Highly β(Z)-Selective Hydrosilylation of Terminal Alkynes Catalyzed by Thiolate-Bridged Dirhodium Complexes

Article abstract of DOI:10.1021/acs.orglett.8b02267

A series of novel monothiolate-bridged dirhodium complexes, [Cp Rh(μ-SR)(μ-Cl)₂RhCp ][BF₄] {Cp? = ??⁵-C₅Me₅, R = tertiary butyl (^tBu), 1a; R = ferrocenyl (Fc), 1b; R = adamantyl (Ad), 1c} were designed and successfully synthesized, which can smoothly facilitate highly regioselective and stereoselective hydrosilylation of terminal alkynes to afford β(Z) vinylsilanes with good functional group compatibility. Furthermore, the hydride bridged dirhodium complex [Cp Rh(μ-S^tBu)(μ-Cl)(μ-H)RhCp ][BF₄] (5) as a potential intermediate was obtained by the reaction of 1a with excess HSiEt₃.

Full text of DOI:10.1021/acs.orglett.8b02267

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Highly β(Z)‑Selective Hydrosilylation of Terminal Alkynes Catalyzed
by Thiolate-Bridged Dirhodium Complexes
Xiangyu Zhao,† Dawei Yang,*,† Yahui Zhang,† Baomin Wang,† and Jingping Qu†,‡
†State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, People’s Republic of China
‡Key Laboratory for Advanced Materials, East China University of Science and Technology, Shanghai 200237, People’s Republic of China
S
* Supporting Information
ABSTRACT: A series of novel monothiolate-bridged dirho-
dium complexes, [Cp*Rh(μ-SR)(μ-Cl)2RhCp*][BF4] {Cp* =
η5-C5Me5, R = tertiary butyl (tBu), 1a; R = ferrocenyl (Fc), 1b;
R = adamantyl (Ad), 1c} were designed and successfully
synthesized, which can smoothly facilitate highly regioselective
and stereoselective hydrosilylation of terminal alkynes to afford
β(Z) vinylsilanes with good functional group compatibility.
Furthermore, the hydride bridged dirhodium complex [Cp*Rh(μ-StBu)(μ-Cl)(μ-H)RhCp*][BF4] (5) as a potential inter-
mediate was obtained by the reaction of 1a with excess HSiEt3.
inylsilanes are a versatile category of building blocks for
attention for the distinctive cooperative effect between two
Vthe synthesis of various functional organic compounds metallic centers,13 which are also common in the catalytic
cycles mediated by metalloenzymes in biology.14 For instance,
carboxylate-bridged dirhodium complexes were well-developed
as excellent catalysts for highly site-selective and stereoselective
functionalization of nonactivated C−H bonds.15 Compared
with extensive development in mononuclear metal catalysts
for hydrosilylation of terminal alkynes, the rational design of
bimetallic catalysts toward the catalytic precise synthesis of
β(Z) vinylsilanes is still rare.
and polymeric organsilicon materials.1 The hydrosilylation of
alkynes promoted by transition-metal complexes is one of the
most straightforward and effective approaches to access such
compounds.2 However, using this synthetic method with ter-
minal alkynes commonly generates three regioselective and
stereoselective isomers, namely, β(Z), β(E), and α vinylsilanes,
as shown in Scheme 1. Except for this key issue, some other
Our group focuses on the construction of novel thiolate-
bridged bimetallic or multimetallic complexes for inert small
molecule activation and catalytic transformation,16 especially bio-
mimetic nitrogen fixation using the diiron scaffold.17 To further
develop richer functions of catalytic conversion, we also extended
to other transition metals in Group VIII, such as Ru, Ni, and
Co.18 Herein, we design and synthesize a series of monothiolate-
bridged dirhodium complexes, which can smoothly facilitate
Z-selective anti-Markovnikov hydrosilylation of terminal alkynes
with tertiary silanes.
Scheme 1. Catalytic Hydrosilylation of Terminal Alkynes
side reactions such as alkyne hydrogenation3 make product
distribution more complicated. Therefore, the precise control
of regioselective and stereoselective synthesis of only one
desired isomer with high activity is still challenging.
Initially, to provide stable reaction framework and enough
interaction space for catalytic transformation of organic
substrates, we constructed a series of monothiolate-bridged
dirhodium complexes [Cp*Rh(μ-SR)(μ-Cl)2RhCp*][BF4]
Over past decades, several late-transition-metal complexes
have been reported for effective alkyne hydrosilylation with
high selectivity, such as classic noble Ru,4 Rh,5 Ir,6 and Pt7
catalysts. Recently, because of increasing concerns of sustain-
able development, Earth-abundant base-metal catalysts, such as
iron and cobalt, were investigated for alkyne hydrosilylation.8,9
Particularly, recent explosively growing Co complexes were
proven to be effective for facilitating alkyne hydrosilylation
to selectively produce β(Z),10 β(E),11 and α12 vinylsilanes.
In both β vinylsilanes, highly selective synthesis for thermo-
dynamically unstable β(Z) vinylsilanes is regarded as a more
challenging task,4a,b,e−n,6b−d,7e,10 because β(Z) product is
usually prone to isomerize to the stable β(E) isomer.5c,6a,b,10a
In recent decades, highly selective organic transformations
promoted by bimetallic complexes have attracted considerable
t
(R = Bu, 1a; R = Fc, 1b; R = Ad, 1c). As illustrated in
Scheme 2, treatment of precursor [Cp*Rh(μ-Cl)3RhCp*]-
[BF4]19 with 1 equiv of sodium thiolate with large sterically
hindered substituents, such as tBu, Fc, and Ad, from −78 °C to
room temperature gave the corresponding desired complexes
1a−1c in good yields. These complexes were fully charac-
terized by 1H NMR, 13C NMR, ESI-HRMS, and elemental ana-
lysis, as well as single-crystal X-ray diffraction (XRD) analysis.
Received: July 20, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02267
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
a
Scheme 2. Synthesis of Dirhodium Complexes
Table 1. Catalytic Activity of Rhodium Complexes
b
b
entry
catalyst
yield (%)
β(Z)/β(E)/α
c
1
2
3
4
5
6
7
8
(PPh3)3RhCl
41
22
11
2
99
99
99
98
6:93:1
89:9:2
17:83:0
19:70:11
50:50:0
98:1:1
c
[Cp*Rh(MeCN)3][PF6]2
[(cod)2Rh][PF6]/2PPh3
[(cod)Rh(μ-Cl)]2
[Cp*Rh(μ-Cl)Cl]2
c
Only one proton signal at ∼1.65 ppm and two carbon signals
at ∼10 and 97 ppm suggest that complexes 1a−1c are all in
a symmetric arrangement in the solution state. Furthermore,
ESI-HRMS and elemental analysis data provide strong evi-
dence for the determination of their molecular composition.
The above spectroscopic data are in good agreement with their
solid-state structures. As shown in Figure 1, the crystal struc-
tures of 1a−1c reveal that the two Rh centers are linked by two
chloride groups and one thiolate ligand in trigonal bipyramid
geometry. The Rh1−Rh2 distances of 1a−1c are 3.2417(3),
3.2026(4), and 3.2225(4) Å, which are obviously longer than
those observed in other reported thiolate-bridged dirhodium
complexes.20 The two Cp* ligands in 1a−1c are almost parallel,
with dihedral angles of 15.33(10)°, 9.74(17)°, and 18.96(15)°.
This minor discrepancy is attributed to different steric hindrance
effect of substituents in the bridging thiolate ligands. These
monothiolate-bridged dirhodium complexes represent a novel
family with only one {Rh−S−Rh} active fragment, which are
obviously different from conventional dithiolate or trithiolate-
bridged cyclopentadienyl dirhodium complexes.21
Next, to examine the catalytic activity of monothiolate-
bridged dirhodium complexes 1a−1c, the hydrosilylation of
terminal alkynes was investigated as a model reaction, and
the experimental results are summarized in Table 1. Initially, to
obtain deep insight into the influence of bimetallic cooperative
effect on catalytic activity and reaction selectivity, some repre-
sentative monorhodium and dirhodium complexes were also
chosen as catalysts. In the presence of 4 mol % monorhodium
complexes as catalysts, the reactions of phenylacetylene (2a)
with HSiEt3 (3a) at room temperature exhibited poor catalytic
activity and irregular product distributions (Table 1, entries
1−3). When using 2 mol % dirhodium complex [(cod)Rh(μ-
Cl)]25a,22 (cod = 1,5-cyclooctadiene), lower activity and poorer
selectivity were observed (Table 1, entry 4). Interestingly,
when employing Cp* to substitute cod as the auxiliary ligand,
the activity was remarkably improved; however, the selectivity
1a
1b
1c
98:1:1
94:5:1
a
Reaction conditions: phenylacetylene (0.5 mmol), HSiEt3 (0.6
mmol, 1.2 equiv), catalyst (0.01 mmol, 2 mol %), CDCl3 (0.6 mL), rt,
b
for 4 h. Yields and selectivity are calculated, based on mellithene as
c
an internal standard, via 1H NMR. Catalyst concentration = 4 mol %.
still was not very good, with two main products β(Z)/β(E)
formed at a ratio of 1:1 (Table 1, entry 5). To our delight,
complexes 1a−1c were proven to be efficient catalysts for
hydrosilylation of terminal alkynes without any additives under
the same conditions. With these dinuclear systems, β(Z) vinyl-
silanes were all selectively obtained in excellent yields (Table 1,
entries 6−8), which indicates the steric effect of substituent in
thiolate ligands has only a small influence on product selec-
tivity. In addition, solvent effect was also not observed in this
catalytic system, except for MeCN (see the Supporting Infor-
mation (Table S1, entries 3−7)). It is especially noteworthy
that there was no obvious isomerization of β(Z) when the
reaction time was prolonged to 24 h, which indicates that the
thiolate-bridged dirhodium complexes can efficiently hinder
the isomerization (Table S1, entries 8−10).
In addition, we chose 1a as the catalyst and investigated
reactions of phenylacetylene and a series of silanes for activity
and regioselectivity (see Table 2). When utilizing tertiary silanes
such as HSiEt3 or HSiPh3 (Table 2, entries 1 and 2), excellent
yields and selectivity were observed. When the substituent of
tertiary silanes was replaced by an electron-withdrawing alkoxyl
such as OMe or OEt, the yields were significantly decreased;
however, β(Z) vinylsilanes were still obtained as the main pro-
ducts. Interestingly, when secondary silane H2SiPh2 (Table 2,
entry 5) was adopted, the main product was β(E) vinylsilane.
However, there is no hydrosilylation product observed using
H2SitBu2.
Figure 1. ORTEP (ellipsoids at 50% probability) diagrams of complexes 1a (left), 1b (middle), 1c (right). All hydrogen atoms and counterion
−
BF4 are omitted for the sake of clarity.
B
DOI: 10.1021/acs.orglett.8b02267
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Letter
dirhodium catalytic system exhibits good functional group
tolerance under mild conditions. Furthermore, we also explored
the hydrosilylation of internal alkynes under similar conditions.
Taking diphenylacetylene as an example, the conversion and
yield are obviously reduced in contrast with terminal alkynes. In
addition, the selectivity is also poor, with a β(Z)/β(E)/α ratio of
40:58:2.
Table 2. Hydrosilylation of Phenylacetylene with Different
Silanes Catalyzed by Complex 1a
a
b
b
b
entry
[Si]
conversion (%) yield (%) β(Z)/β(E)/α
To shed light on the possible mechanism, the stoichiometric
reactions of 1a with phenylacetylene and HSiEt3 were performed.
No expected ligand exchange reaction between complex 1a
and phenylacetylene was observed, which may be attributed to
poor coordination capacity of neutral alkyne, compared to
negative bridged ligands in 1a. This result is absolutely distinct
from the reactivity of thiolate-bridged diiron complexes toward
alkynes.23 To our delight, treatment of 1a with excess HSiEt3
in CH2Cl2 at 35 °C for 24 h afforded a rare monothiolate-
bridged dirhodium hydride complex [Cp*Rh(μ-StBu)(μ-
Cl)(μ-H)RhCp*][BF4] (5) in 33% yield (see Scheme 4).
1
2
3
4
5
HSiEt3 (3a)
HSiPh3 (3b)
HSi(OMe)3 (3c)
HSi(OEt)3 (3d)
H2SiPh2 (3e)
100
100
94
86
100
99
99
68
69
54
98:1:1
99:0:1
85:15:0
98:2:0
1:94:5
a
Reaction conditions: phenylacetylene (0.5 mmol), silane (0.6 mmol,
1.2 equiv), 1a (0.01 mmol, 2 mol %), CDCl3 (0.6 mL), rt, for 4 h.
The conversion of phenylacetylene, the yields of vinylsilanes
containing β(Z), β(E), α and the selectivity were calculated, based
b
1
on using mellithene as an internal standard, via H NMR.
With the optimal reaction conditions in hand, we next
investigated the substrate scope of the hydrosilylation reaction
using complex 1a (2 mol %) as the catalyst and HSiEt3 as the
silane source. As shown in Scheme 3, complex 1a was proven
Scheme 4. Reaction of Complex 1a with HSiEt3
Scheme 3. Hydrosilylation of Terminal Alkynes Catalyzed
a
by Complex 1a
Notably, thiolate-bridged diiron,17 dicobalt,24 and nickel−iron25
complexes cannot activate tertiary silane HSiEt3 under similar
1
conditions. The H NMR spectrum of 5 exhibits a character-
istic resonance for the bridging hydride as a triplet at δ =
−10.48 ppm with 1JRh,H = 24 Hz in the high-field region, which
is similar to those found in the previously reported dirhodium
hydride bridged complexes.21d,26 In addition, the ESI-HRMS
data provides further experimental evidence for the existence
of a hydride subunit.
As expected, the molecular structure of complex 5 shown
in Figure 2 was fully consistent with the structure predicted
Figure 2. ORTEP (ellipsoids at 50% probability) diagram of complex
−
5. All hydrogen atoms and BF4 are omitted for the sake of clarity,
except for the bridging hydride.
a
General reaction conditions: Terminal alkynes (0.5 mmol), HSiEt3
(0.6 mmol, 1.2 equiv), 1a (0.01 mmol, 2 mol %), CDCl3 (0.6 mL), rt,
for 4 h; yield and selectivity isomers were determined, based on
mellithene as an internal standard, via H NMR. 1,1,2,2-tetrachloro-
ethane was used as an internal standard.
1
b
by the above spectroscopic data. Because of the formation of
three-center two-electron bond between two Rh centers and a
bridging hydride, the Rh1−Rh2 distance (2.7280(9) Å) of 5 is
significantly shortened, compared with that of 1a (3.2417(3) Å).
In contrast with 1a, another obvious structural feature is the
change of dihedral angle between the two Cp* ligands from
15.33(10)° to 49.00(24)°. Importantly, complex 5 also shows
good catalytic activity toward selectively furnishing β(Z)
vinylsilanes (86% yield, β(Z)/β(E)/α ratio = 87:9:4) although
to be effective in the selective formation of β(Z) vinylsilanes
from aryl- or alkyl-substituted terminal alkynes in high yields.
Whether terminal alkynes were modified by electron-with-
drawing or electron-donating groups such as alkoxyl, halogen,
amine, hydroxyl, ester, and β(Z) vinylsilanes were always
selectively obtained in high yields. These results indicate this
C
DOI: 10.1021/acs.orglett.8b02267
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
ORCID
the selectivity is not better than those of complexes 1a−1c
under the same conditions.
Dawei Yang: 0000-0002-7646-3891
Baomin Wang: 0000-0001-9058-4983
Jingping Qu: 0000-0002-7576-0798
Furthermore, we also conducted deuterium-labeling experi-
ments using PhCCD or DSiEt3 under similar conditions.
Hydrosilylation of PhCCD with HSiEt3 in the presence of
1a as the catalyst exhibits excellent selectivity with a β(Z)/
Notes
1
2
β(E)/α ratio of 97:2:1. The H and H NMR spectra of
d-β(Z)-4a clearly confirmed that the deuterium atom is almost
intact on the C atom bearing the silyl group. Moreover, the
deuteriosilylation of phenylacetylene with DSiEt3 also displays
high β(Z)-selectivity and the deuterium atom is located at the
C atom linked to the phenyl group. The above results suggest
that this hydrosilylation of terminal alkynes promoted by
dirhodium complexes proceeds through a trans-addition of
silane to the CC bond.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (Nos. 21571026, 21690064, 21231003)
and the Program for Changjiang Scholars and Innovative
Research Team in University (No. IRT13008), and the “111”
project of the Ministry of Education of China.
Based on the above experimental results and some classic
mechanistic proposals for hydrosilylation of alkyne such as the
Chalk−Harrod mechanism,2a we proposed a potential pathway
for β(Z)-selective hydrosilylation of terminal alkyne catalyzed by
thiolate-bridged dirhodium complexes. First, heterolytic splitting
of the silane mediated by dirhodium centers and terminal alkynes
gave an intermediate hydride-bridged complex 5 and active
species [R3Si−CHC−R′]+. Then, nucleophilic attack of the
bridging hydride over [R3Si−CHC−R′]+ facilely gives β(Z)
vinylsilanes, which is similar to the recently reported mono-
nuclear Rh system.3a In this process, the cooperative effect of
dirhodium centers and steric effect of two Cp* ligands are
essential to access the high regioselectivity and stereoselectivity of
hydrosilylation of terminal alkynes, and a similar strategy was also
adopted in the thiolate-bridged diruthenium systems.27
In summary, based on the steric and electronic effects of
thiolate and Cp* ligands, a class of novel monothiolate-bridged
dirhodium complexes were precisely designed and successfully
constructed. By the cooperative effect between the two Rh
centers, highly regioselective and stereoselective hydrosilyla-
tion of terminal alkynes to afford β(Z)-vinylsilanes was achieved
in high yields and isomerization from β(Z) to β(E) isomer was
efficiently suppressed. Moreover, this bimetallic catalytic system
exhibits good functional group compatibility and broad substrate
scope. Unexpectedly, the hydride bridged dirhodium complex 5
as an intermediate species was successfully synthesized, which
is essential to reveal the actual reaction pathway. Further
mechanistic investigations for this catalytic transformation and
other catalytic applications are in progress.
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ASSOCIATED CONTENT
* Supporting Information
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S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.8b02267.
Experimental details and spectroscopic data (PDF)
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AUTHOR INFORMATION
Corresponding Author
■
́
́
Nicolas, M.; Sanz Miguel, P. J.; Polo, V.; Fernandez-Alvarez, F. J.;
́
Perez-Torrente, J. J.; Oro, L. A. Chem. Commun. 2012, 48, 9480−
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*E-mail: yangdw@dlut.edu.cn.
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DOI: 10.1021/acs.orglett.8b02267
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