Tetrahedron
Letters
Tetrahedron Letters 45 (2004)25–27
Cationic rhodium(I)/PPh complex-catalyzed dehydrogenation of
3
alkanethiols to disulfides under inert atmosphere
*
Ken Tanaka and Kaori Ajiki
Department of Applied Chemistry, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan
Received 1 October 2003; revised 20 October 2003; accepted 23 October 2003
Abstract—A cationic rhodium(I)/PPh
symmetrical disulfides under inert atmosphere. The dependence of reactivity on the structure of thiols is examined in detail.
2003 Elsevier Ltd. All rights reserved.
3
complex is an effective catalyst for dehydrogenation of primary or secondary alkanethiols to
ꢀ
Selective oxidative coupling of thiols to disulfides is of
interest from the viewpoint of both biological and
We first examined various rhodium catalysts to facilitate
this transformation and determined that a cationic
1
chemical processes. Thiols are among functional
groups, which can be easily over-oxidized and, there-
fore, extensive methods have been reported for their
3
rhodium(I)/PPh complex showed the highest catalytic
7
activity. The rhodium-catalyzed dehydrogenation of
various thiols was investigated in detail as shown in
2
–6
8
controlled oxidation.
involve the use of a stoichiometric amount of oxidants
(
Most of the existing methods
Table 1. The reaction of a primary alkanethiol is highly
dependent on reaction temperature and reaction time.
The highest yield of disulfides was obtained at 4 ꢁC for
1 h (entry 1). Interestingly, longer reaction time (16 h) at
4 ꢁC decreased the yield of disulfides (entry 2). Fur-
thermore, when the reactions were conducted at 4 ꢁC for
1 h followed by 25 ꢁC for 15 h or at 4 ꢁC for 16 h fol-
lowed by 25 ꢁC for 24 h, the yield of disulfides decreased
2
3
4
5
e.g. metal oxidants, peroxides, halogens, and air )
and suffer from effluence, flammability, explosion, or
undesirable side reactions due to their reagents.
Selective metal-catalyzed dehydrogenation of thiols to
disulfides under inert atmosphere is a possible alterna-
tive to avoid the use of oxidants. Ogawa et al. reported
that diphenyl disulfide was formed as a by-product in
9
further and thiols were regenerated (entries 3 and 4).
These results indicate that this dehydrogenation reaction
may be reversible and the formation of disulfide is a
the reaction of benzenethiol and 1-octyne using a cata-
6
10
lytic amount of RhCl(PPh
3
)
3
.
However, transition
kinetically favorable process. The reaction of a sec-
metal complex-catalyzed dehydrogenation of thiols to
disulfides remains unexplored. The present study
describes cationic rhodium(I)/PPh complex-catalyzed
3
dehydrogenation of alkanethiols to disulfides under
inert atmosphere (Eq. 1).
ondary alkanethiol is also dependent on reaction tem-
perature and reaction time (entries 7–10). The highest
yield of disulfides was obtained at 4 ꢁC for 1 h (entry 7).
Highly functionalized alkanethiols are also suitable
substrates to furnish the corresponding disulfides in
good yield (entries 5 and 6). However, tertiary alkane-
thiol 1e or arenethiol 1f is not a suitable substrate, fur-
nishing disulfide in very low yield or not at all (entries 11
and 12).
catalytic
[
Rh(cod) ]BF /8 PPh
2 4 3
RSH
(RS)
2
ð1Þ
CH Cl , 4 °C, 1 h
2
2
under Ar, –H
2
The dependence of reactivity on the structure of thiols
suggested to us that we might be able to effect selective
dehydrogenation of two structurally different thiols to
disulfides (Eq. 2). Treatment of a mixture of primary
alkanethiol 1a and tertiary alkanethiol 1e predominantly
furnished primary alkyl disulfide 2a, and disulfide 3 or
2e was obtained in low yield or not at all. Interestingly,
Keywords: rhodium; dehydrogenation; thiol; disulfide.
*
Corresponding author. Tel./fax: +81-42-388-7037; e-mail: tanaka-k@
cc.tuat.ac.jp
0
040-4039/$ - see front matter ꢀ 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2003.10.120