IR (neat) ν (cm−1): 1659; anal. calcd for
(C12H14OS): C, 69.86; H, 6.84; S, 15.54; found: C, 69.80; H,
Butyl 3-(benzoylthio)propanoate (4)
Colorless oil; 1H NMR (250 MHz, CDCl3) δ 7.95–7.91 (m, 6.88; S, 15.61.
2H), 7.54–7.51 (m, 1H), 7.44–7.38 (m, 2H), 4.09 (t, J = 6.6 Hz,
2H), 3.30 (t, J = 7.0 Hz, 2H), 2.71 (t, J = 7.0 Hz, 2H), 1.65–1.54
Supporting Information
(m, 2H), 1.43–1.31 (m, 2H), 0.90 (t, J = 7.3 Hz, 3H); 13C NMR
(62.5 MHz, CDCl3) δ 191.5, 171.8, 136.8, 133.5, 128.6, 127.2,
64.7, 34.5, 30.6, 24.1, 19.1, 13.7; IR (neat) ν (cm−1): 1732
(C=O ester), 1666 (C=O thioester); anal. calcd for
(C14H18O3S): C, 63.13; H, 6.81; S, 12.04; found: C, 63.19; H,
6.85; S, 11.98.
Supporting Information File 1
Analytical data.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-11-141-S1.pdf]
Supporting Information File 2
NMR spectra.
S-Decyl benzothioate (17)
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-11-141-S2.pdf]
1H NMR (250 MHz, CDCl3) δ 7.91–7.86 (m, 2H), 7.51–7.33
(m, 3H), 2.99 (t, J = 7.3 Hz, 2H), 1.65–1.54 (m, 2H), 1.38–1.19
(m, 14H), 0.83–0.78 (m, 3H); 13C NMR (62.5 MHz, CDCl3) δ
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Beilstein J. Org. Chem. 2015, 11, 1265–1273.
25.Yi, C.-L.; Huang, Y.-T.; Lee, C.-F. Green Chem. 2013, 15, 2476–2484.
doi:10.1039/c3gc40946e
Acknowledgements
We gratefully acknowledge the support of this study by the
26.Abbasi, M. Synth. Commun. 2013, 43, 1759–1765.
doi:10.1080/00397911.2012.667183
Persian Gulf University Research Council.
27.Basu, B.; Paul, S.; Nanda, A. K. Green Chem. 2010, 12, 767–771.
doi:10.1039/b925620b
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