Highly stereoselective anti-Markovnikov hydrothiolation of alkynes and electron-deficient alkenes by a supported Cu-NHC complex

Article abstract of DOI:10.1039/c4gc00642a

A practical, efficient, and low-cost heterogeneous catalyst consisting of a Cu-NHC (N-heterocyclic carbene) complex grafted to SBA-15 silica for the catalytic hydrothiolation of alkynes and electron-deficient alkenes under mild reaction conditions has been developed. The heterogeneous catalyst displays higher activity and stereoselectivity to Z-anti-Markovnikov isomers compared with the homogeneous analog under otherwise identical reaction conditions. The catalytic system is applicable to a broad range of alkynes and thiols and is recyclable without significant loss in catalytic performance. High activity and perfect selectivity to alkyl sulfides formed by the addition of electron-deficient alkenes to various thiols catalyzed by the supported Cu-NHC complex were also realized. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4gc00642a

Green Chemistry
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Highly stereoselective anti-Markovnikov
hydrothiolation of alkynes and electron-deficient
alkenes by a supported Cu-NHC complex†
Cite this: Green Chem., 2014, 16,
3916
Yong Yang*‡ and Robert M. Rioux*
A practical, efficient, and low-cost heterogeneous catalyst consisting of a Cu-NHC (N-heterocyclic
carbene) complex grafted to SBA-15 silica for the catalytic hydrothiolation of alkynes and electron-
deficient alkenes under mild reaction conditions has been developed. The heterogeneous catalyst displays
higher activity and stereoselectivity to Z-anti-Markovnikov isomers compared with the homogeneous
analog under otherwise identical reaction conditions. The catalytic system is applicable to a broad range
of alkynes and thiols and is recyclable without significant loss in catalytic performance. High activity and
perfect selectivity to alkyl sulfides formed by the addition of electron-deficient alkenes to various thiols
catalyzed by the supported Cu-NHC complex were also realized.
Received 9th April 2014,
Accepted 26th May 2014
DOI: 10.1039/c4gc00642a
www.rsc.org/greenchem
High stereoselectivity to the E-isomer is observed in most
1. Introduction
cases.2i,j,n,o,p In contrast, there are fewer demonstrations of cata-
Vinyl sulfides are prevalent in many natural products and lytic hydrothiolation for the preparation of stereoselective
pharmaceuticals and are versatile intermediates for the syn- Z-isomers. Kondoh et al.4b reported the synthesis of Z-vinyl sul-
thesis of biologically active compounds, organic building fides catalyzed by Cs2CO3 as a base for alkyne hydrothiolation.
blocks, and new materials.1 Many approaches have been develo- However, such a process required the presence of 2,2,6,6-tetra-
ped to construct vinyl sulfide compounds, such as the addition methylpiperidine-N-oxyl (TEMPO) as a radical inhibitor, and
of thiols to alkynes in the presence of transition metals,2 free demonstrated limited substrate versatility since it was only
radicals,3 or bases,4 Wittig olefination,5 direct nucleophilic sub- applicable to alkyl thiols. Frech8 reported a Pd complex
stitution with vinyl halides,6 and reaction of sulfonyl hydrazides (dichlorobis[1-(dicyclohexylphosphanyl)piperidine]palladium)
with terminal arylacetylenes using a DBU-based ionic liquid.7 for the synthesis of Z-vinyl sulfides with good substrate gene-
Despite considerable progress, these approaches generally rality. Very recently, copper-catalyzed processes for the highly
suffer from harsh reaction conditions, costly starting materials, stereoselective synthesis of Z-vinyl sulfides have been reported
or low regio- and stereoselectivity. It remains a challenge to by Beletskaya,9 Zhang,10a and Liu10b through CuI-catalyzed
develop a highly regio- and stereo-controlled synthesis of vinyl addition of alkynes with thiols, CO2-mediated reductive coup-
sulfides under mild reaction conditions. ling of alkynes and thiols, and decarboxylative C–S coupling
Transition metal catalyzed addition of the S–H bond to reaction, respectively.
alkynes is a straightforward and atom-efficient method for the
Due to reasons of commercial availability, lower cost, and
formation of stereo- and regio-defined vinyl sulfides com- abundance of copper, compared with noble metals, there is an
pounds. Markovnikov2a–f,p and anti-Markovnikov2g–p selectivity incentive to replace these noble metal catalysts with low-cost
has been observed in transition metal catalyzed processes. In copper complexes. Copper complexes have been extensively
the case of anti-Markovnikov addition in the presence of tran- developed for applications in organic synthesis and catalysis.11
sition metals, either Z (cis-configuration) or E (trans-configur- Among the many complexes investigated, Cu-NHC (N-hetero-
ation) isomer or a mixture of Z and E isomers is produced. cyclic carbene) complexes have attracted great interest because
the strong σ-donating NHC ligand offers an opportunity to tune
the reactivity and selectivity of transition metal catalysts.12
Department of Chemical Engineering, The Pennsylvania State University, University
Cu-NHC complexes have been successfully employed in hydro-
boration,13 alkene hydrothiolation,14 hydrosilylation,15a oxi-
Park, PA 16802, USA. E-mail: yangyo@ices.a-star.edu.sg, rioux@engr.psu.edu
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
dative coupling,15b and carboxylation reactions.16 No studies
c4gc00642a
known to us have pursued the hydrothiolation of alkynes cata-
‡Current address: Organic Chemistry, Institute of Chemical
& Engineering
lyzed by Cu-NHC complexes. Due to the unique electronic and
Sciences (ICES), 11 Biopolis Way, Helios Blk, #03-08, 138667 Singapore.
3916 | Green Chem., 2014, 16, 3916–3925
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steric properties of NHC ligands, we envisioned that Cu-NHC (700–900 m2 g−1), was synthesized according to the previous
complexes might be capable of facilitating alkyne hydrothiola- literature.20 Preparation of the Cu-NHC-SBA-15 catalyst was
tion in a regio- and stereoselective fashion. Although homo- performed via a covalent attachment strategy in a step-by-step
geneous Cu-NHC complexes have advantages such as high manner under a N2 atmosphere, as shown in Scheme 1. The
activity and selectivity, their recovery and reuse detracts from terminal CvC bonds in the Cu-NHC complex coupled with
their adoption in practical applications. Furthermore, the –SH groups tethered on the SBA-15 surface in the presence of
residual metal along with the products could induce serious AIBN (2–2′-azoisobutyronitrile) as an initiator in toluene,
problems in the synthesis of bioactive and functional sub- resulting in the immobilization of the Cu-NHC complex onto
strates. Heterogeneous catalyst offer potential advantages the surface of SBA-15. Characterization results from physical
including easy product separation, long lifetime, and enhanced adsorption, X-ray diffraction (XRD), and high resolution trans-
stability. Immobilization of transition metal complexes onto a mission electron microscopy (HR-TEM) (ESI, Fig. S1, 2 and 3,
solid support to produce a molecular heterogeneous catalyst is Table S1†) for a Cu-NHC-SBA-15 catalyst demonstrate preser-
one way to make such a catalyst practical.17 The immobilization vation of the ordered mesoporous structure upon functionali-
of metal complexes through covalent bond formation with func- zation and immobilization of the Cu-NHC complex within the
tional groups on high surface area silica materials is the most pores of SBA-15.
widely employed approach to form surface organometallic cata-
Fig. 1 shows the FT-IR spectra for the homogeneous
lysts that are applicable for a variety of catalytic chemistries. The Cu-NHC complex, SH-SBA-15, and the supported Cu-NHC-SBA-15
structure of metal complexes upon immobilization on the silica catalyst, respectively. Upon immobilization of the Cu-NHC
surface not only reduce metal leaching from the support and complex onto the support SH-SBA-15, the characteristic FT-IR
subsequent metal contamination of the products, and provide peaks at 1678 and 2580 cm−1, assigned to the stretching of the
an enhancement in the reactivity and selectivity.18 The chemical CvC bond in the Cu-NHC complex (spectrum a) and –SH of
bonding between metal complexes and functional groups of the grafted thiol (spectrum b), on SBA-15 disappeared,
the support can help to maintain the isolated nature of metal suggesting the effective coupling between the CvC bond and
complexes with controllable loadings, which can influence the –SH groups in the presence of AIBN. The characteristic peaks
catalytic performance in such a manner that the homogeneous of the Cu-NHC complex appeared upon immobilization onto
analogue does not appear in solution.19
Recently, we developed well-defined supported Rh covalently tethered to the surface of SBA-15. During the prepa-
SBA-15 (spectrum c), indicating that the Cu-NHC complex was
a
complex on SBA-15 for a broad range of alkyne hydrothiola- ration of the SH-SBA-15 material, the nominal loading of
tions in high yield with excellent stereoselectivity to E-vinyl sul- (3-mercaptopropyl)trimethoxysilane was 3.0 molar excess rela-
fides.2p The objective of this study was to develop
a
tive to the amount of Cu-NHC complex added to the
heterogeneous catalyst comprised of an inexpensive, earth- SH-SBA-15 (accounting for the 2 : 1 stoichiometry between
abundant element with the capability of producing the Z-vinyl –SH : Cu-NHC; see Scheme 1) (see ESI† for experimental
sulfide product. Herein, we report a facile, efficient, and low- details). Elemental analysis of the supernatant demonstrated
cost supported Cu-NHC complex on SBA-15 for regio- and that the nominal amount of Cu added during synthesis was
stereoselective hydrothiolation of alkynes and electron- retained in the final Cu-NHC-SBA-15 catalyst. The enlarged
deficient alkenes to produce Z-vinyl sulfides and alkyl sulfides regions where the –SH vibration occurs demonstrate that a
with high activity and recyclability.
weak –SH resonance is observable in SH-SBA-15 (Fig. 1b) but
absent in Cu-NHC-SBA-15 (Fig. 1c). The decreased concen-
tration of grafted –SH groups after coupling reaction of the
Cu-NHC complex with SH-SBA-15 reduces the infrared –SH
signal below the noise level of the instrument since –SH groups
were grafted in excess of the nominal amount of the Cu-NHC
complex added during the synthesis. The maximum surface
density of –SH groups on the surface of Cu-NHC-SBA-15 after
2. Results and discussion
2.1. Characterization of the supported Cu-NHC complex
SBA-15, a mesoporous hexagonal silica with large tunable
uniform pores (6–9 nm) and high specific surface area
Scheme 1 Procedure for the preparation of the Cu-NHC-SBA-15 catalyst.
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sition in the spectrum of the grafted copper complex (curve b)
indicates the presence of a carbene ligand, Cu(I) oxidation
state and a local coordination that resembles the Cu-NHC
complex.
Further evidence for the successful immobilization was pro-
vided by 13C and 29Si solid-state CP-MAS NMR results (Fig. 3
and 4). The appearance of new peaks at δ = −67 and −58 ppm
in the 29Si solid-state NMR spectrum (Fig. 3, spectra b and c),
corresponding to T2 (RSi(OSi)2(OH)) and T3 (RSi(OSi)3) sites,22
respectively, revealed that (3-mercaptopropyl)trimethoxysilane
reacted with SiOH groups on the SBA-15 surface and formed
covalent linkages of approximately 73% –Si(OSi)3 and 27%
–Si(OSi)2(OCH3) type. Two signals around −100, and −110 ppm
were observed for the pure SBA-15 (Fig. 3, spectrum a), charac-
teristic of Q3 and Q4 silicon sites of the SiO4-substructures (Qn =
Si(OSi)n(OH)4−n, n = 2–4) in the silica framework.22 Upon graft-
ing (3-mercaptopropyl)trimethoxysilane to the surface of silica,
there are no significant differences among the Q3 and Q4
species with the exception of a reduction in intensity of the Q3
species, consistent with previous studies of silicate materials.23,24
The peak at δ = 117 ppm in the 13C NMR spectrum (Fig. 4,
spectrum c), characteristic of a terminal carbon in a CvC
bond, completely disappeared upon immobilization of the
Cu-NHC complex onto the SH-SBA-15 support due to the coup-
ling reaction in the presence of AIBN, consistent with FT-IR
results. Fig. 4b and d demonstrate that the other characteristic
peaks associated with the mercapto linker and Cu-NHC
Fig. 1 FT-IR spectra for (a) Cu-NHC complex; (b) SH-SBA-15; and
(c) Cu-NHC-SBA-15. Insets are magnified views of the –SH stretching
region for SH-SBA-15 and Cu-NHC-SBA-15, respectively. The inset
shows the expanded spectra ranging from 1500–3000 cm−1
.
grafting is 0.195 nm−2 compared with 0.415 nm−2 for the
SH-SBA-15 (these surface densities are normalized by the BET
surface area for the corresponding material, see ESI Table S1†).
Fig. 2 shows Cu 2p3/2 XPS spectra for the homogeneous
Cu-NHC complex and the supported Cu-NHC-SBA-15. Cu 2p3/2
XPS analysis showed that the oxidation state of Cu does not
change after immobilization (Fig. 2b). We have provided
additional confirmation that the electronic structure of the
Cu(I) center is minimally perturbed when grafted to the
SH-SBA-15 support utilizing UV-Vis spectroscopy (see ESI
Fig. S4†). In the UV/Vis spectra, similar peaks before and after
immobilization were observed at 363 nm (Cu d → carbene π)
and 498 nm (Cl p → Cu d).21 The presence of the d–π tran-
complex were retained. As expected, the peak widths in the 13
C
CP-MAS spectra broadened in the solid-state. Featureless
peaks at ~17 ppm and 70 ppm, respectively, are associated
with resonances from carbon in Pluronic P123 tri-block co-
polymer used in the template synthesis of the SBA-15 silica.
Fig. 2 Cu 2p3/2 XPS spectra for (a) Cu-NHC complex and (b) Cu-
Fig. 3 29Si solid-state NMR spectra for (a) SBA-15; (b) SH-SBA-15; and
NHC-SBA-15.
(c) Cu-NHC-SBA-15.
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Fig. 4 13C liquid and solid-state NMR spectra for (a) SH(CH2)3Si(OCH3)3; (b) SH-SBA-15; (c) Cu-NHC complex; and (d) Cu-NHC-SBA-15.
Although these samples have been ethanol-washed and cal- SH-modified SBA-15 silica. In total, the three techniques
cined at 550 °C for 12 h, residual polymer still remains. It was provide evidence that the grafting reaction occurred.
previously determined that the peak at ~17 ppm has been
2.2. Hydrothiolation of phenylacetylene with thiophenol over
different catalysts
attributed to –CH3 resonance in the poly(propylene oxide)
block and the peak at 70 ppm is associated with –CH2–CH–
from the poly(propylene oxide) block and the –CH2– from the
poly(ethylene oxide) block for ethanol-washed SBA-15.25
The loading of the Cu-NHC-SBA-15 catalyst was determined
to be 0.69 wt% (Cu) by ICP-OES, which corresponds to a
copper density of 0.2 nm−2. This rather low density ensured a
high probability that individually-grafted Cu-NHC complexes
were site-isolated.
Initial efforts concentrated on screening Cu catalysts for the
stereo-defined synthesis of vinyl sulfides. The addition of
phenylacetylene to thiophenol was chosen as a model reaction;
a summary of the results is compiled in Table 1. The addition
proceeded quite slowly and gave poor conversion with a
Table 1 Activity and selectivity of homogeneous and heterogeneous
Cu-based catalystsa
The occurrence of the grafting reaction between the
Cu-NHC complex and –SH groups on the surface of silica is
confirmed by a combination of infrared spectroscopy, 13C
solid-state NMR spectroscopy and Cu elemental analysis. None
of the techniques individually are sufficient enough to confirm
the occurrence of grafting since the spectra become complex
when the Cu-NHC complex is grafted to the silica surface due
to broad spectral contributions of silanol and siloxane
stretches in the infrared spectra and the inherent broadening
observed in solid-state NMR. However, the apparent disappear-
d
Time
(h)
Selectivityc (%) TOFini
Entry Catalyst
Conv.b Z : E
(h−1
)
1
2
3
4
5
6
7
Blanke
24
24
24
3
41.2
39.6
40.8
>99.0
79.2
98.0
71.0
51 : 49
—
—
—
65.9
3.2
28.7
10.8
SBA-15f
SH-SBA-15g
CuClh
52 : 48
56 : 44
58 : 42
72 : 28
89 : 11
81 : 19
ance of the –SH (∼2580 cm−1) and CvC (∼1678 cm−1
)
vibrations coupled with the solid-state 13C NMR results
demonstrates that the characteristic resonance of the terminal
CvC peak (117 ppm) in the Cu-NHC disappears after grafting
to the SH-functionalized surface (Fig. 4d) and is confirmation
that the reaction between the grafted alkylthiol and the
Cu-NHC complex occurred.
Cu-NHCi
24
Cu-NHC-SBA-15j 20
k
Cu-NHC–SiO2
24
a Reaction conditions: 1.0 mmol phenylacetylene, 1.1 mmol
thiophenol, 2 mL 1,2-DCE, 70 °C. b Conversion of phenylacetylene.
c Determined by 1H NMR analysis of the crude mixture. d The initial
turnover frequency for phenylacetylene disappearance at conversion
<30%. e In the absence of catalyst. f 50 mg SBA-15. g 50 mg SH-SBA-15.
h 20 μmol CuCl. i 10 μmol Cu-NHC complex (as-synthesized, see the
ESI for details). j 50 mg Cu-NHC-SBA-15 (5.3 μmol Cu). k 50 mg
Cu-NHC–SiO2 (5.4 μmol Cu) (see the ESI for details).
Elemental analysis of the immobilised Cu catalyst demon-
strated that the actual weight loading of Cu was 0.69 wt%
which agreed with the nominal weight loading. Therefore, all
of the Cu-NHC complex was grafted to the surface of the
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mixture of (E + Z) vinyl sulfide (Z/E ratio = 51/49, 52/48, 56/44,
respectively) in the absence of catalyst, in the presence of the
support SBA-15 or SH-SBA-15 (entries 1–3) in 1,2-dichloro-
ethane (1,2-DCE), respectively. Upon the addition of the pre-
cursor Cu(I)Cl to the reaction mixture, the activity increased
dramatically, affording complete conversion but with roughly
identical stereoselectivity to Z and E vinyl sulfide isomers after
3 h (entry 4). The Cu-NHC complex as a homogeneous catalyst,
synthesized from Cu(I)Cl and the N-heterocyclic carbene
(1,3-bis-(4-allyl-2,6-diisopropyl-phenyl)imidazolium chloride)
ligand in the presence of NaOtBu (see the Experimental
section), showed a relatively low reaction rate (79.2% conver-
sion after 24 h) compared with Cu(I)Cl under otherwise identi-
cal reaction conditions. However, there was a substantial
change in stereoselectivity; the Z-vinyl sulfide was the predomi-
nant product, demonstrating that the N-heterocyclic carbene
ligand facilitates stereoselective-reaction control. Upon immo-
bilization of the Cu-NHC complex onto SBA-15, the catalytic
activity was significantly enhanced under otherwise identical
reaction conditions. The initial turnover frequency of Cu-
NHC-SBA-15 was 9 times higher than that of the Cu-NHC
complex. More importantly, the stereoselectivity to Z-vinyl
sulfide increased with the increase in the activity.
Fig. 5 TON as a function of reaction time for the reaction of phenyl-
acetylene and thiophenol catalyzed by (a) the homogeneous Cu-NHC
complex; (b) Cu-NHC–SiO2; and (c) Cu-NHC-SBA-15. The inset shows
the initial reaction rate for both catalysts. Reaction conditions: 1.0 mmol
phenylacetylene, 1.1 mmol thiophenol, 2 mL 1,2-DCE, 70 °C.
believed to be small since there was no evidence of Cu precipi-
tation. Further work is necessary to completely characterize
the Cu species formed during the reaction of the homo-
geneous Cu-NHC complex. Extended X-ray absorption fine
structure (EXAFS) measurements would be required for con-
firming the presence of Cu–Cu bonding which is beyond the
intended scope of this work.
The rates of both the homogeneous and heterogeneous
reactions were proven to be free of transport artifacts. For the
homogeneous reactions, the initial turnover frequency (nor-
malized to the moles of Cu) does not change with catalyst
loading, and reactions were conducted in a regime where the
stirring speed had no influence on the measured rate of reac-
tion. For the heterogeneous reaction, we applied the Weisz–
Prater test26a,b to confirm that the rates over the heterogeneous
catalyst were not influenced by mass transfer. Utilizing an
identical procedure to that used by Mukherjee and Vannice,26c
calculated values for the W-P parameter were on the order of
∼10−2, demonstrating transport limitations were insignificant
in the supported catalyst system.
Initial rate experiments were carried out to shed light on
the differences in catalytic performance between the Cu-NHC
complex and Cu-NHC-SBA-15. The results, shown in Fig. 5,
demonstrate that the Cu-NHC complex exhibited a lower reac-
tion rate and that the reaction proceeded sluggishly without
deactivation of the catalyst, while the Cu-NHC-SBA-15 catalyst
displayed a higher initial reaction rate. We postulate that the
higher catalytic activity of the supported Cu-NHC-SBA-15 orig-
inates from the formation of isolated catalytically active sites
on the surface of SBA-15, while the slower reaction rate of
the homogeneous Cu-NHC complex is likely due to the for-
mation of an inactive Cu species through bimolecular conden-
Apart from this, we observed that the Cu-NHC-SBA-15 cata-
lyst displayed higher stereoselectivity to the Z-vinyl sulfide
isomer than the homogeneous catalyst, which is possibly
due to the confinement of the Cu-NHC complex in the nano-
pores of SBA-15 (5.8 nm). This was further supported by the
catalytic performance of the Cu-NHC–SiO2 catalyst (0.71 wt%
Cu loading) which had a relatively lower surface area (166 m2
g
−1) and larger pore size (9.7 nm) (Table 1, entry 7; and Fig. 5).
It is apparent from Table 1 that a significant background reac-
tion occurred over the time period of 24 h in the absence of
catalysts which was not influenced by the presence of SBA-15
and SH-SBA-15. The homogeneous hydrothiolation was
limited to ∼40% conversion based on the concentration of
phenylacetylene and a Z : E selectivity of ∼50 : 50 which was
invariant with time (i.e., there was no conversion between the
Z and E isomers with time).
Upon the addition of catalyst, the conversion of phenyl-
acetylene increased significantly and the Z : E isomer was
invariant with time with a Z stereoselectivity of ∼90% (Fig. 6).
Therefore the supported Cu-NHC-SBA-15 catalyst either con-
trols the overall hydrothiolation kinetics or is a sufficient Z
stereoselective addition and E/Z isomerization catalyst. We
have previously demonstrated for hydrothiolation reactions in
1,2-DCE that a radical mechanism is not operative because
selectivity and yield results were unaffected when galvinoxy (gal-
vinoxy/catalyst molar ratio = 3) was included in the reaction.2p
sation reactions between Cu-NHC complexes.
A control
experiment involving the as-synthesized Cu-NHC complex in
1,2-DCE without the addition of thiophenol at 70 °C demon-
strated that the color of the solution changed to dark brown
after reaction for 24 h. UV-Vis spectroscopic analysis of the
solution contained a small plasmon band at ∼565 nm, which
is consistent with the literature27 and suggestive of the for-
mation of small metallic Cu particles. These particles are
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Table 2 Cu-NHC-SBA-15 catalyzed hydrothiolation of various alkynes
with thiophenola
Temp
(°C)
Time
(h)
Selectivityc
(%) Z : E
Entry
R1
R2
Yieldb
1
2
3
4
5
6
7
8
Ph
H
H
H
H
H
H
H
H
H
H
H
H
H
Me
Ph
70
70
70
70
70
70
70
70
100
100
100
100
100
70
20
20
20
20
20
20
20
20
72
72
72
72
72
48
72
98.0
98.0
>99.0
>99.0
>99.0
>99.0
84.1
50.9
>99.0
85.6
64.5
>99.0
83.2
88.0
8.0
89 : 11
87 : 13
90 : 10
91 : 9
91 : 9
91 : 9
94 : 6
93 : 7
91 : 9
92 : 8
88 : 12
89 : 11
90 : 10
91 : 9
91 : 9
4-Me–Ph
2-Me–Ph
3-Me–Ph
4-MeO–Ph
2-MeO–Ph
4-F–Ph
4-Br–Ph
n-C5H11
n-C10H21
HO–(C2H4)
Cl–(C2H4)
Cyclohexene
Ph
9
10
11
12
13
14
15
Ph
100
Fig. 6 Stereoselectivity (Z/E) ratio as a function of time for the reaction
of phenylacetylene and thiophenol catalyzed by (a) the homogeneous
Cu-NHC complex; (b) Cu-NHC–SiO2; and (c) Cu-NHC-SBA-15. Reaction
conditions: 1.0 mmol phenylacetylene, 1.1 mmol thiophenol, 2 mL
1,2-DCE, 70 °C.
a Reaction conditions: 1.0 mmol alkyne, 1.1 mmol thiophenol, 50 mg
Cu-NHC-SBA-15 (5.3 μmol Cu),
2
mL 1,2-DCE. b Isolated yield.
c Determined by 1H NMR analysis.
acetylene was performed (entry 15). A variety of functional
groups, such as fluoro, chloro, hydroxyl, methoxy, and olefinic,
were tolerated under the reaction conditions, and the corres-
ponding addition products were obtained in high yield. In all
cases listed in Table 2, no disulfide adduct formed, and no
double bond dimerization reaction occurred.
A similar result is anticipated in the case of the supported Cu
complex studied in this work; therefore we discounted a free
radical based mechanism in the presence of the supported Cu
catalyst.
We next explored the influence of the thiol substrate on the
hydrothiolation reaction. Aliphatic and aromatic thiols reacted
with phenylacetylene efficiently under similar reaction con-
ditions. Electron-rich substituents on the phenyl ring of aro-
matic thiols provided high yield and good stereoselectivity to
Z-vinyl sulfides (Table 3, entries 1 and 2). Electron-poor substi-
2.3. Scope of alkynes and thiols for hydrothiolation over the
Cu-NHC-SBA-15 catalyst
Initial screening results indicated that higher stereoselectivity
to the Z-isomer was achieved when phenylacetylene hydrothio-
lation was catalyzed by the Cu-NHC-SBA-15 catalyst. We sub-
sequently examined the scope of alkynes using thiophenol as
the substrate in the presence of the Cu-NHC-SBA-15 catalyst in
1,2-DCE, and the results are summarized in Table 2. A wide
range of alkynes including aromatic, aliphatic, and internal
was hydrothiolated to the Z-anti-Markovnikov products in
moderate to high yield with excellent regio- and stereo-selecti-
vity irrespective of the nature of the substrate. Electron-rich
substituents at m, o, and p positions on the phenyl ring of aro-
matic alkynes all provided high yields (entries 2–6). Electron-
poor substituents dramatically decreased the activity but
caused no alteration in the high stereoselectivity (entries 7 and
8). Aliphatic alkynes reacted with thiophenol to afford the
corresponding Z-anti-Markovnikov vinyl sulfides as the major
product in satisfactory yield, but required both longer reaction
time and elevated temperature (entries 9–13). The internal
alkyne, 1-phenyl-1-propyne, underwent stereoselective hydro-
thiolation but required longer reaction time (entry 14);
however, only a small amount of the addition products was
Table 3 Cu-NHC-SBA-15 catalyzed hydrothiolation of phenylacetylene
with various thiolsa
Temp
(°C)
Time
(h)
Yieldb
(%)
Selectivityc
(%) Z : E
Entry
R3
1
2
3
4
5
6
7
4-Me–Ph
4-MeO–Ph
4-Cl–Ph
4-Br–Ph
PhCH2
Cyclohexane
CH3(CH2)5
70
70
70
20
20
20
20
72
72
72
>99.0
>99.0
75.4
86.3
52.5
56.9
79.5
91 : 9
87 : 13
90 : 10
85 : 15
81 : 19
80 : 20
82 : 18
70
100
100
100
a Reaction conditions: 1.0 mmol alkyne, 1.1 mmol thiophenol, 50 mg
Cu-NHC-SBA-15 (5.3 μmol Cu),
2
mL 1,2-DCE. b Isolated yield.
isolated when the addition of thiophenol with diphenyl- c Determined by 1H NMR analysis.
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Scheme 2 Gram scale hydrothiolation of phenylacetylene with thiophenol.
tuents decreased the activity but did not cause any modifi-
cation to the stereoselectivity (entries 3 and 4). Aliphatic thiols
reacted with phenylacetylene to afford the corresponding
Z-vinyl sulfides as the major product in satisfactory yield after
longer reaction times and elevated temperatures (entries 5–7).
Additionally, we investigated the capability of the
Cu-NHC-SBA-15 catalyst for gram-scale synthesis (Scheme 2).
The reaction of 1.02 g (10 mmol) of phenylacetylene with 1.1
equivalents of thiophenol under the optimized conditions
yielded 1.92 g (91% of isolated yield) of the desired vinyl
sulfide with high selectivity to Z-isomer product after 26 h.
Table 4 Cu-NHC-SBA-15 catalyzed hydrothiolation of electron-
deficient alkenes with various thiols.a
Entry
1
Thiol
PhSH
Alkene
Yieldb [%]
99.0
2
3
PhSH
99.0
97.2
2.4. Hydrothiolation of electron-deficient alkenes over the
Cu-NHC-SBA-15 catalyst
4-MeOPhSH
As demonstrated above, the Cu-NHC-SBA-15 catalyst exhibited
good activity and high stereoselectivity for hydrothiolation over
a broad substrate scope with respect to alkyne and thiol.
Further investigation of alkene hydrothiolation catalyzed by
the Cu-NHC-SBA-15 catalyst was conducted to extend its poten-
tial applications in organic synthesis. Utilizing a series of
α,β-unsaturated carbonyls as substrates, we demonstrate that
the heterogeneous Cu-NHC-SBA-15 catalyst is effective for the
hydrothiolation of electron-deficient alkenes with thiophenol,
as shown in Table 4. Unfortunately, the addition of alkenes
(1-hexene and styrene, results not shown) to thiophenol did
not proceed even with prolonged reaction times and elevated
reaction temperatures. However, thiophenol reacted quantita-
tively with the electron-deficient alkene, 2-cyclohexene-1-one,
in the presence of the Cu-NHC-SBA-15 catalyst in 1,2-DCE to
afford the corresponding anti-Markovnikov β-sulfido carbonyl
product (Table 4, entry 1), an important organic intermediate
for synthesis of bioactive compounds.28 Based on this finding,
a set of thiophenols bearing electron-rich or electron-poor sub-
stituents (Table 4, entries 3 and 4) were reacted with 2-cyclo-
hexene-1-one to yield the corresponding β-sulfido carbonyl
products in high yield without the formation of any side reac-
tion products such as those derived from dimerization and/or
polymerization reactions which proceed simultaneously with
the addition reaction in the presence of acid or base.29 The
addition of aliphatic thiols to electron-deficient alkenes gene-
rated the corresponding β-sulfido carbonyl products in satisfac-
tory yield (Table 4, entries 5–7).
4
5
6
7
4-ClPhSH
98.3
62.1
69.1
60.6
CH3(CH2)4SH
PhCH2SH
PhCH2SH
a Reaction conditions: 1.0 mmol electron-deficient alkene, 1.1 mmol
thiol, 50 mg Cu-NHC-SBA-15 (5.3 μmol Cu), 2 mL 1,2-DCE, 100 °C,
48 h. b Yield of the desired product.
solution, we carried out the addition of phenylacetylene to
thiophenol and removed the catalyst from the reaction
mixture by hot filtration at approximately 40% conversion of
phenylacetylene (Fig. 7). After removal of the Cu-NHC-SBA-15
catalyst, the filtrate was again held at 70 °C under an atmos-
phere of N2. In this case, no significant increase in the
conversion was observed, indicating that leached copper
species from the catalyst (if any) are not responsible for the
observed activity. It was confirmed by ICP–OES analysis that
no copper species could be detected in the filtrate (below
the detection limit). Recycling studies confirm the stability of
the Cu-NHC-SBA-15 catalyst under reaction conditions
(Table 5). The supported catalyst exhibited good stability
and was recycled 6 times without significant loss of activity
and selectivity with the exception of 10% of loss in activity
after the first recycle, which is most likely due to the cleavage
of a trace amount of tethered Cu-NHC complex from the
support under reaction conditions or the loss of the catalyst
during recovery.
2.5. Recyclability of the Cu-NHC-SBA-15 catalyst
To verify whether the observed catalysis was due to the hetero-
geneous Cu-NHC-SBA-15 catalyst or leached copper species in
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4. Experimental section
4.1. Synthesis of the Cu-NHC complex
4.1.1. Synthesis of 4-allyl-2,6-diisopropylaniline.30 Allyl
chloride (5.80 g, 75.8 mmol) and 2,2-diisopropylaniline
(15.1 g, 85.1 mmol) were mixed in 40 mL xylene and heated to
120 °C for 12 h. After the mixture was cooled to room tempera-
ture, it was stirred with 20% NaOH (20 mL), washed with satu-
rated NaCl (3 × 20 mL), and then dried by distilling off the
xylene. Fractional distillation afforded the pure N-allyl-2,6-di-
isopropylaniline as a colorless liquid.
N-Allyl-2,6-diisopropylaniline (5.80 g, 26.7 mmol) and ZnCl2
(4.0 g, 29.4 mmol) were mixed in 15 mL of xylene and heated
to reflux for 4 h to induce an aza-Claisen rearrangement. The
mixture was cooled to room temperature and then stirred with
NaOH (4.5 g in 20 mL H2O). The organic layer was extracted
with ether, washed with saturated NaCl, and dried over anhy-
drous MgSO4. The pure 4-allyl-2,6-diisopropylaniline was sep-
arated from the unreacted N-allyl-2,6-diisopropylaniline
through fractional distillation as a colorless liquid (2.68 g,
46%). 1H NMR (CDCl3, 300 MHz): δ 6.85 (s, 2H, ArH),
5.95–6.06 (m, 1H, CHvCH2), 5.05–5.15 (m, 2H, CHvCH2),
3.71 (s, 2H, NH2), 3.35 (d, 2H, Ph–CH2), 2.93–3.02 (m, 2H,
CH(CH3)2), 1.29 (d, 12H, CH(CH3)2). 13C NMR (CDCl3,
75 MHz): δ 138.9, 133.1, 130.2, 123.4, 115.4, 40.5, 28.5, 22.9.
The spectral data matched those previously reported.30
Fig. 7 Effect of removal of the Cu-NHC-SBA-15 catalyst during the
hydrothiolation of phenylacetylene with thiophenol (a) after removal of
Cu-NHC-SBA-15 and (b) without removal of Cu-NHC-SBA-15. The
arrow indicates the time when the Cu-NHC-SBA-15 catalyst was
removed from the reaction mixture. Reaction conditions: 1.0 mmol
phenylacetylene, 1.1 mmol thiophenol, 50 mg Cu-NHC-SBA-15
(5.3 μmol Cu), 2 mL 1,2-DCE, 70 °C.
4.1.2. Synthesis of N,N′-bis-(4-allyl-2,6-diisopropyl-phenyl)-
ethanediimine.31 A 50 mL round-bottom flask was charged
with 4-allyl-2,6-diisopropylaniline (1.32 g, 6.08 mmol), 40%
aqueous solution of glyoxal (0.44 g, 3 mmol) and 25 mL of
absolute isopropyl alcohol. A few drops of formic acid were
added as a catalyst. The color of the reaction mixture turned
from colorless to yellow gradually, and a yellow precipitate
appeared after a few hours. The reaction mixture was stirred
for 2 days at room temperature. The solid was collected by fil-
tration and thoroughly washed with cold methanol, followed
by drying under vacuum (2.25 g, 81%). 1H NMR (CDCl3,
300 MHz): δ 8.12 (s, 2H, NvCH), 7.03 (s, 4H, ArH), 5.98–6.11
(m, 2H, CHvCH2), 5.11–5.19 (m, 4H, CHvCH2), 3.42 (d, 4H,
Ph–CH2), 2.89–3.01 (m, 4H, CH(CH3)2), 1.23 (d, 24H,
CH(CH3)2). 13C NMR (CDCl3, 75 MHz): δ 163.7, 146.6, 138.1,
137.2, 136.9, 123.7, 116.1, 40.6, 28.4, 23.8. The spectral data
matched those previously reported.31
Table 5 Recyclability of the Cu-NHC-SBA-15 catalyst for hydrothiola-
tion of phenylacetylene with thiophenola
Run
1
2
3
4
5
6
Yieldb (%)
98
86
86
84
85
85
a Reaction conditions: 1.0 mmol phenylacetylene, 1.1 mmol
thiophenol, 50 mg Cu-NHC-SBA-15 (5.3 μmol Cu), 2 mL 1,2-DCE,
70 °C, 24 h. b Isolated yield.
3. Conclusions
We developed a new heterogeneous Cu-NHC complex co-
valently linked to SBA-15 for the highly stereoselective hydro-
thiolation of alkynes and electron-deficient alkenes under
mild reaction conditions. The Cu-NHC-SBA-15 is an efficient
and recyclable heterogeneous hydrothiolation catalyst exhibit-
ing broad substrate scope and higher activity and product
stereoselectivity to Z-vinyl sulfides than its homogeneous
analog. In addition, the supported Cu-NHC-SBA-15 catalyst
demonstrated high activity and perfect selectivity for the
hydrothiolation of electron-deficient alkenes. The process
represents a green methodology for synthesizing regio-defined
C–S bond compounds and extends the synthetic utility of
hydrothiolation reactions.
4.1.3. Synthesis of 1,3-bis-(4-allyl-2,6-diisopropyl-phenyl)-
imidazolium chloride.31 Under N2, chloromethylethyl ether
(0.49 mL, 5.2 mmol) in 10 mL THF was added to N,N′-bis-
(4-allyl-2,6-diisopropyl-phenyl)ethanediimine (1.20 g, 2.6 mmol)
in 10 mL THF, and then 2 drops of water were added. The
mixture was stirred at 40 °C for 16 h. The solvent was removed
to obtain an oil mixture which was purified using a SiO2
column with the eluent CH2Cl2–MeOH (15/1, v/v) to obtain a
1
yellow solid (0.53 g, 41%). H NMR (CDCl3, 300 MHz): δ 9.93
(s, 1H, NC(HCl)N), 8.14 (s, 2H, NvCH), 7.15 (s, 4H, ArH),
5.93–6.04 (m, 2H, CHvCH2), 5.05–5.20 (m, 4H, CHvCH2),
3.47 (d, 4H, Ph–CH2), 2.40–2.44 (m, 4H, CH(CH3)2), 1.25 (d,
24H, CH(CH3)2). 13C NMR (CDCl3, 75 MHz): δ 151.1, 145.3,
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144.7, 139.4, 136.5, 128.4, 127.6, 125.3, 117.5, 40.9, 29.5, 25.3,
24.3. The spectral data matched those previously reported.31
4.1.4. Synthesis of Cu-NHC complex.32 An oven-dried
25 mL Schlenk flask was charged with 1,3-bis-(4-allyl-2,6-diiso-
propyl-phenyl)imidazolium chloride (0.50 g, 0.99 mmol), CuCl
(0.098 g, 0.99 mmol), NaOtBu (0.095 g, 0.99 mmol), and 10 mL
anhydrous THF. The resulting mixture was stirred for 20 h at
room temperature. The reaction mixture was filtered over
Celite, and the solvent was removed in vacuo. The product was
dried under vacuum to obtain a pale brown powder (0.48 g,
85%). 1H NMR (CDCl3, 300 MHz): δ 7.13 (s, 4H, ArH),
5.93–6.05 (m, 2H, CHvCH2), 5.17–5.24 (m, 4H, CHvCH2),
3.49 (d, 4H, Ph–CH2), 2.52–2.59 (m, 4H, CH(CH3)2), 1.31 (d,
12H, CH(CH3)2), 1.24 (d, 12H, CH(CH3)2). 13C NMR (CDCl3,
75 MHz): δ 186.0, 145.9, 142.8, 137.1, 124.7, 123.6, 117.0, 40.8,
29.1, 25.3, 24.3. C33H44ClCuN2 (567.71): calcd C 69.82, H 7.81,
N 4.93; found C 69.93, H 7.93, N 4.56.
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