Heterogeneous hydrothiolation of alkynes with thiols catalyzed by diphosphino-functionalized MCM-41 anchored rhodium complex

Article abstract of DOI:10.1007/s10562-011-0732-x

A novel diphosphino-functionalized MCM-41 anchored rhodium complex [MCM-41-2P-RhCl(PPh₃)] was conveniently synthesized by the reaction of diphosphino-functionalized MCM-41 (MCM-41-2P) with RhCl(PPh₃) ₃. It was found that this rhodium complex is an efficient catalyst for the hydrothiolation of terminal alkynes with thiols and can be recovered and recycled by a simple filtration of the reaction solution and used for at least ten consecutive trials without any decreases in activity.

Full text of DOI:10.1007/s10562-011-0732-x

Catal Lett (2012) 142:138–142
DOI 10.1007/s10562-011-0732-x
Heterogeneous Hydrothiolation of Alkynes with Thiols Catalyzed
by Diphosphino-Functionalized MCM-41 Anchored Rhodium
Complex
•
Hong Zhao Jian Peng Mingzhong Cai
•
Received: 22 August 2011 / Accepted: 31 October 2011 / Published online: 22 November 2011
Ó Springer Science+Business Media, LLC 2011
Abstract A novel diphosphino-functionalized MCM-41
anchored rhodium complex [MCM-41-2P-RhCl(PPh3)] was
conveniently synthesized by the reaction of diphosphino-
functionalized MCM-41 (MCM-41-2P) with RhCl(PPh3)3.
It was found that this rhodium complex is an efficient cat-
alyst for the hydrothiolation of terminal alkynes with thiols
and can be recovered and recycled by a simple filtration of
the reaction solution and used for at least ten consecutive
trials without any decreases in activity.
important synthetic intermediates in total synthesis and are
versatile building blocks for many functionalized mole-
cules owing to the versatile reactivity of the sulfonyl group
and the carbon–carbon double bond [6–9]. The addition of
thiols to alkynes is one of the most straightforward routes
to vinyl sulfides. Thiols are known to add to alkynes, in the
presence of a radical initiator, to afford regioselectively the
anti-Markovnikov products, usually as a stereoisomeric
mixture [10]. The addition of thiols to alkynes catalyzed by
transition metals has provided a straightforward and atom-
efficient method for the formation of stereo- and regiode-
fined vinyl sulfides [11–20]. Among these transition metal
complexes reported, rhodium complexes have attracted
much attention, due to their high catalytic activity and
regio- and stereoselectivity for either Markovnikov or anti-
Markovnikov products under mild reaction conditions [12–
14, 16–18]. Ogawa and Love groups [12, 16] have reported
that Wilkinson’s catalyst [RhCl(PPh3)3] exhibits excellent
catalytic ability toward the anti-Markovnikov addition of
thiols to terminal alkynes, which affords regio- and stere-
oselectively the corresponding (E)-vinyl sulfides. However,
the problem with homogeneous catalysis is the difficulty to
separate the catalyst from the reaction mixture and the
impossibility to reuse it in consecutive reactions. In con-
trast, 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 high costs of the transition metal
catalysts coupled with toxic effects associated with many
transition metals has led to an increased interest in
immobilizing catalysts onto a support. Heterogeneous
catalysis also helps to minimize wastes derived from
reaction workup, contributing to the development of green
chemical processes [21, 22]. So far, supported palladium
catalysts have successfully been used for the Heck reaction,
Keywords Supported catalyst Á MCM-41 Á
Hydrothiolation Á Phosphine rhodium complex Á
Heterogeneous catalysis
1 Introduction
Although, organic thiols have been widely employed as the
sources of ligands for various transition metals [1, 2], the
transition-metal-catalyzed reactions with thiols as sub-
strates have been scarcely developed because organic sul-
fur compounds are often considered to be incompatible
with metal-catalyzed reactions due to the fact that sulfur
containing substrates can bind strongly to transition metals
and often poison the catalysts [3–5]. Vinyl sulfides are
H. Zhao (&)
School of Chemistry and Chemical Engineering,
Guangdong Pharmaceutical University,
510006 Guangzhou, People’s Republic of China
e-mail: zhaohong1001@sina.com
J. Peng Á M. Cai (&)
Department of Chemistry, Jiangxi Normal University,
330022 Nanchang, People’s Republic of China
e-mail: caimzhong@163.com
123

Heterogeneous Hydrothiolation of Alkynes with Thiols
139
the Suzuki–Miyaura reaction, the Sonogashira reaction,
and the Stille reaction, etc. [23–25]. However, carbon–
carbon bond or carbon-heteroatom bond formation reac-
tions catalyzed by heterogeneized rhodium complexes have
received less attention.
with benzene (5 9 10 mL), and dried at 70 °C/26.7 Pa
under an argon atmosphere for 3 h to give 2.34 g of the
light yellow rhodium complex [MCM-41-2P-RhCl(PPh3)].
The phosphine and rhodium content was 1.74 mmol/g and
0.39 mmol/g, respectively.
Developments on the mesoporous material MCM-41
provided a new possible candidate for a solid support for
immobilization of homogeneous catalysts [26]. MCM-41
has a regular pore diameter of ca.5 nm and a specific surface
area [ 700 m2g-1 [27]. Its large pore size allows passage of
large molecules such as organic reactants and metal com-
plexes through the pores to reach to the surface of the channel
[28–30]. Shyu et al. [31] reported phosphinated MCM-41-
supported rhodium complex for catalytic hydrogenation of
olefins and found that it is an excellent hydrogenation cata-
lyst with turnover frequency (TOF) three times higher than
that of RhCl(PPh3)3 in the hydrogenation of cyclohexene.
However, to the best of our knowledge, no hydrothiolation
reaction of alkynes with thiols catalyzed by a heterogeneized
rhodium complex has been reported until now. In this paper,
we wish to report the synthesis of diphosphino-functiona-
lized MCM-41 anchored rhodium complex [abbreviation:
MCM-41-2P-RhCl(PPh3)] and its catalytic properties in the
hydrothiolation reaction of alkynes with thiols.
2.2 General Procedure for the Hydrothiolation
of Alkynes with Thiols
In a 20 mL two-necked glass flask with a magnetic stirring
bar under an argon atmosphere were placed MCM-41-2P-
RhCl(PPh3) (77 mg, 0.03 mmol), EtOH (2 mL), and
alkyne (1.0 mmol). Then thiol (1.1 mmol) was added
dropwise to the solution over 1 h at 40 °C. The reaction
was continued with magnetic stirring for 24 h at 40 °C.
The mixture was diluted with Et2O (30 mL). The MCM-
41-2P-RhCl(PPh3) catalyst was separated from the mixture
by filtration, washed with EtOH (2 9 10 mL), Et2O
(2 9 10 mL) and reused in the next run. The solvent was
removed in vacuo, and the residue was purified by Flash
chromatography on silica gel to give the desired product.
3 Results and Discussion
The novel diphosphino-functionalized MCM-41 anchored
rhodium complex [MCM-41-2P-RhCl(PPh3)] was conve-
niently synthesized by the reaction of diphosphino-func-
tionalized MCM-41 (MCM-41-2P) with RhCl(PPh3)3
(Scheme 1). Small angle X-ray powder diffraction (XRD)
analysis of the MCM-41-2P-RhCl(PPh3) indicated that, the
100 reflection of MCM-41-2P-RhCl(PPh3) had lower
intensity compared to that of the parent MCM-41, while
the 110 and 200 reflections became weak and diffuse,
which could be due to contrast matching between the sil-
icate framework and organic moieties which are located
inside the channels of MCM-41. Therefore, the basic
structure of the parent MCM-41 was not damaged in the
whole process of catalyst preparation. Elemental analyzes
and solid state 31P NMR were used to characterize the
supported rhodium complex [MCM-41-2P-RhCl(PPh3)].
The P:Rh mole ratio of the MCM-41-2P-RhCl(PPh3) was
determined to be 4:5. Blumel et al. [33, 34] investigated the
silica-supported phosphine rhodium complexes by solid
state 31P NMR spectroscopy. Solid state 31P NMR of
MCM-41-2P showed a signal at d -23.1 ppm, which fur-
ther indicates that the mesoporous material MCM-41-2P
contains phosphorus. Solid state 31P NMR of MCM-41-2P-
RhCl(PPh3) showed three signals at d -23.1, 12.2, and
31.3 ppm, respectively (Fig. 1b). One of these corresponds
to the unreacted anchoring ligand while the other two are
assigned to the anchoring ligand and triphenylphosphine
ligand coordinated to the Rh complex since they are shifted
2 Experimental
The diphosphino-functionalized mesoporous material
MCM-41-2P was prepared according to our previous pro-
cedure, the phosphine content was 1.44 mmol/g [32]. Other
reagents were obtained from commercial suppliers and
purified by distillation. All hydrothiolation products were
characterized by comparison of their spectra and physical
data with authentic samples. IR spectra were determined on
a Perkin–Elmer 683 instrument. 1H NMR spectra were
recorded on a Bruker AC-P400 (400 MHz) spectrometer
with TMS as an internal standard in CDCl3 as solvent.
13C NMR spectra were recorded on a Bruker AC-P400
(100 MHz) spectrometer in CDCl3 as solvent. X-ray
powder diffraction patterns were obtained on Damx-rA
(Rigaku). 31P one-pulse experiments were performed on a
Bruker AMX 400 spectrometer at a 31P frequency of
161.98 MHz at room temperature. Chemical shifts were
referenced to Na2HPO4 at 0 ppm. Microanalyses were
obtained using a Perkin–Elmer 240 elemental analyzer.
2.1 Preparation of MCM-41-2P-RhCl(PPh3)
To a solution of RhCl(PPh3)3 (1.109 g, 1.2 mmol) in
benzene (50 mL) was added MCM-41-2P (2.04 g). The
mixture was stirred under an argon atmosphere at 25 °C for
48 h. The solid product was filtered by suction, washed
123

140
H. Zhao et al.
OSiMe3
O
OSiMe3
O
PPh2
PPh2
PPh2
RhCl(PPh3)
PPh2
RhCl(PPh3)3
Benzene,
Si(CH2)3N
Si(CH2)3N
O
r.t., 48 h
O
OEt
MCM-41-2P
Scheme 1 Preparation of MCM-41-2P-RhCl(PPh3) complex
OEt
MCM-41-2P-RhCl(PPh3)
R
H
H
3 mol% MCM-41-2P-RhCl(PPh3)
EtOH, 40 oC
R
+
ArSH
SAr
1
2
3
Scheme 2 Stereoselective synthesis of (E)-vinyl sulfides
with PhSH to afford the corresponding (E)-vinyl sulfides
in good to high yields (entries 2–5). The catalytic activity
of MCM-41-2P-RhCl(PPh3) is comparable to that of
RhCl(PPh3)3 [12]. For example, the preparation of (E)-1-
(4-methylphenyl)-2-phenylthioethene (3b) was successful
using 3 mol% MCM-41-2P-RhCl(PPh3) as catalyst in
ethanol at 40 °C and gave a 93% yield after 24 h (entry 2),
the same reaction catalyzed by 3 mol% RhCl(PPh3)3 gave
a 94% yield after 20 h. The procedure can be applied to a
variety of aliphatic alkynes, the corresponding (E)-vinyl
sulfides 3h–r was obtained in good yields (entries 8–18).
Similarly, arenethiols bearing 4-methyl, 4-methoxy and
4-chloro groups can add to terminal alkynes regio- and
stereoselectively, but the reactivity of arenethiols with
electron-donating substituents was lower than that of
arenethiols with electron-withdrawing substituents (entries
6, 7, 9, 10). In contrast, the hydrothiolation reaction of
terminal alkynes with alkanethiols such as butanethiol and
cyclohexanethiol did not occur at all under the same con-
ditions and most of the starting materials were recovered
unchanged. Functionalities such as chloro, fluoro, hydroxy,
methyl, methoxy, and olefinic groups tolerate the reaction
conditions (entries 2–7, 9, 10, 12–18). In all cases listed in
Table 1, the addition reaction proceeded with excellent
regio- and stereoselectivity to provide only the E isomer of
the anti-Markovnikov adduct 3. Although the solid sup-
ports such as H-Rho-Zeolite and Montmorillonite K 10 are
known to catalyze regioselectively the addition of thiols
to olefins in high yields in an anti-Markovnikov or a
Markovnikov manner [35, 36], no the addition of thiols to
alkynes catalyzed by the solid supports has been reported
until now. We also found that the addition reaction of thiols
to alkynes did not occur in the presence of MCM-41 or
MCM-41-2P support without rhodium.
Fig. 1 Solid state 13P NMR spectra of a MCM-41-2P and b MCM-
41-2P- RhCl(PPh3). Signals arising from side bands are marked with
asterisks
relative to the 31P signal of RhCl(PPh3)3. However, exact
assignments cannot be concluded. These observations
indicate that Rh has been successfully immobilized on the
mesoporous material MCM-41.
In order to evaluate the catalytic activity of the novel
diphosphino-functionalized MCM-41 anchored rhodium
complex [MCM-41-2P-RhCl(PPh3)], the hydrothiolation
reaction of alkynes with thiols was studied under the
conditions similar to those used in the corresponding
homogeneous reactions (Scheme 2), ethanol was selected
as solvent according to Ogawa’s work [12]. The experi-
mental results are summarized in the Table 1. As seen from
the Table 1, the hydrothiolation reaction of phenylacety-
lene with PhSH proceeded very smoothly at 40 °C in the
presence of 3 mol% MCM-41-2P-RhCl(PPh3) complex in
EtOH to give (E)-2-(phenylthio)styrene in excellent yield
(entry 1). Substituted phenylacetylenes such as 4-methyl-
phenylacetylene, 4-chlorophenylacetylene, 4-fluorophe-
nylacetylene and, 4-methoxyphenylacetylene also reacted
Compounds 3a–r were characterized by their FT-IR, 1H
NMR, 13C NMR and MS spectra. Investigation of the crude
1
products 3 by H NMR spectroscopy (400 MHz) showed
their isomeric purities of more than 98% and no Z-isomer
123

Heterogeneous Hydrothiolation of Alkynes with Thiols
141
Table 1 Regio- and stereoselective addition of thiols to terminal alkynes catalyzed by MCM-41-2P-RhCl(PPh3)a
Entry
R
Ar
Product
Yieldb (%)
1
Ph
Ph
3a
3b
3c
3d
3e
3f
95
93
80
74
88
87
97
81
85
70
69
73
76
72
63
67
70
74
2
4-CH3C6H4
4-ClC6H4
4-FC6H4
Ph
3
Ph
4
Ph
5
4-CH3OC6H4
Ph
Ph
6
4-CH3C6H4
4-ClC6H4
Ph
7
Ph
3g
3h
3i
8
n-C6H13
9
n-C6H13
4-ClC6H4
4-CH3OC6H4
Ph
10
11
12
13
14
15
16
17
18
n-C6H13
3j
i-C5H11
3k
3l
i-C5H11
4-ClC6H4
4-ClC6H4
Ph
PhCH2CH2
1-Cyclohexenyl
HOCH2CH2CH2
CH3OCH2CH2CH2
ClCH2CH2CH2
ClCH2CH2CH2
3m
3n
3o
3p
3q
3r
Ph
Ph
Ph
4-ClC6H4
Bold values represent different structures of compounds 3a–3r according to Scheme 1
a
Reaction conditions: Terminal alkyne (1 mmol), ArSH (1.1 mmol), rhodium catalyst (0.03 mmol), EtOH (2 mL), 40 °C, 24 h
Isolated yield based on the terminal alkyne used
b
Table 2 Hydrothiolation reaction of phenylacetylene with PhSH
catalyzed by recycled catalyst
The catalyst filtration was performed at the reaction tem-
perature (40 °C) in order to avoid possible recoordination
or precipitation of soluble rhodium upon cooling. We
found that, after this filtration, no further reaction was
observed in the filtrate.
Ph
H
H
0.3 mmol MCM-41-2P-RhCl(PPh3)
EtOH, 40 oC, 24 h
+
PhSH
Ph
10 mmol
SPh
11 mmol
3a
Entry
Catalyst cycle
Isolated
yield (%)
TON
The MCM-41-2P-RhCl(PPh3) complex can be easily
recovered by a simple filtration of the reaction solution. We
also investigated the possibility to reuse of the catalyst by
using the hydrothiolation reaction of phenylacetylene with
PhSH. In general, the continuous recycle of resin-supported
transition-metal catalysts is difficult owing to leaching of
the metal species from the polymer supports, which often
reduces their activity within five recycles. However, when
the reaction of phenylacetylene with PhSH was performed
even with 3 mol% of MCM-41-2P-RhCl(PPh3), the cata-
lyst could be recycled 10 times without any loss of activity.
The reaction promoted by the 10th recycled catalyst gave
3a in 93% yield (entry 2, Table 2). The average yield of 3a
in consecutive reactions promoted by the first through the
tenth recycled catalyst was 94% (entry 3, Table 2). The
high stability and excellent reusability of the catalyst result
from the chelating action of the bidentate phosphine ligand
on rhodium and the mesoporous structure of the MCM-41
support. The result is important from a practical point of
view. The high catalytic activity, excellent reusability and
the easy accessibility of the MCM-41-2P-RhCl(PPh3)
1
2
3
1st
95
31.7
31
10th
93
1st to 10th
consecutive
av. 94
Total of 313.3
was isolated. The stereochemistry of the addition was
readily apparent from the 1H NMR spectra of compounds 3
which showed a doublet at d = 6.04–6.94 with a coupling
constant of 15.6–16.4 Hz, fully in accord with an E geom-
etry and overall cis addition of S–H bond to acetylenes.
A further objective of our studies was to determine
whether the catalysis was due to the MCM-41-2P-
RhCl(PPh3) complex or to a homogeneous rhodium com-
plex that comes off the support during the reaction and then
returns to the support at the end. To test this, we focused on
the hydrothiolation reaction of phenylacetylene with PhSH.
We filtered off the MCM-41-2P-RhCl(PPh3) complex after
2 h of reaction time and allowed the filtrate to react further.
123

142
H. Zhao et al.
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4 Conclusions
We have successfully developed a novel, practical and
environmentally friendly method for the synthesis of
(E)-vinyl sulfides through the addition reaction of thiols
to terminal alkynes by using diphosphino-functionalized
MCM-41 anchored rhodium complex [MCM-41-2P-RhCl
(PPh3)] as catalyst under mild reaction conditions. The
reactions generated the corresponding (E)-vinyl sulfides
regio- and stereoselectively in good to high yields. In
addition, the catalyst could be easily recovered and reused
for ten cycles without significant loss of activity or selec-
tivity, thus making this procedure environmentally more
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