Reductive coupling of disulfides and diselenides with alkyl halides catalysed by a silica-supported phosphine rhodium complex using hydrogen as a reducing agent

Article abstract of DOI:10.3184/174751913X13796950361813

The reductive coupling of disulfides and diselenides with alkyl halides was achieved in THF at 65 °C in the presence of 3 mol% of a silica-supported phosphine rhodium complex and triethylamine using hydrogen as a reducing agent, affording a variety of unsymmetrical sulfides and selenides in high yields. The heterogeneous rhodium catalyst can be recovered by a simple filtration and reused several times without significant loss of activity. Reaction with an acyl halide was also observed.

Full text of DOI:10.3184/174751913X13796950361813

JOURNAL OF CHEMICAL RESEARCH 2013
OCTOBER, 645–647
RESEARCH PAPER 645
Reductive coupling of disulfides and diselenides with alkyl halides
catalysed by a silica-supported phosphine rhodium complex using
hydrogen as a reducing agent
Hean Zhanga*, Mangen Hua and Mingzhong Caib
aDepartment of Chemistry, College of Science, Nanchang University, Nanchang 330031, P.R. China
bDepartment of Chemistry, Jiangxi Normal University, Nanchang 330022, P.R. China
The reductive coupling of disulfides and diselenides with alkyl halides was achieved in THF at 65 °C in the presence of 3 mol%
of a silica-supported phosphine rhodium complex and triethylamine using hydrogen as a reducing agent, affording a variety of
unsymmetrical sulfides and selenides in high yields. The heterogeneous rhodium catalyst can be recovered by a simple filtration
and reused several times without significant loss of activity. Reaction with an acyl halide was also observed.
Keywords: supported catalyst, phosphine rhodium complex, sulfide, selenide, heterogeneous catalysis
Organic sulfides and selenides have been used as versatile
reagents in organic synthesis.1–4 Recently, transition-metal-
catalysed carbon–heteroatom cross-coupling reactions have
providedconvenientroutesforthepreparationofunsymmetrical
sulfides and selenides. For instance, unsymmetrical diaryl
sulfides could be obtained by copper-catalysed carbon-sulfur
coupling of thiophenols with aryl iodides or aryl boronic
acids;5–7 unsymmetrical diorganyl selenides can be synthesised
by ruthenium-catalysed reactions of dibenzyl or diphenyl
diselenides with alkyl halides in the presence of zinc.4 The
reductive coupling of disulfides or diselenides with alkyl
or aryl halides is an important reaction in the preparation of
unsymmetrical sulfides and selenides, which can avoid the use
of unstable and odoriferous thiols and selenols.8–10 Although
some efficient reductive coupling reactions using disulfides
or diselenides have been reported, they require stoichiometric
amount of metal reducing agents.11–16 Tanaka et al.17 described
a RhCl(PPh3)3-catalysed reductive coupling of disulfides or
diselenides with alkyl halides using hydrogen as a reducing
agent, providing a convenient new route for the synthesis of
unsymmetrical sulfides and selenides from disulfides or
diselenides instead of thiols and selenols. However, industrial
applications of homogeneous rhodium complexes remain a
challenge because they are expensive, cannot be recycled and
are difficult to separate from the product mixture, which is a
particularly significant drawback for their application in the
pharmaceutical industry. In contrast, heterogeneous catalysts
can be easily separated from the reaction mixture by simple
filtration and reused in successive reactions provided that the
active sites have not become deactivated. The immobilisation of
catalytically active species, i.e. organometallic complexes, onto
solid supports to produce molecular heterogeneous catalysts
has attracted much attention because they can combine the
advantages of easy catalyst recovery, characteristic of a
heterogeneous catalyst, with the high activity and selectivity
of soluble complexes.18 Heterogeneous catalysis also helps to
minimise wastes derived from reaction workup, contributing to
the development of green chemical processes.19,20
The immobilisation of rhodium complexes through covalent
bond formation with functional groups on silica is the most
commonly employed method to prepare surface organometallic
catalysts. So far, silica-supported rhodium catalysts have
successfully been used for hydrogenation reactions,21–23
hydroformylation
reactions,24–26
and
hydrosilylation
reaction.27,28 However, carbon–heteroatom bond formation
reaction catalysed by heterogeneous rhodium complexes has
received less attention.29 In continuing our efforts to develop
greener synthetic pathways for organic transformations, in this
paper, we have designed and synthesised a new silica-supported
phosphine rhodium(I) complex [SiO2–P–RhCl(PPh3)2], which is
used as an effective rhodium catalyst for the reductive coupling
of disulfides or diselenides with alkyl halides using hydrogen
as a reducing agent.
The novel silica-supported phosphine rhodium complex
[SiO2–P–RhCl(PPh3)2] was conveniently synthesised by the
reaction of triphenylphosphine-functionalised silica (SiO2–P)30
with RhCl(PPh3)3 (Scheme 1). In our initial screening
experiments, the reductive coupling reaction of di-p-tolyl
disulfide with 1-bromododecane was investigated to optimise
the reaction conditions, and the results are summarised in
Table 1. At first, the temperature effect was examined and a
significant effect was observed. For the temperatures studied
[25, 50, 65, and 80 °C], 65 °C gave the best result and the
reaction proceeded slowly at 25 °C. Other reaction temperatures
such as 50 and 80 °C also gave good results. Our next studies
focused on the effect of solvent on the model reaction. Among
the solvents used [toluene, benzene, THF, and dioxane], THF
was the best choice. Increasing the amount of rhodium catalyst
could shorten the reaction time, but did not increase the yield
of p-tolyl dodecyl sulfide (entry 9). Low rhodium concentration
usually led to a long period of reaction, as indicated by our
experimental result (entry 10). No reaction was observed in the
absence of rhodium catalyst or hydrogen. An excellent result
was obtained when the reductive coupling reaction was carried
out with 3 mol% of the rhodium catalyst in a mixture of THF
and Et3N (5:1, v/v) at 65 °C (entry 6).
O
O
Si
O
Ph
RhCl(PPh3)3
O
Si
PP
h2
SiO2
SiO2
O
P RhCl(PPh3)2
,
,
4
8 h
enzene r.t.
B
O
P
h
-
P
SiO2
SiO2-P-RhCl(PPh3)2
Scheme 1
* Correspondent. E-mail: zhanghean96@sina.com
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646 JOURNAL OF CHEMICAL RESEARCH 2013
Table 1 Screening of reaction conditions for the coupling reaction of di-
Table 2 Reductive coupling of disulfides and diselenides with alkyl
a
a
p-tolyl disulfide with 1-bromododecane catalysed by SiO2–P–RhCl(PPh3)2
halides catalysed by SiO2–P–RhCl(PPh3)2
R1
3 mol% SiO2-P-RhCl(PPh3)2
Tol-p
(R1Y)2
R2X
+
3 mol% SiO2-P-RhCl(PPh3)2
Y
(p-TolS)2
+
CH3(CH2)11Br
S
R2
3
H2 (1 atm), THF-Et N (5:1, v/v)
H2 (1 atm), solvent-Et3N (5:1, v/v)
(CH2)11CH3
65 oC, 24 h
1
2
3
Entry
Solvent
Temp./°C
Time/h
Yield/%b
Entry
(R1Y)2
R2–X
Product
Yield/%b
1
2
3
4
5
6
7
8
Toluene
Toluene
Toluene
Toluene
THF
25
50
65
80
50
65
65
65
65
65
36
30
24
16
30
24
24
24
18
48
53
81
89
86
87
93
80
77
92
90
1
2
3
4
5
(p-TolS)2
(4-ClC6H4S)2
(4-MeOC6H4S)2 CH3(CH2)11Br
(PhCH2S)2
(n-C12H25S)2
(p-TolS)2
(p-TolS)2
(p-TolS)2
(PhSe)2
CH3(CH2)11Br
CH3(CH2)11Br
3a
3b
3c
3d
3e
3f
3g
3h
3i
93
90
91
83
86
87
89
56
91
90
82
CH3(CH2)11Br
CH3(CH2)11Br
PhCH2Cl
THF
6
Benzene
Dioxane
THF
7c
8c
9
PhCOCl
EtO2CCH2Br
CH3(CH2)11Br
PhCH2Cl
9c
10d
THF
10
11
(PhSe)2
(PhSe)2
3j
3k
aReaction was carried out with di-p-tolyl disulfide (0.2 mmol),
1-bromododecane (0.44 mmol) in THF (1.0 mL) and Et3N (0.2 mL).
bIsolated yield based on disulfide.
PhCOCl
aReaction was carried out with disulfide or diselenide (0.2 mmol), alkyl
halide or acyl halide (0.44 mmol) in THF (1.0 mL) and Et3N (0.2 mL).
bIsolated yield based on disulfide or diselenide.
c5 mol% Rh catalyst was used.
d2 mol% Rh catalyst was used.
cReaction was conducted with 5 mol% catalyst in toluene at 100 °C for 36 h.
With this promising result in hand, we started to investigate
the scope of this reaction under the optimised conditions. A
series of disulfides and alkyl halides (and one acyl halide) were
subjectedtotheaboveoptimalreactionconditionsandtheresults
are summarised in Table 2. As shown in Table 2, the reductive
coupling reactions of aryl, benzyl, and alkyl disulfides with
1-bromododecane proceeded smoothly under mild conditions in
good to excellent yields (entries 1–5). The reaction of di-p-tolyl
disulfide with benzyl chloride also proceeded effectively to give
the corresponding benzyl p-tolyl sulfide in 87% yield (entry 6).
However, when ethyl bromoacetate and benzoyl chloride were
used as the substrates, the reactions required a high catalyst
loading (5 mol%) and high reaction temperature (100 °C) and
good to high yields were obtained on a prolonged reaction
time (36 h) (entries 7 and 8). We were pleased to find that the
heterogeneous rhodium-catalysed reductive coupling protocol
can be applicable to the preparation of various selenides. The
reactions of diphenyl diselenide with 1-bromododecane, benzyl
chloride, and benzoyl chloride proceeded smoothly under the
conditions optimised for disulfides to afford the corresponding
selenides in good to excellent yields (entries 9–11).
To verify whether the observed catalysis was due to the
heterogeneous catalyst SiO2–P–RhCl(PPh3)2 or a leached
rhodium species in solution, we performed a hot filtration
test.31 We focused on the reductive coupling reaction of di-
p-tolyl disulfide with 1-bromododecane. We filtered off the
SiO2–P–RhCl(PPh3)2 after 12 h of reaction time and allowed the
filtrate to react further. The catalyst filtration was performed
at the reaction temperature (65 °C) in order to avoid possible
recoordination or precipitation of soluble rhodium upon cooling.
We found that, after this hot filtration, no further reaction was
observed and no rhodium could be detected in the hot filtered
solution by ICP-AES. This result suggests that the rhodium
catalyst remains on the support at elevated temperatures
during the reaction and points to a process of heterogeneous
nature. The reusability of the SiO2–P–RhCl(PPh3)2 complex
was tested through the repeated reaction of di-p-tolyl disulfide
with 1-bromododecane. The SiO2–P–RhCl(PPh3)2 complex
can be easily recovered by a simple filtration of the reaction
solution and washed with THF and diethyl ether. After being
air-dried, it can be reused directly in the next run without
further purification. The recovered catalyst was used in the
next run and almost consistent activity was observed for five
consecutive cycles (Table 3). This result is important from a
practical point of view.
In summary, we have successfully developed a novel,
practical and environmentally friendly method for the synthesis
of a variety of unsymmetrical sulfides and selenides through
the reductive coupling of disulfides and diselenides with alkyl
halides by using a silica-supported phosphine rhodium complex
[SiO2–P–RhCl(PPh3)2] as catalyst and hydrogen as a reducing
agent under mild reaction conditions. The reactions generated
the corresponding unsymmetrical sulfides and selenides in good
to excellent yields. This heterogeneous rhodium catalyst could
be easily recovered and recycled by a simple filtration of the
reaction solution and reused for five cycles without significant
loss of activity, thus making this procedure environmentally
more acceptable.
Table 3 Reusability study of the SiO2–P–RhCl(PPh3)2 in the reductive
coupling reaction of di-p-tolyl disulfide with 1-bromododecanea
Cycle
Yield/%b
1
93
2
92
3
91
4
91
5
90
aReaction conditions: di-p-tolyl disulfide (0.2 mmol), 1-bromododecane
(0.44 mmol), SiO2–P–RhCl(PPh3)2 (3 mol%) in THF (1.0 mL) and Et3N
(0.2 mL) at 65 °C for 24 h under 1 atm of H2.
bIsolated yield.
Experimental
THF was distilled from sodium prior to use, Et3N was dried over
KOH and distilled before use. All other reagents were used as
received without further purification. All reaction were carried out
under an atmosphere of Ar in oven-dried glassware with magnetic
1
stirring. H NMR spectra were recorded on a Bruker Avance 400
(400 MHz) spectrometer with TMS as an internal standard using
CDCl3 as the solvent. 13C NMR spectra were recorded on a Bruker
Avance 400 (100 MHz) spectrometer using CDCl3 as the solvent.
The triphenylphosphine-functionalised silica (SiO2–P) was prepared
according to a literature method,30 the phosphine content was
determined to be 0.71 mmol g–1 by elemental analysis.
Synthesis of the SiO2–P–RhCl(PPh3)2 complex
SiO2–P (2.0 g) was added to a solution of RhCl(PPh3)3 (1.109 g,
1.2 mmol) in benzene (50 mL). The mixture was stirred under an argon
atmosphere at 25 °C for 48 h. The solid product was filtered by suction,
washed with benzene (5×10 mL), and dried at 70 °C/26.7 Pa under
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JOURNAL OF CHEMICAL RESEARCH 2013 647
an argon atmosphere for 3 h to give the light yellow rhodium complex
[SiO2–P–RhCl(PPh3)2] (2.68 g). The phosphine and rhodium content
was 1.34 mmol g–1 and 0.38 mmol g–1, respectively.
Benzyl phenyl selenide (3j): Oil.36 1H NMR (400 MHz, CDCl3): δ
7.46–7.40 (m, 2H), 7.31–7.19 (m, 8H), 4.11 (s, 2H); 13C NMR (100 MHz,
CDCl3): δ 138.6, 133.6, 130.5, 128.9, 128.8, 128.5, 127.4, 126.8, 32.3.
Benzoyl phenyl selenide (3k): Oil.36 IR (film): ν (cm–1) 3062, 1692, 1578,
1474, 1446; 1H NMR (400 MHz, CDCl3): δ 7.96–7.90 (m, 2H), 7.64–7.55
(m, 3H), 7.52–7.37 (m, 5H); 13C NMR (100 MHz, CDCl3): δ 193.4, 138.6,
136.4, 133.8, 129.4, 129.0, 128.9, 127.4, 125.8.
Reactions of disulfides and diselenides with alkyl halides; general
procedure
A 25 mL, three-necked round-bottom flask equipped with a reflux
condenser and a magnetic stir bar was charged sequentially with
SiO2–P–RhCl(PPh3)2 (16 mg, 0.006 mmol), disulfide or diselenide
(0.2 mmol), alkyl halide (0.44 mmol), THF (1.0 mL), and Et3N
(0.2 mL) under argon. Then hydrogen was introduced to the resulting
suspension. The mixture was stirred at 65 °C for 24 h. After being
cooled to room temperature, the mixture was diluted with Et2O (10 mL)
and filtered. The SiO2–P–RhCl(PPh3)2 complex was washed with
THF (2×5 mL) and Et2O (2×5 mL) and reused in the next run. The
ether solution was concentrated under a reduced pressure, and the
residue was purified by preparative TLC (hexane) to afford the desired
product.
We thank the National Natural Science Foundation of China
(20862008) for financial support.
Received 30 June 2013; accepted 18 August 2013
Paper 1302033 doi: 10.3184/174751913X13796950361813
Published online: 7 October 2013
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