Hydrotalcite clay-catalysed air oxidation of thiols

Article abstract of DOI:10.1039/a808922a

Hydrotalcite is an efficient catalyst for air oxidation of a variety of aromatic, aliphatic and alicyclic thiols in hexane, affording the corresponding disulfides in excellent to quantitative yields under mild and neutral conditions.

Full text of DOI:10.1039/a808922a

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374 J. CHEM. RESEARCH (S), 1999
J. Chem. Research (S),
1999, 374^375y
Hydrotalcite Clay-catalysed Air Oxidation of Thiolsy
Masao Hirano,* Hiroyuki Monobe, Sigetaka Yakabe and
Takashi Morimoto
Department of Applied Chemistry, Faculty of Technology, Tokyo University of Agriculture and
Technology (TUAT), Koganei, Tokyo 184-8588, Japan
Hydrotalcite is an efficient catalyst for air oxidation of a variety of aromatic, aliphatic and alicyclic thiols in hexane,
affording the corresponding disulfides in excellent to quantitative yields under mild and neutral conditions.
Air
Use of supported reagents and catalysts for organic syntheses
is current research interest, since reactions under
Hydrotalcite
Hexane
2 R–SH
R–S–S–R
solid^solution biphasic conditions have many practical
advantages unavailable by conventional solution-phase
counterparts.1 Of support materials frequently examined,
aluminosilicate clays,2 especially bentonites, have enjoyed
extensive use as solid acid catalysts. On the other hand, inves-
tigations into solid bases and their application to organic
reactions have apparently been limited. It has very recently
30 ˚C, 0.5–2.5 h
1
2
1, 2
R
1, 2
R
1, 2
R
a
b
c
Ph
g
h
i
2-ClC6H4
3-ClC6H4
4-ClC6H4
m
n
C8H17
4-MeOC6H4
2-MeC6H4
C10H21
C12H25
o
d
e
f
3-MeC6H4
4-MeC6H4
4-BrC6H4
j
k
l
4-O2NC6H4
PhCH2
p
q
r
C14H29
c-C5H9
c-C6H11
d
been reported, however, that the epoxidation of alkenes3a
and Baeyer^Villiger oxidation of ketones3e;f are favorably
e¡ected by synthetic basic clays, hydrotalcites, some of which
exert interesting shape-selective catalysis3d as observed in
well-known zeolite catalysed reactions.4
C6H13
Scheme 1 Oxidative coupling of thiols
Oxidative coupling of thiols 1 to disul¢des 2 has been a
subject of extensive research and many reagents and reagent
systems are currently available.5 It is well known that the ease
with which thiols are oxidised is dependent on their acidities
and therefore they undergo base catalysis.6 Thus basic
alumina7 and DMSO,8;9 for instance, can lead to e¤cient
oxidation of a number of thiols, owing to their nucleophilic
identical conditions achieved 100% conversion and, after
simple work-up and chromatographic isolation, gave 2a in
97% yield (Table 1, entry 1). The higher activity of dry
hydrotalcite than that of the untreated clay may be related
to its increased surface area and/or enhanced basicity.10
A
activity. Accordingly, it is reasonably expected that
a
control run under an inert atmosphere gave only 11% of
2a and 86% of 1a remained unreacted even in the presence
of dry hydrotalcite (Table 1, entry 2), indicating clearly that
the clay catalyses the reaction and air is essential for e¤cient
oxidation. These observations are markedly in contrast to
those from an earlier basic alumina-catalysed air oxidation7
claiming that the dehydration of commercial alumina does
not give any improvement, and also that small scale exper-
iments (< 5mmol of thiols) give the same results whether
or not air is involved in the reaction system. Treatment of
2a in place of 1a under the conditions of entry 1 led to com-
plete recovery of the disul¢de, the mp of which showed no
depression upon mixing with 2a, suggesting that the dry
hydrotalcite clay could be an e¤cient catalyst for the
oxidation of thiols.
Attempted reactions of benzenethiol 1a were carried out at
30 8C by simply stirring 1a in hexane under a gentle £ow of
air in the absence and presence of commercial (untreated)
hydrotalcite. However, reactions were sluggish and diphenyl
disul¢de 2a was not obtained in a synthetically acceptable
yield within a reasonable period of time; GLC analyses of
crude mixtures from 30 min reactions showed that conver-
sions of 1a were 5.3 and 12%, respectively, giving only 2.0
and 5.0% of 2a, respectively. On the other hand, oxidation
with the use of predried hydrotalcite under otherwise
Table 1 Oxidative coupling of thiols 1 to disulfides 2 with air and calcined hydrotalcitea
Entry
Clay/g
t/h
Yield of disulfide(%)b
Entry
Clay
t/h
Yield of disulfide(%)b
1
0.5
0.5
15
0.7
0.5
0.5
0.5
0.7
0.7
0.7
0.5
0.5
1.5
2
0.75
0.75
0.75
2
2a(97)
2a(11)
2a(94)
2b(96)
2c(96)
2d(95)
2e(97)
2f(92)
2g(92)
2h(90)
11
12
13
14
15
16
17
18
19
20
0.7
0.7
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
2
2
2
2.5
2.5
2.5
2
2
2.5
2.5
2i(96)
2j(90)
2c
3d
4
2k(98)
21(95)
2m(quant.)
2n(98)
2o(98)
2p(96)
2q(98)
2r(95)
5
6
7
8
9
10
2.5
2.5
a Under a gentle flow of air (ca. 25 ml min 1), at 308C; 1 mmol of thiol 1 and 5 ml of hexane were used in every run. b Isolated yield of
chromatographically purified disulfide 2 based on the starting 1. c Under a gentle flow of argon; 86% of 1a remained unreacted.
d 1a (30 mmol, 3.3 g), hexane (150 ml).
hydrotalcite/air system lacks the ability to convert the di-
* To receive any correspondence.
sul¢de to a higher oxidation product such as the sulfonic
acid. A test reaction of 1a at 20 8C led to decreased
conversion and product yield (92 and 87%, respectively).
y This is a Short Paper as de¢ned in the Instructions for Authors,
Section 5.0 [see J. Chem. Research (S), 1999, Issue 1]; there is
therefore no corresponding material in J. Chem. Research (M).

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J. CHEM. RESEARCH (S), 1999 375
ensure smooth reaction and to attain reproducible results. After
cooling to room temp., the reaction mixture was ¢ltered through
a sintered glass funnel and the ¢lter cake was washed thoroughly
with portions of dry diethyl ether (total 50 ml). Rotary evaporation
of the combined solvent left diphenyl disul¢de 2a, which was imme-
diately chromatographed on a silica gel column (Merck silica gel
60; hexane^AcOEt, 10:1) to a¡ord pure (1H NMR, GLC and TLC)
2a in 97% yield (0.106 g, mp 57.5^58 8C; lit.,13 58±60 8C).
Oxidations of the other thiols 1b ^r were carried out as above, the
conditions (quantities of dry hydrotalcite and reaction periods) of which
were determined on the basis of the reactivity of 1 and yield of disul¢des
2. Each reaction achieved 100% conversion and, after usual work-up
and a single chromatography, gave the disul¢de with satisfactory purity
ꢀ> 99%†. Disul¢des thus obtained were known compounds, physical
properties of which were consistent with literature data. A multigram
scale experiment was performed with 1a (3.3 g, 30 mmol), dry
hydrotalcite (15 g) and hexane (150 ml) at 30 8C for 1.5 h, giving 2a
in 94% isolated yield (Table 1, entry 3).
Another set of comparative experiments showed that
hexane is superior as solvent to ethanol, dichloromethane,
carbon or tetrachloride, acetonitrile or diethyl ether.
The dry hydrotalcite/air system successfully gave the
high-yielding oxidation of benzenethiols 1a ^j, regardless of
electronic properties of substituents (Table 1, entries 1, 4, 7,
8, 11 and 12) and their positions on the benzene ring (entries
5^7 and 9^11). Aralkyl 1k (entry 13), aliphatic 1l ^p (entries
14^18), and alicyclic thiols 1q,r (entries 19 and 20) also under-
went smooth oxidation. In addition, although a prolonged
period of time was required to e¡ect the multigram scale
oxidation of 1a as compared with the 1 mmol scale exper-
iment in entry 1, it can easily be achieved without appreciable
decrease in the yield of 2a (entry 3).
Commercial hydrotalcites are readily available and inex-
pensive and safe to handle, some of which, including the
one examined here, are used as pharmaceuticals, e.g. antiacid
agents,11 and thus non-toxic. The present hydrotalcite-based
procedure can favorably be compared to a similar basic
alumina/air oxidation system in benzene7 in terms of smaller
quantities of the solid catalyst, slightly superior yields of di-
sul¢des and especially shorter reaction times. Indeed,
oxidations of 1a, k, l and r (Table 1, entries 1, 13, 14 and
20, respectively) with 0.5 g of dry hydrotalcite per mmol
of thiol for 0.5^2.5 h gave 2a, k, l and r in 97, 98, 95
and 95% yields, respectively, whereas the latter system
required 1g of the alumina per mmol of thiol and gave
96, 91, 89 and 93% yields of 2a, k, l and r, respectively, from
4^6 h reactions. In addition, from economical and environ-
mental points of view, hexane is a more favorable solvent than
the aromatic solvent.
Received, 16th November 1998; Accepted, 24th February 1999
Paper E/8/08922A
References
1
ꢀa† J. H. Clark, A. P. Kybett and D. J. Macquarrie, Supported
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In conclusion, the dry hydrotalcite/air system in hexane
gives a mild, inexpensive and high-yielding oxidation of a
wide range of thiols. Interesting results demonstrated here
coupled with the ability of hydrotalcites to e¡ect various
oxidations3 and easy reaction performance might make
hydrotalcites practically attractive as basic solid catalysts
2
3
and/or supports, providing
synthesis.
a new strategy for organic
Experimental
1H NMR spectra were recorded with a JEOL PMX-60 (60 MHz)
spectrometer for solutions in CDCl3 using TMS as an internal
standard. Analytical GLC was performed on a Shimadzu GC-4CM
instrument, equipped with a FID via a 2 m  5 mm diameter glass
column packed with 3% Silicone OV-17 on Uniport HP and interfaced
with a Shimadzu Chromatopac C-R6A integrator, with temperature
programming. Melting points were determined on a Yanagimoto
MP-S3 melting point apparatus and are uncorrected. Mass spectra
were determined on a JEOL SX-102A mass spectrometer coupled
to a Hewlett Packard GC5890 Series II GC apparatus via a heated
capillary column. Thiols 1a ^r were used as received from commercial
sources. A free-£owing synthetic hydrotalcite powder, Kyowaad1
500SH [formulated as Mg6Al2ꢀOH†16CO3 Á4H2OŠ,12 was a gift from
Kyowa Chemical Industry, which was oven-dried (500 8C, 1 h; dry
hydrotalcite) and stored in a desiccator, the activity of which was
maintained at least for a month.
4
5
S. M. Csicsery and P. Laszlo, in ref. 1(d), part VII, ch. 22.
X. Wu, R. D. Rieke and L. Zhu, Synth. Commun., 1996, 26,
191; N. Iranpoor, P. Salehi and F. Shiriny, Org. Prep. Proced.
Int., 1995, 27, 216; G. Capozzi and G. Modena, in The Chem-
istry of the Thiol Group, ed. S. Patai, Wiley, London, 1974,
ch. 17.
6
C. F. Cullis, J. D. Hopton and D. L. Trimm, J. Appl. Chem.,
1968, 18, 330; C. F. Cullis, J. D. Hopton, C. J. Swan and
D. L. Trimm, J. Appl. Chem., 1968, 18, 335; C. J. Swan
and D. L. Trimm, J. Appl. Chem., 1968, 18, 340.
Oxidation Procedure.öA general procedure is described for the
oxidation of benzenethiol 1a (Table 1, entry 1). To a mixture of 1a
7
8
K.-T. Liu and Y.-C. Ton, Synthesis, 1978, 669.
C. N. Yiannios and J. V. Karabinos, J. Org. Chem., 1963, 28,
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Am. Chem. Soc., 1964, 86, 2018; T. J. Wallace and J. J.
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1965, 30, 1502; T. J. Wallace and H. A. Weiss, Chem. Ind.
(London), 1966, 1558.
(0.110 g, 1mmol) and hexane (5 ml) in
a 30 ml two-necked
roundbottom £ask was added dry hydrotalcite (0.5 g) in a dry box.
The £ask was quickly equipped with a Te£on-coated stirrer bar, a
gas-inlet tubing connected to a dry air supplierz and a re£ux con-
denser, the top of the latter was linked to a liquid para¤n trap
via
a £exible silicone-rubber tubing. The cloudy heterogeneous
9
W. W. Epstein and F. W. Sweat, Chem. Rev., 19677, 247.
mixture was kept at 30 8C for 30 min under
a gentle ¯ow of
air (ca. 25 ml min 1) while e¤cient stirring was continued in order to
10 S. Miyata, T. Kumura, H. Hattori and K. Tanabe, Nippon
Kagaku Zasshi, 1971, 92, 514 (Chem. Abstr., 1971, 75, 70781.
11 Technical report from Kyowa Chemical Industry Co., Ltd (in
Japanese).
12 Aldrich Catalog Handbook of Fine Chemicals, 1996^1997, pp.
1159.
z A simple, small electric air pump (100 V, 4.5 W) for tropical ¢sh
breeding coupled with drying tubes (H2SO4 and NaOH tablets)
can conveniently be used for this purpose.