S-Acylation of aliphatic and aromatic thiols with carboxylic acids and their esters over solid acid catalysts in the gas phase at temperatures above 200 °c

Article abstract of DOI:10.1016/j.apcata.2013.06.011

Benzenethiol is reacted with acetic acid in a hydrogen stream over [(Mo₆Cl₈)Cl₄(H₂O) ₂]·6H₂O. Catalytic activity of the clusters appears above 200 °C, yielding S-phenyl thioacetate. The selectivity is 98% at 300 °C. Niobium, tantalum, and tungsten halide clusters with the same octahedral metal framework also catalyze the reaction. Benzoic acid and aliphatic carboxylic acids afford the corresponding S-phenyl carbothioates by reaction with benzenethiol. Aliphatic thiols are also S-acylated to yield the corresponding S-alkyl carbothioates. When carboxylic esters are applied to the reaction with benzenethiol over [(Nb₆Cl₁₂)Cl ₂(H₂O)₄]·4H₂O at 450 °C, the sterically unhindered moiety of the ester is incorporated into the products: S-phenyl thioacetate or methyl phenyl sulfide is obtained selectively. A Br?nsted acid site developed on the cluster complex by thermal activation is the active site of the catalyst. Hence, solid acids such as silica-alumina, zeolites, and heteropoly acids that are stable above 200 °C also catalyze these reactions.

Full text of DOI:10.1016/j.apcata.2013.06.011

Applied Catalysis A: General 464–465 (2013) 332–338
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
S-Acylation of aliphatic and aromatic thiols with carboxylic acids and
their esters over solid acid catalysts in the gas phase at temperatures
above 200 ◦C
Sayoko Nagashimaa, Hitomi Yamazakia, Kentaro Kudoa,
Satoshi Kamiguchib, Teiji Chiharaa,∗
a Graduate School of Science and Engineering, Saitama University, Shimo-Okubo, Sakura-Ku, Saitama City, Saitama 338-8570, Japan
b The Institute of Physical and Chemical Research (RIKEN), Wako, Saitama 351-0198, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Benzenethiol is reacted with acetic acid in a hydrogen stream over [(Mo6Cl8)Cl4(H2O)2]·6H2O. Catalytic
activity of the clusters appears above 200 ◦C, yielding S-phenyl thioacetate. The selectivity is 98% at
300 ◦C. Niobium, tantalum, and tungsten halide clusters with the same octahedral metal framework
also catalyze the reaction. Benzoic acid and aliphatic carboxylic acids afford the corresponding S-phenyl
carbothioates by reaction with benzenethiol. Aliphatic thiols are also S-acylated to yield the correspond-
ing S-alkyl carbothioates. When carboxylic esters are applied to the reaction with benzenethiol over
[(Nb6Cl12)Cl2(H2O)4]·4H2O at 450 ◦C, the sterically unhindered moiety of the ester is incorporated into
the products: S-phenyl thioacetate or methyl phenyl sulfide is obtained selectively. A Brønsted acid site
developed on the cluster complex by thermal activation is the active site of the catalyst. Hence, solid
acids such as silica–alumina, zeolites, and heteropoly acids that are stable above 200 ◦C also catalyze
these reactions.
Received 24 February 2013
Received in revised form 6 May 2013
Accepted 12 June 2013
Available online 21 June 2013
Keywords:
S-Acylation
Thiol
Carboxylic acid
Carboxylic ester
Brønsted acid
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
catalytic acylations of alcohols, phenols, and amines with car-
boxylic acids have been reported, acylation of thiols with carboxylic
Acylation of alcohols, phenols, amines, and thiols is a fre-
quently used transformation in organic synthesis, and it provides
an efficient and inexpensive means of protecting the groups in
a multistep synthetic process. Acylation is mostly carried out by
use of acid anhydrides or acyl chlorides in the presence of acid
catalysts or basic reagents such as 4-(dimethylamino)pyridine, 4-
pyrrolidinopyridine, pyridine, or tertiary amines [1]. Esters are
often used as alkylating agents, as they have higher solubilities and
lower melting points than the corresponding acids. Direct acyla-
tion with carboxylic acids in the absence of any reagents is the best
method for performing the acylation reaction, as the atomic econ-
omy is high and only water is produced as a by-product. Another
advantage is the relatively cheaper route for industrial purposes
when compared with the use of acid anhydrides or acid chlorides.
When a catalyst is employed, a solid catalyst is preferable because it
allows easy separation from the reaction mixture. Although many
acids is quite rare. In acylation, the reactivity of the thiol is very
much lower than that of an alcohol or amine [2]. There is only
one report of this reaction: benzenethiol, as well as alcohols and
amines, was S-acetylated with acetic acid at 110 ◦C in the presence
of yttria–zirconia-based Lewis acid [3]. S-Acylation with carboxylic
ester has not been reported.
We have been studying catalysis by halide clusters of Group
5–7 transition metals such as [(Nb6Cl12)Cl2(H2O)4]·4H2O (1) and
[(Mo6Cl8)Cl4(H2O)2]·6H2O (2) with an octahedral metal framework
[4,5]. These halide clusters have several characteristic features:
metal–metal bonds that are similar to those in bulk metals, multi-
center and multielectron systems, unique intermediate oxidation
states of the metal atoms between 1+ and 3+, a diversity of metal
atoms and halogen ligands, high thermal stability, and low vapor
pressures and high melting points as halide complexes. Halide
clusters have solid and molecular states, which can be used in het-
erogeneous and homogeneous systems. We recently reported the
alkylation of benzenethiol with various alkylating reagents such as
alcohols, ethers, esters, alkyl halides, and olefins over halide clus-
ters [6]. When alkyl acetates were used for the reaction, selective
S-alkylation or S-acetylation of benzenethiol proceeded depend-
ing on the catalyst (Eq. (1)). This prompted us to study S-acylation
of thiols. Herein, we report a universally applicable simple and
∗
Corresponding author at: Saitama University, Graduate School of Science and
Engineering, Shimo-Okubo 255, Sakura-ku, Saitama-city, Saitama, 338-8570, Japan.
Tel.: +81 48 858 3521; fax: +81 48 858 3521.
E-mail addresses: chihara@apc.saitama-u.ac.jp, chiyara@cj9.so-net.ne.jp
(T. Chihara).
0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2013.06.011

S. Nagashima et al. / Applied Catalysis A: General 464–465 (2013) 332–338
333
practical method for the acylation of both aliphatic and aromatic
100
80
60
40
20
0
thiols with carboxylic acids or their esters over various solid acid
catalysts that are stable above 200 ◦C (Eq. (2)).
C6H5SH + CH3COOR
→ C6H5SR + C6H5SCOCH3 (R = CH3, C2H5, C3H7)
(1)
R1SH + R2COOR3 → R1SCOR2 + HOR3
(R1 = C5–C8alkyl, C6H5;R2 = C1–C4alkyl, C6H5;
R3 = H, C1–C3alkyl)
(2)
0
2
4
6
8
2. Experimental
t / h
2.1. Materials
Fig. 1. A typical reaction profile for the S-acetylation of benzenethiol with acetic
acid over (H3O)2[(Mo6Cl8)Cl6]·6H2O (2)/SiO2. Following the pretreatment of 2/SiO2
(30 mg) in a hydrogen stream (300 mL/h) at 300 ◦C for 1 h, reaction was initiated
by the introduction of a mixture of benzenethiol (102 ␮L/h, 1.0 mmol/h) and acetic
acid (114 ␮L/h, 2.0 mmol/h) to the hydrogen stream without changing the temper-
ature. Conversion (᭹), S-phenyl thioacetate (ꢀ), diphenyl sulfide (ꢁ), and diphenyl
disulfide (ꢀ).
Crystals
of
the
molecular
cluster
complexes
of
[(Nb6Cl12)Cl2(H2O)4]·4H2O (1) [7], (H3O)2[(Mo6Cl8)Cl6]·6H2O (2)
[8], [(Ta6Cl12)Cl2(H2O)4]·4H2O [7], and (H3O)2[(W6Cl8)Cl6]·6H2O
[9] were synthesized using the published procedures. The general
procedure for the preparation of a silica-supported catalyst is
described for 1. Methanol (330 mL) was added to a 1 L flask
containing 1 (1.0 g). After dissolution of the clusters, silica gel
(Nippon Aerosil, Tokyo; Aerosil 380 m2/g, 19.0 g) was added and
allowed to stand for 1 h with occasional shaking. Then, the solvent
was evaporated to dryness under reduced pressure at ambient
temperature. Samples of the dried silica gel were crushed and
screened to 166−200 mesh. All of the clusters were supported
on the silica gel in the same way at 5.0% by weight. No peaks
attributable to the cluster complexes were observed in XRD
pattern of the silica gel supported catalysts. The clusters were sup-
ported in well dispersed form. Crystalline heteropoly acids were
gifted from Nippon Inorganic Colour & Chemical Co., Ltd. (Tokyo):
phosphomolybdic acid H3[PMo12O40]·nH2O (n ≈ 30), phospho-
tungstic acid H3[PW12O40]·nH2O (n ≈ 30), and silicotungstic acid
H4[SiW12O40]·nH2O (n ≈ 24). They were crushed and screened to
166−200 mesh. The other chemicals were commercial products
and used as received: silica–alumina (high alumina), Catalysts &
Chemicals Ind. Co., Ltd. (JRC-SHA-1, Catalysis Society of Japan);
H-Y zeolite, Tosoh Corporation (JRC-Z-HY5.5, Catalysis Society of
Japan), H-beta, Sud-Chemie Catalysts Japan, Inc. (JRC-Z-HB25(1),
Catalysis Society of Japan); H-mordenite, Tosoh Corporation
(JRC-Z-HM20(4), Catalysis Society of Japan); active alumina, GL
Sciences Inc. (Tokyo).
six-way valve kept at 150 ◦C followed by analysis using an online
gas–liquid chromatograph (GLC) with a methyl silicone column.
The reactor effluent was collected in an ice trap containing tetrahy-
drofuran for subsequent analyses by GLC with a poly(ethylene
glycol) capillary column or a dimethylpolysiloxane capillary col-
umn and gas chromatography–mass spectrometry (GC–MS) with
a dimethylpolysiloxane capillary column. Catalytic reactions using
the other thiols, alkylating reagents, and catalysts were performed
in the same way. In this paper, conversion and selectivity are
defined as follows: conversion = products/(products + recovered
thiol) × 100 (%), and selectivity = product/(total amount of prod-
ucts) × 100 (%) based on thiol.
3. Results and discussion
3.1. S-Acetylation of benzenethiol with acetic acid
Some acetylating reagents were reacted with benzenethiol.
When a mixture of two equivalents of acetyl chloride with ben-
zenethiol was allowed to stand for 3 h under neat conditions at
ambient temperature in the absence of any catalyst, S-acetylation
proceeded almost quantitatively to yield S-phenyl thioacetate.
Under the same reaction conditions, acetic anhydride yielded S-
phenyl thioacetate exclusively with about 50% conversion. Similar
results have been reported [11]. However, acetic acid and methyl
acetate did not afford even a trace amount of the thioacetate
under the same reaction conditions. Harsh reaction conditions are
required for the S-acetylation with carboxylic acids and their esters.
We have reported that S-acetylation of benzenethiol with acetic
esters proceeded at 400 ◦C over halide cluster catalysts [6]. Then,
S-acetylation of benzenethiol with acetic acid was attempted at
elevated temperatures in the gas phase in the presence of halide
cluster catalysts.
2.2. Catalytic measurements
The general procedure for the catalytic reaction is described
for the benzenethiol/acetic acid/1 system. A conventional ver-
tical glass fixed-bed microreactor with a continuous gas-flow
system was operated at atmospheric pressure [10]. In each exper-
iment, a weighed supported sample of 1/SiO2 (30.0 mg) was
packed in a borosilicate glass tube (3 mm i.d.) with the aid of
quartz glass and placed in the center of an electric furnace.
The supported cluster sample was initially heated from room
temperature to a fixed temperature between 50 and 500 ◦C in
15 min in a hydrogen stream (300 mL/h), and then it was held
at that temperature for 45 min for activation. Then, the reaction
was initiated by feeding a mixture of benzenethiol (102 ␮L/h,
1.0 mmol/h) and acetic acid (114 ␮L/h, 2.0 mmol/h) into the hydro-
gen stream without changing the temperature. The reaction was
monitored every 30 min by sampling the reaction gas (1 mL) with a
A
typical reaction profile of benzenethiol with acetic
acid in
a
hydrogen stream at 300 ◦C in the presence of
(H3O)2[(Mo6Cl8)Cl6]·6H2O (2)/SiO2 as catalyst is shown in
Fig. 1. The activity decreased with time, probably because of
fouling by carbonaceous compounds or coke. When 1/SiO2 was
reduced in the hydrogen stream for 1 h by stopping the sup-
ply of the reactants at 3 h after the start of the reaction, the

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S. Nagashima et al. / Applied Catalysis A: General 464–465 (2013) 332–338
+
100
80
60
40
20
0
O
O
H
O
H
S
C6H5
H+
• •
CH3
C
O
H
CH3
C
H
– H+
O
C
S
H
O
O
C
S
H
O
+
C6H5
CH3
H
CH3
H
C6H5
Scheme 1. S-Acetylation mechanism with acetic acid over a simple Brønsted acid
site.
3.2. S-Acetylation of benzenethiol with acetic acid over various
acid catalysts
0
100
200
300
400
Various halide clusters and solid acids were applied to S-
acetylation of benzenethiol with acetic acid at 300 ◦C; the data
are listed in Table 1. The clusters of niobium (1), tantalum, and
tungsten with an octahedrally arranged M6 metal core also selec-
tively catalyzed the S-acetylation. Although the S-acetylation over
2/SiO2 proceeded faster in a helium stream, selectivity for the
S-acetylation was a little lower than in a hydrogen stream. Dehy-
drogenative coupling of benzenethiol to yield diphenyl disulfide,
which is a spontaneous reaction, was suppressed under hydro-
gen atmosphere. As Table 1 shows, acetic acid did not afford
acetic anhydride under the reaction conditions, which preludes
acid anhydride intermediates.
We have reported the thermal activation of the halide clusters,
which proved to be a unique inorganic oxo acid stable above 200 ◦C.
When the halide clusters were treated above 150 ◦C in a helium
or hydrogen stream, Brønsted acid sites developed [13,14]. Titra-
tion of the clusters activated at 300 ◦C revealed the formation of an
equimolar number of weak Brønsted acid sites (H0 ≈ +1.3) derived
from the developed hydroxo ligand [15,16], as exemplified by Eqs.
(3)–(5) for niobium cluster 1. The progress of S-acetylation can be
explained on the basis of Brønsted acid catalysis: protonation of
the carbonyl group of the acetic acid would facilitate nucleophilic
attack of benzenethiol to yield S-phenyl thioacetate (Scheme 1).
T / °C
Fig. 2. Temperature effect on the S-acetylation of benzenethiol with acetic acid over
(H3O)2[(Mo6Cl8)Cl6]·6H2O (2)/SiO2. Pretreatment and following reaction temper-
atures were altered concomitantly. Other reaction conditions are the same as in
Fig. 1. Conversion (᭹), S-phenyl thioacetate (ꢀ), diphenyl sulfide (ꢁ), and diphenyl
disulfide (ꢀ) at 3 h after the start of the reaction.
catalytic activity was restored to 126% of the original activity
at 3 h. Catalyst deactivation by coke formation is inevitable in
gas–solid-phase reactions at higher temperatures [12]. In contrast,
the selectivity was practically unchanged from the beginning of
the reaction. Clusters of 2 catalyzed S-acetylation of benzenethiol
to yield S-phenyl thioacetate almost exclusively. Side reactions
were the coupling of benzenethiol yielding diphenyl sulfide and
diphenyl disulfide with 1.2% and 0.6% selectivity, respectively.
The turnover frequency per Mo6 cluster unit during a period of
2–4 h was 150 h−1, assuming that all of the cluster molecules were
active.
The effect of the amount of acetic acid to benzenethiol was
examined. When equimolar amounts of acetic acid and benzene
thiol were used under the same reaction conditions as in Fig. 1, the
conversion was 6.9% and the selectivity for S-phenyl thioacetate
was 89.1% at 3 h after the start of the reaction. When the amount
of acetic acid was increased from two to 10 molar equivalents, the
conversion increased to 20–30% but the selectivity leveled off to
96.1–98.2%. Two equivalents of acylating reagents were employed
thereafter.
The effect of the reaction temperature is presented in Fig. 2,
in which the activation temperature of the clusters and the
successive reaction temperature were changed concomitantly.
Practically no catalytic activity for the S-acetylation was observed
below 150 ◦C, although there was considerable spontaneous
oxidative coupling of benzenethiol to yield diphenyl disulfide.
Catalytic activity for the S-acetylation developed above 200 ◦C
and increased with increasing temperature. Over the temperature
range 200–450 ◦C, selectivity for S-phenyl thioacetate was more
than 94%.
Many examples of S-acylation of thiols with acyl halides or car-
boxylic anhydrides have been reported. However, examples with
carboxylic acids are quite rare, presumably because few exper-
iments have been attempted at higher temperatures. The single
example reported is S-acetylation of benzenethiol with acetic acid:
in a batch reaction system a mixture of benzenethiol and acetic
acid in a molar ratio of 1:5 was heated at 110 ◦C for 8 h under
neat conditions in the presence of 20 wt% of yttria–zirconia-based
Lewis acid catalyst to yield S-phenyl thioacetate in 94% yield
[3].
[(Nb6Cl12)Cl2(H2O)4]·4H2O(1) → [(Nb6Cl12)Cl2(H2O)4] + 4H2O
(3)
[(Nb6Cl12)Cl2(H2O)4] → [(Nb6Cl12)Cl(OH)(H2O)3] + HCl
[(Nb6Cl12)Cl(OH)(H2O)3] → H+ + [(Nb6Cl12)Cl(O)(H2O)3]−
(4)
(5)
Table 1 also lists the catalytic activities of silica–alumina, zeo-
lites, and heteropoly acids; their acidity is stable above 200 ◦C.
All these acids selectively catalyzed S-acetylation of benzenethiol,
as expected. On the other hand, alumina (a Lewis acid) exhibited
essentially no catalytic activity for the acetylation. Thus, Brønsted
acids catalyzed the S-acetylation at temperatures above 200 ◦C.
Stability above 200 ◦C is a requisite of the catalyst for the reac-
tion. However, phosphoric acid polymerizes to diphosphoric acid or
polyphosphoric acid above 200 ◦C, and Nafion-H (a perfluorinated
resin-sulfonic acid) is not stable above 200 ◦C.
3.3. S-Acylation of benzenethiol with various carboxylic acids
Several aliphatic and aromatic carboxylic acids were applied to
the S-acylation of benzenethiol over 2/SiO2 at 300 ◦C. As Table 2
shows, S-acylation with C2–C5 carboxylic acids proceeded; the
longer and bulkier the alkyl chain, the lower the reactivity and
selectivity. The selectivities for S-acylation were more than 91%

S. Nagashima et al. / Applied Catalysis A: General 464–465 (2013) 332–338
335
Table 1
S-Acetylation of benzenethiol with acetic acid over various catalysts.a
Catalyst
Conversion (%)
Selectivity (%)
S-Phenyl thioacetate
Diphenyl sulfide
Diphenyldisulfide
[(Nb6Cl12)Cl2(H2O)4]·4H2O (1)/SiO2
(H3O)2[(Mo6Cl8)Cl6]·6H2O (2)/SiO2
(H3O)2[(Mo6Cl8)Cl6]·6H2O (2)/SiO2
(H3O)2[(Mo6Cl8)Cl6]·6H2O (2)/SiO2
[(Ta6Cl12)Cl2(H2O)4]·4H2O/SiO2
(H3O)2[(W6Cl8)Cl6]·6H2O/SiO2
SiO2–Al2O3 (high alumina)
H-Y zeolite
18.5
18.7
27.4
0.0c
11.9
17.6
15.7
14.5
14.2
6.4
96.9
98.2
88.2
–
0.4
1.2
0.6
–
2.7
0.6
11.1
–
2.3
2.0
1.1
1.3
2.3
5.8
b
96.1
97.8
93.6
96.4
94.3
92.5
88.5
92.0
88.1
41.6
76.8
86.9
56.8
1.7
0.2
5.3
2.3
3.4
1.7
3.7
5.8
1.8
0.0
1.6
0.4
30.2
H-beta
H-mordenite
H3[PMo12O40]·30H2O
2.5
7.8
2.2
H3[PW12O40]·30H2O
23.3
11.4
0.9
1.1
1.8
H3[SiW12O40]·24H2O
Al2O3
10.1
58.4
21.6
12.7
13.0
Mo metald
d,e
SiO2
No catalyst
1.9
a
After treatment of catalyst (30.0 mg) in a hydrogen stream (300 mL/h) at 300 ◦C for 1 h, reaction was initiated by introduction of a mixture of benzenethiol (102 ␮L/h,
1.0 mmol/h) and acetic acid (114 ␮L/h, 2.0 mmol/h) to the hydrogen stream without changing the temperature. At 3 h after the start of the reaction.
b
In a helium stream (300 mL/h).
c
The conversion of acetic acid when acetic acid was allowed to react singly. The formation of acetic anhydride was not detected.
166–200 mesh.
Silica gel used for the support.
d
e
Table 2
S-Acetylation of benzenethiol with various carboxylic acids over (H3O)2[(Mo6Cl8)Cl6]·6H2O (2)/SiO2.a
Reagent
Conversion (%)
Selectivity (%)
S-Phenyl carbothioate
Diphenyl sulfide
Diphenyl disulfide
Acetic acid
Propionic acid
Butyric acid
Isobutyric acid
Valeric acid
Isovaleric acid
2-Methylbutanoic acid
Pivalic acid
18.7
13.7
10.9
8.4
8.8
6.8
9.6
4.3
3.8
98.2
96.3
94.9
93.8
97.6
98.4
91.3
77.0
79.8
1.2
2.7
3.3
1.5
0.0
0.0
0.0
5.5
13.7
0.6
1.0
1.8
4.7
2.4
1.6
8.7
17.5
6.5
Benzoic acid
a
Reaction conditions are the same as in Table 1.
when carboxylic acids bearing ␣-hydrogen were used. Even car-
boxylic acids having no ␣-hydrogen, pivalic acid and benzoic acid,
afforded the S-acylation products with selectivities of more than
77%. S-Formylation of benzenethiol was unsuccessful under these
reaction conditions, probably because of the thermolability of the
product. Thus, halide clusters were proved to be efficient catalysts
for S-acylation of benzenethiol with both aliphatic and aromatic
carboxylic acids, except for formic acid.
100
80
60
40
20
0
3.4. S-Acetylation and S-methylation with methyl acetate
Esters are occasionally used for acylation of alcohols and amines.
Methyl acetate was applied to S-acetylation of benzenethiol in
the presence of niobium cluster 1 at various temperatures, and
the results are shown in Fig. 3. Catalytic activity developed above
200 ◦C and increased with increasing temperature. Over the tem-
perature range 300–400 ◦C, selective S-methylation proceeded, and
at around 450 ◦C, S-acetylation was replaced to yield S-phenyl
thioacetate preferentially. Then, methyl acetate was applied to the
reaction at 450 ◦C over various catalysts, and the data are listed
in Table 3. Clusters of niobium (1) and tantalum, Group 5 transi-
tion metals, catalyzed S-acetylation selectively to provide S-phenyl
thioacetate. Silica–alumina, H-Y zeolite, H-beta, and H-mordenite
were also effective catalysts for the reaction. In the cases of het-
eropoly acids, phosphotungstic acid catalyzed S-acetylation. The
0
100
200
300
400
500
T / °C
Fig. 3. Temperature effect on the reaction of benzenethiol with methyl acetate in the
presence of (H3O)2[(Nb6Cl12)Cl2(H2O)4]·4H2O (1)/SiO2. Pretreatment and follow-
ing reaction temperatures were altered concomitantly. A mixture of benzenethiol
(102 ␮L/h, 1.0 mmol/h) and methyl acetate (795 ␮L/h, 10.0 mmol/h) was reacted.
Other reaction conditions are the same as in Fig. 1. Conversion (᭹), S-phenyl thioac-
etate (ꢀ), methyl phenyl sulfide (♦), diphenyl sulfide (ꢁ), and diphenyl disulfide (ꢀ)
at 3 h after the start of the reaction.

336
S. Nagashima et al. / Applied Catalysis A: General 464–465 (2013) 332–338
Table 3
S-Acetylation and S-methylation of benzenethiol with methyl acetate over various catalysts.a
Catalyst
Conversion (%)
Selectivity (%)
S-Phenyl thioacetate
Methyl phenyl sulfide
Diphenyl sulfide
Diphenyl disulfide
[(Nb6Cl12)Cl2(H2O)4]·4H2O (1)/SiO2
[(Nb6Cl12)Cl2(H2O)4]·4H2O (1)b
(H3O)2[(Mo6Cl8)Cl6]·6H2O (2)/SiO2
[(Ta6Cl12)Cl2(H2O)4]·4H2O/SiO2
(H3O)2[(W6Cl8)Cl6]·6H2O/SiO2
SiO2–Al2O3
H-Y zeolite
H-beta
H-mordenite
46.8
29.1
84.7
35.8
44.1
44.5
44.6
37.7
31.1
28.6
5.8
13.6
4.5
2.5
2.3
11.4
2.4
86.9
72.8
4.1
12.0
7.5
0.6
3.2
1.0
4.1
1.1
0.7
1.6
0.7
4.0
0.5
16.5
0.2
0.6
0.9
0.8
0.9
1.6
1.1
0.9
63.3
9.1
23.5
27.9
43.2
9.4
94.7
14.8
39.9
30.2
9.3
26.9
12.9
74.4
9.0
14.7
17.1
23.6
21.3
30.7
26.3
80.6
58.1
68.3
88.3
70.8
82.0
19.9
14.7
57.4
28.0
8.4
H3[PMo12O40]·30H2O
H3[PMo12O40]·30H2Oc
H3[PW12O40]·30H2O
H3[SiW12O40]·24H2O
Mo metald
4.8
13.0
18.8
31.4
40.1
18.2
5.8
Nb metald
17.3
54.1
9.0
d,e
SiO2
No catalyst
14.7
50.0
a
After treatment of catalyst at 450 ◦C for 1 h, reaction was initiated by introduction of a mixture of benzenethiol (102 ␮L/h, 1.0 mmol/h) and methyl acetate (795 ␮L/h,
10.0 mmol/h) at 450 ◦C. Other reaction conditions are the same as in Table 1.
b
Unsupported crushed crystalline cluster (30.0 mg, 166–200 mesh).
In a helium stream (300 mL/h).
166–200 mesh.
Silica gel used for the support.
c
d
e
(771.5 kJ mol−1) [18].
progress of S-acetylation with methyl acetate can be explained as
simple Brønsted acid catalysis: the carbonyl group of the ester acti-
vated by a proton is attacked by benzenethiol to afford S-phenyl
thioacetate (Scheme 2).
Clusters of molybdenum (2), a Group 6 transition metal, cat-
alyzed S-methylation to afford methyl phenyl sulfide almost
exclusively. Tungsten halide clusters also catalyzed S-methylation,
though less selectively. With heteropoly acids, phosphomolybdic
acid catalyzed S-methylation preferentially.
(H3O)2[(Mo6Cl8)Cl6]·6H2O(2) → [(Mo6Cl8)Cl4(H2O)2]·6H2
O + 2HCl
(6)
[(Mo6Cl8)Cl4(H2O)2]·6H2O → [(Mo6Cl8)Cl4(H2O)2] + 6H2O (7)
We have reported the thermal activation process for molybde-
num clusters 2 [13,17], which will be summarized as follows. When
2 was treated in a helium or hydrogen stream at ambient temper-
ature, hydrated countercations were incorporated into the cluster
anion with liberation of hydrogen chloride to yield neutral clus-
ters [(Mo6Cl8)Cl4(H2O)2]·6H2O (Eq. (6)). Treatment at 100–150 ◦C
removed the crystallization water to afford [(Mo6Cl8)Cl4(H2O)2]
(Eq. (7)). When 2 was heated at 250–400 ◦C, it lost aqua ligands
and changed to poorly crystallized solid-state clusters containing
[(Mo6Cl8)Cl4(H2O)2] → [Mo6Cli8]Cla2Cla−a4/2 + 2H2O
(8)
Under hydrogen, MoO3 is reported to be reduced to a suboxide
of chemical composition between Mo2O and MoO at 300 ◦C and
to molybdenum metal at 600 ◦C [19]. Thus, coordinatively unsatu-
rated sites should appear in part on phosphomolybdic acid under
the reaction conditions. Table 3 shows that S-methylation over
phosphomolybdic acid was obviously depressed under a helium
stream.
S-Methylation over 2 and phosphomolybdic acid under hydro-
gen can be explained on the basis of a Brønsted acid site in concert
with the coordinatively unsaturated site. The carbonyl group of
methyl acetate is activated by protonation, and the oxygen atom
of the methoxy group coordinates to the molybdenum atom. Then,
the electron-deficient methyl group is attacked by benzenethiol to
afford the S-methyl sulfide (Scheme 3).
defective [Mo6Cli8]Cla2Cla−a
moieties, in which molybdenum
4/2
atoms lacking bridging Cla−a ligands are formed (Eq. (8)). Thus,
aqua ligands of the molybdenum clusters were removed by rapid
heating above 250 ◦C, and partially coordinatively unsaturated
molybdenum atoms were formed. Similarly, tungsten halide clus-
ters (H3O)2[(W6Cl8)Cl6]·6H2O of the same molecular structure
provided coordinatively unsaturated sites by thermal activation
[10,13]. With niobium clusters 1, formation of coordinatively unsat-
urated sites is difficult because of the high bond energy of Nb–O
+
O
C
O
C
H
O
H
S
C6H5
H+
+
• •
• •
O
C
O
C
H
H+
Mo
CH3
O
CH3
CH3
CH3
H
S
C6H5
– H+
CH3
O
CH3
CH3
CH3
O
CH3
– H+
• •
Mo
S
O
C
S
H
O
O
C
S
H
O
O
H
O
+
C6H5
CH3
CH3
CH3
CH3
+
CH3
C6H5
C
C6H5
Scheme 3. S-Methylation mechanism with methyl acetate over
Brønsted acid site and coordinatively unsaturated site.
a combined
Scheme 2. S-Acetylation mechanism with methyl acetate over a simple Brønsted
acid site.

S. Nagashima et al. / Applied Catalysis A: General 464–465 (2013) 332–338
337
Table 4
S-Acylation and S-alkylation of benzenethiol with esters over [(Nb6Cl12)Cl2(H2O)4]·4H2O (1)/SiO2.a
Reagent
Conversion (%)
Selectivity (%)
S-Phenyl carbothioate
Alkyl phenyl sulfide
Diphenyl sulfide
Diphenyl disulfide
Methyl acetate
Ethyl acetate
46.8
45.0
48.0
36.9
40.4
44.8
14.7
86.9
88.7
98.3
69.0
29.5
27.9
33.2
12.0
9.7
0.8
30.6
62.2
62.1
58.8
0.6
0.4
0.7
0.0
8.0
8.5
2.2
0.5
1.2
0.3
0.4
0.3
1.5
5.8
n-Propyl acetate
Methyl propionate
Methyl butyrate
Methyl valerate
Methyl benzoate
a
Reaction conditions are the same as in Table 3.
Table 5
S-Acetylation of 1-hexanethiol with acetic acid over various catalysts.a
Catalyst
Conversion (%)
Selectivity (%)
S-Hexyl thioacetate
Hexene
Dihexyl sulfide
Dihexyl disulfide
[(Nb6Cl12)Cl2(H2O)4]·4H2O (1)/SiO2
[(Nb6Cl12)Cl2(H2O)4]·4H2O (1)/SiO2
(H3O)2[(Mo6Cl8)Cl6]·6H2O (2)/SiO2
[(Ta6Cl12)Cl2(H2O)4]·4H2O/SiO2
(H3O)2[(W6Cl8)Cl6]·6H2O/SiO2
SiO2–Al2O3
42.8
36.3
52.5
23.7
42.6
29.7
31.1
26.5
86.9
–
5.3
74.3
66.3
9.4
20.2
9.6
1.1
18.3
11.8
1.5
5.1
4.0
6.7
7.5
1.8
14.8
3.8
6.1
b
20.2
74.3
70.9
80.3
81.6
84.9
H-Y Zeolite
H3[PW12O40]·30H2O
10.0
10.0
0.7
0.4
7.7
4.7
a
A mixture of 1-hexanethiol (141 ␮L/h, 1.0 mmol/h) and acetic acid (114 ␮L/h, 2.0 mmol/h) was reacted. Other reaction conditions are the same as in Table 1.
1-Hexanethiol was singly reacted.
b
Table 6
S-Acetylation of aliphatic thiols with various carboxylic acids over [(Nb6Cl12)Cl2(H2O)4]·4H2O (1)/SiO2.a
Aliphatic thiol
Carboxylic acid
Conversion (%)
Selectivity (%)
S-Alkyl carbothioate
Olefin
Sulfide
Disulfide
1-Pentanethiol
1-Pentanethiol
1-Pentanethiol
1-Hexanethiol
1-Hexanethiol
1-Hexanethiol
1-Hexanethiol
1-Hexanethiol
1-Octanethiol
1-Octanethiol
1-Octanethiol
Acetic acid
28.9
26.3
36.1
42.8
49.3
70.6
63.6
37.0
35.1
36.2
38.8
91.2
91.4
88.6
86.9
90.3
92.7
95.5
79.6
96.3
97.9
97.1
5.5
3.3
2.3
5.3
1.0
1.4
0.6
2.9
0.7
0.3
0.8
0.2
0.5
0.5
1.1
1.0
0.8
0.4
1.4
0.4
0.3
0.5
3.1
4.8
8.6
6.7
7.6
5.1
3.4
16.1
2.6
1.5
1.7
Propionic acid
Butyric acid
Acetic acid
Propionic acid
Butyric acid
Valeric acid
Benzoic acid
Acetic acid
Propionic acid
Butyric acid
a
A mixture of aliphatic thiol (1.0 mmol/h) and carboxylic acid (2.0 mmol/h) was reacted. Other reaction conditions are the same as in Table 1.
3.5. S-Acylation and S-alkylation with various esters over
[(Nb6Cl12)Cl2(H2O)4]·4H2O (1)/SiO2
smaller acyl or alkyl moiety of the ester was incorporated into the
thiol.
3.6. S-Acylation of aliphatic thiols with carboxylic acids
Several carboxylic esters were reacted with benzenethiol over
1/SiO2, and the data are listed in Table 4. Acetyl esters provided
S-acetate almost exclusively. On the other hand, methyl esters of
butyric, valeric, and benzoic acids afforded S-methyl sulfide pref-
erentially. Methyl propionate showed an intermediate selectivity
to afford both S-acyl and S-alkyl products. Accordingly, a relatively
It is known that aliphatic thiols bearing a ␤-hydrogen atom
are susceptible to decomposing to olefins by eliminating hydro-
gen sulfide at elevated temperatures, particularly in the presence
of acid catalysts [20,21], as in the case of alcohols decomposing
to olefins. Attempts were made to S-acetylate 1-hexanethiol with
acetic acid over acid catalysts under identical reaction conditions
for acetylation of benzenethiol (Table 1), and the data are listed in
Table 5. Over 1/SiO2, the S-acetylation proceeded to yield S-hexyl
thioacetate selectively. Selectivity for 1-hexene was as low as 5%.
In contrast, when 1-hexanethiol was reacted alone over 1/SiO2,
decomposition to 1-hexene proceeded with 74% selectivity.
H
H+
Mo
C4H9 CH CH2
H
S
H
C4H9 CH CH2 S+
H
• •
H
Mo
H
Table
5 also shows that all the acid catalysts tested,
except molybdenum cluster 2, were effective catalysts for the
S-acetylation. Cluster 2 catalyzed decomposition to hexene pref-
erentially. Coordinatively unsaturated sites developed on 2 could
cooperate with the Brønsted acid sites to catalyze the elimination
of hydrogen sulfide (Scheme 4).
C4H9 CH CH2
+
S
H
– H+
Scheme 4. The mechanism of elimination of hydrogen sulfide over a combined
Brønsted acid site and coordinatively unsaturated site.

338
S. Nagashima et al. / Applied Catalysis A: General 464–465 (2013) 332–338
Several aliphatic thiols were reacted with aliphatic and aro-
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Acknowledgement
The authors are grateful to Nippon Inorganic Colour & Chemi-
cal Co., Ltd. (Tokyo Japan) for supplying the crystalline heteropoly
acids.