Carboxylate-Assisted β-(Z) Stereoselective Hydrosilylation of Terminal Alkynes Catalyzed by a Zwitterionic Bis-NHC Rhodium(III) Complex

Article abstract of DOI:10.1021/acscatal.0c01582

The zwitterionic compound [Cp*RhCl{(MeIm)2CHCOO}] is an efficient catalyst for the hydrosilylation of terminal alkynes with excellent regio- and stereoselectivity toward the less thermodynamically stable β-(Z)-vinylsilane isomer under mild reaction conditions. A broad range of linear 1-alkynes, cycloalkyl acetylenes, and aromatic alkynes undergo the hydrosilylation with HSiMe2Ph to afford the corresponding β-(Z)-vinylsilanes in quantitative yields in short reaction times. The reaction of aliphatic alkynes with HSiEt3 is slower, resulting in a slight decrease of selectivity toward the β-(Z)-vinylsilane product, which is still greater than 90%. However, a significant selectivity decrease is observed in the hydrosilylation of aromatic alkynes because of the β-(Z) → β-(E) vinylsilane isomerization. Moreover, the hydrosilylation of bulky alkynes, such as t-Bu-CCH or Et3SiCCH, is unselective. Experimental evidence suggests that the carboxylate function plays a key role in the reaction mechanism, which has been validated by means of density functional theory calculations, as well as by mass spectrometry and labeling studies. On the basis of previous results, we propose an ionic outer-sphere mechanism pathway in which the carboxylate fragment acts as a silyl carrier. Namely, the hydrosilylation mechanism entails the heterolytic activation of the hydrosilane assisted by the carboxylate function to give the hydrido intermediate [Cp*RhH{(MeIm)2CHCOO-SiR3}]+. The transference of the silylium moiety from the carboxylate to the alkyne results in the formation of a flat β-silyl carbocation intermediate that undergoes a hydride transfer from the Rh(III) center to generate the vinylsilane product. The outstanding β-(Z) selectivity results from the minimization of the steric interaction between the silyl moiety and the ligand system in the hydride transfer transition state.

Full text of DOI:10.1021/acscatal.0c01582

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Article
Carboxylate-Assisted #-(Z) Stereoselective Hydrosilylation of Terminal
Alkynes Catalyzed by a Zwitterionic Bis-NHC Rhodium(III) Complex
Raquel Puerta-Oteo, Julen Munarriz, Victor Polo, M. Victoria Jiménez, and Jesús J. Pérez-Torrente
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.0c01582 • Publication Date (Web): 05 Jun 2020
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Carboxylate-Assisted -(Z) Stereoselective
Hydrosilylation of Terminal Alkynes Catalyzed by
a Zwitterionic Bis-NHC Rhodium(III) Complex.
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Raquel Puerta-Oteo,a Julen Munarriz,b,c Víctor Polo,b M. Victoria Jiménez,a,* and Jesús J.
Pérez-Torrente.a,*
(a) Departamento de Química Inorgánica, Instituto de Síntesis Química y Catálisis Homogénea
(ISQCH), Universidad de Zaragoza–CSIC, Facultad de Ciencias, C/ Pedro Cerbuna 12, 50009
Zaragoza, Spain.
(b) Departamento de Química Física, Instituto de Biocomputación y Física de Sistemas
Complejos (BIFI), Universidad de Zaragoza, Facultad de Ciencias, C/ Pedro Cerbuna 12, 50009
Zaragoza, Spain.
(c) Department of Chemistry & Biochemistry, University of California Los Angeles
(UCLA), Los Angeles, California 90095, United States.
KEYWORDS: Hydrosilylation, Alkynes, Vinylsilanes, Rhodium, N-heterocyclic carbenes
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ABSTRACT
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The zwitterionic compound [Cp*RhCl{(MeIm)2CHCOO}] is an efficient catalyst for the
hydrosilylation of terminal alkynes with excellent regio- and stereoselectivity towards the less
thermodynamically stable β-(Z)-vinylsilane isomer under mild reaction conditions. A broad
range of linear 1-alkynes, cycloalkyl acetylenes and aromatic alkynes undergo the
hydrosilylation with HSiMe2Ph to afford the corresponding β-(Z)-vinylsilanes in quantitative
yields in short reaction times. The reaction of aliphatic alkynes with HSiEt3 is slower, resulting
in a slight decrease of selectivity towards the β-(Z)-vinylsilane product, which is still greater
than 90%. However, a significant selectivity decrease is observed in the hydrosilylation of
aromatic alkynes due to the β-(Z)β-(E) vinylsilane isomerization. Moreover, the
hydrosilylation of bulky alkynes, such as t-Bu-C≡CH or Et3SiC≡CH, is unselective.
Experimental evidences suggest that the carboxylate function plays a key role in the reaction
mechanism, which has been validated by means of DFT calculations, as well as by mass
spectrometry and labelling studies. On the basis of previous results, we propose an ionic outer-
sphere mechanism pathway in which the carboxylate fragment acts as a silyl carrier. Namely,
the hydrosilylation mechanism entails the heterolytic activation of the hydrosilane assisted by
the carboxylate function to give the hydrido intermediate [Cp*RhH{(MeIm)2CHCOO-SiR3}]+.
The transference of the silylium moiety from the carboxylate to the alkyne results in the
formation of a flat -silyl carbocation intermediate that undergoes a hydride transfer from the
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Rh(III) center to generate the vinylsilane product. The outstanding -(Z) selectivity results from
the minimization of the steric interaction between the silyl moiety and the ligand system
in the hydride transfer transition state.
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INTRODUCTION
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Vinylsilanes are valuable building blocks in organic synthesis, polymer chemistry and
materials science because of their versatility, ease of handling, low toxicity, and reasonable
stability relative to other vinyl-metal species.1 They have been traditionally prepared by
stoichiometric methods from organometallic reagents and the development of more sustainable
synthetic strategies for their production is still an important current scientific challenge.
Transition-metal-catalyzed hydrosilylation of alkynes has been regarded as one of the most
straightforward and atom-economical methodologies for accessing versatile vinylsilanes.
However, the hydrosilylation of terminal alkynes can afford several vinylsilanes isomers and
thus, the control of the regio- and stereoselectivity along the H–Si addition process is a major
issue (see Scheme 1).2 The Markovnikov addition results in the formation of the α-vinylsilane
isomer. However, the reaction may as well proceed with anti-Markovnikov regioselectivity to
afford the β-(E)-vinylsilane and β-(Z)-vinylsilane stereoisomers, the syn- and anti-addition
reaction products, respectively. The selective synthesis of α- and β-(Z)-vinylsilanes is
considered to be more challenging than the formation of the thermodynamically more stable β-
(E)-vinylsilane isomer.2d In this context, the β-(Z) isomerization to β-(E)-vinylsilanes under
reaction conditions is an additional hurdle to the development of the β-(Z)-selective alkyne
hydrosilylation catalysts. Furthermore, the formation of the competitive dehydrogenative
silylation products, namely alkynylsilane and the corresponding alkene, has been frequently
observed for some catalysts (Scheme 1).3 Even though a number transition-metal-based
catalysts including both noble-metals, such as Rh,4 Ir5 and Ru,6 and more recently base-metals,
such as Co, Fe7 and Mn,8 for the preparation of β-(Z)-vinylsilanes have been reported, the
development of improved catalysts to overcome the thermodynamic stability of the β-(E)-
vinylsilane with wider applicability is still necessary. In this context, a deeper understanding of
the reaction mechanisms is pivotal for the rational design of more active and selective catalytic
systems.
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The catalytic hydrosilylation of terminal alkynes usually proceeds with anti-Markovnikov
regiochemistry and predominantly syn-addition stereochemistry, which can be rationalized by
the classic inner-sphere Chalk-Harrod mechanism. This mechanism has been put forward to
explain the formation of the β-(E)-vinylsilane isomer in Pt-based transition-metal catalysts.9
However, it cannot account for the formation of the β-(Z)-vinylsilane isomer or the
alkynylsilane product resulting from the dehydrogenative silylation process. The formation of
both types of products can be explained by a modified Chalk−Harrod mechanism that entails
the silylmetalation of the alkyne by migratory insertion into the M−Si bond, instead the M–H
bond, to give a (Z)-silylvinylene metal complex.10 The formation of the β-(Z)-vinylsilane
requires the metal-assisted isomerization of the (Z)-silylvinylene to the thermodynamically
more favorable (E)-silylvinylene through the well-established Crabtree11 and Ojima12
mechanisms, which involve a metallacyclopropene or zwitterionic carbene species,
respectively. However, the metal-catalyzed hydrosilylation reactions can also proceed through
a heterolytic Si–H bond cleavage that gives rise to silylium-type putative catalytic
intermediates.13 This mode of activation of silanes constitutes an important alternative to the
conventional Chalk–Harrod mechanism that entails the oxidative addition of the silane Si–H
bond at the metal center. Such advances have paved the way for the rational design of more
active and selective catalysts.14
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Scheme 1. Hydrosilylation and dehydrogenative silylation of terminal alkynes.
Importantly, ligand design plays a crucial role when proposing novel catalysts, as they are
able to modulate the steric and electronic properties of the metal center. In this sense, N-
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heterocyclic carbenes (NHCs) have become ubiquitous as ligands in transition metal mediated
homogeneous catalysis due the easy access to a variety of topologies with tuned electronic
properties that adapt to the specific requirements of individual catalytic transformations.15 In
this context, there has been a significant interest in the development of alkyne hydrosilylation
rhodium catalysts based on functionalized N-heterocyclic carbene ligands of hemilabile
character.16 Recently, we have proposed a novel ionic outer-sphere mechanism for the
hydrosilylation of terminal alkynes catalyzed by [MIII(bis-NHC)I2]+ (M = Ir, Rh; bis-NHC =
methylene-bis(N-2-methoxyethyl)imidazole-2-ylidene) complexes, featuring a bis-NHC ligand
functionalized with labile 2-methoxyethyl groups at the wingtips, able to explain the selective
formation of the β-(Z)-vinylsilane isomer.17 This outer-sphere mechanism involves the
heterolytic splitting of the hydrosilane assisted by the metal center and a molecule of acetone,
which is the reaction solvent, that acts as a silane-shuttle and transfers the silylium moiety to
the alkyne. In sharp contrast, the related rhodium complex [RhIII(bis-NHC)(CF3COO)2]+ is
selective for the α-vinylsilane isomer in the hydrosilylation of a wide range of terminal alkynes
which is believed to proceed with the assistance of the trifluoroacetate ligands.18 These results
point out the wide variability in the catalytic selectivity when tuning the non-innocent ligands,
as a consequence of their active participation in the catalytic steps.
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Our group has an ongoing research interest on the potential of a carboxylate-functionalized
methylene-bridged bis-NHC ligand for the construction of a versatile metal-ligand platform
with application in catalysis.19 In this regard, we have shown that zwitterionic iridium
complexes such as [Ir(cod){(MeIm)2CHCOO}] and [Cp*IrCl{(MeIm)2CHCOO}] (MeIm = 3-
methylimidazol-2-yliden-1-yl) efficiently catalyze both the hydrogenation of CO2 to formate,20
and the oxidation of water using chemical oxidants.21 In both cases, experimental an theoretical
studies have shown that the κ3-C,C’,O-tridentate coordination mode of the functionalized bis-
NHC ligand plays a key role in the stabilization of catalytic intermediates. However, the
carboxylate function at the linker not only confers hemilabile properties to the ligand but also
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behaves as a reactive site. In this sense, we have shown that the carboxylate moiety in
[Cp*IrCl{(MeIm)2CHCOO}] is involved in the precatalyst activation step leading to the active
species for the reduction of CO2 with hydrosilanes to silylformates.22
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These precedents and the potential of the uncoordinated carboxylate moiety to assist non-
classical hydrosilylation pathways such as a silane-shuttle or proton-shuttle, facilitating the
alkyne-vinylidene tautomerization,23 have prompted us to study the catalytic activity of the
related rhodium(III) compound [Cp*RhCl{(MeIm)2CHCOO}] in the hydrosilylation of
terminal alkynes. Remarkably, the investigation of the reaction mechanism has disclosed the
key role of the carboxylate fragment in the selective formation of the less thermodynamically
stable β-(Z) isomer in the hydrosilylation of terminal alkynes. This catalyst constitutes a step
forward in the exploration of new catalysts with unconventional mechanisms.
RESULTS AND DISCUSSION
Preliminary catalyst screening for terminal alkyne hydrosilylation. The zwitterionic
complexes depicted in Chart 1 were screened as catalyst precursors for the hydrosilylation of
terminal alkynes. The hydrosilylation tests were carried out by using phenylacetylene and oct-
1-yne as terminal alkynes and dimethylphenylsilane as a representative hydrosilane. The
catalytic reactions were performed under an argon atmosphere in CDCl3 at 333 K with 1 mol%
catalyst loading, [catalyst] = 1.54 mM, and routinely monitored by 1H NMR spectroscopy with
anisole as internal standard. The outcome of these preliminary experiments is summarized in
Table 1.
O
O
O
O
+
+
+
M
Ru
M
N
-
-
N
N
N
N
Cl
Cl
N
N
N
N
N
N
N
-
O
O
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2
4
M = Rh, ; Ir,
5
M = Rh, ; Ir,
3
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Chart 1. Selected zwitterionic rhodium, iridium and ruthenium catalyst precursors for
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terminal alkyne hydrosilylation.
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The hydrosilylation of phenylacetylene with HSiMe2Ph catalyzed by the zwitterionic
compound [Cp*RhCl{(MeIm)2CHCOO}] (1) was completed in 24 min with full selectivity for
the β-(Z)-vinylsilane derivative (entry 1). In sharp contrast, the related iridium(III)-based
species, [Cp*IrCl{(MeIm)2CHCOO}] (2), is almost completely inactive and only provides a
2% conversion in 24
h
(entry 2). The ruthenium(II)-based complex [(p-
cymene)RuCl{(MeIm)2CHCOO}] (3) is moderately active, with a 71% conversion in 48 h.
Nonetheless, it lacks selectivity, affording up to 15% of the undesired alkene by-product, which
results from the dehydrogentive silylation of the alkyne (entry 3). Along the rhodium(I) and
iridium(I) series, the rhodium complex [Rh(cod){(MeIm)2CHCOO}] (4) was found to be active
in the polymerization of phenylacetylene (PA) producing polyphenylacetylene (PPA) in the
early stage of the reaction even when the order of addition of the reagents was reversed
1
(HSiMe2Ph before than PA). The PPA was identified in the H NMR spectra of the reaction
mixture due to its characteristic set of resonances at δ 6.9, 6.6 and 5.8 (=CH) ppm.24 In the case
of catalyst precursor 4 the selectivity in PPA was 63% after 1 hour (35% conversion) and 18%
after 24 h (98% conversion) (entries 4 and 5). The iridium-based complex
[Ir(cod){(MeIm)2CHCOO}] (5) does not show activity in polymerization and affords a 95% of
conversion in 24 h. However, it is unselective, providing a 30% of the alkene by-product, as
well as similar proportions of the -(Z), -(E), and -vinylsilane derivatives (entry 6). The
performance of the catalyst precursors in the hydrosilylation of oct-1-yne follows a similar trend
as that observed for phenylacetylene. Noteworthy, catalyst [Cp*RhCl{(MeIm)2CHCOO}] (1)
is by far the most active and selective one, enabling full conversion to β-(Z)-vinylsilane in 24
min (entry 7).
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Table 1. Hydrosilylation of terminal alkynes with HSiMe2Ph catalyzed by zwitterionic
carboxylate bridge-functionalized bis-NHC rhodium, iridium and ruthenium complexes.a,b
time
(h)
conv.
(%)
β-(Z)
(%)
β-(E)
(%)
α
alkene
entry alkyne
catalyst
(%) (%)
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0.4
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48
1
>99
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>99
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---
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74
24
---
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3
---
---
10
8
---
---
15
---
---
30
---
---
2
71
35c
98c
95
>99
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46
13
24
24
0.4
24
24
24
---
8
5
1
2
4
5
32
14
---
---
---
29
>99
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91
95
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a) Experiments were carried out in CDCl3 at 333 K using a HSiMe2Ph/RC≡CH/catalyst ratio
of 100/100/1, [catalyst] = 1.54 mM. b) Conversion and selectivities determined by 1H NMR
using anisole as internal standard. c) HSiMe2Ph was added before phenylacetylene. Selectivity
towards PPA = 63% after 1 h and 18% after 24h.
Remarkably, catalyst precursor 1 is highly soluble in water owing to the presence of an
uncoordinated carboxylate function at the bis-NHC ligand. In this regard, we envisaged it as a
potential pre-catalyst for the hydrosilylation in water, and thus performed an in-depth
investigation of such process. In this context, it is worth mentioning that highly efficient and
stereoselective water-soluble catalysts for the hydrosilylation of terminal alkynes based on
platinum25 and copper26 have recently been reported, paving the way to the development of a
new generation of clean and sustainable catalysts. Unfortunately, 1 exhibited a poor catalytic
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activity in the hydrosilylation of oct-1-yne with HSiMe2Ph in water even when using the phase
transfer agent TEBA (triethylbenzylammonium chloride). Namely, only a 14% alkyne
conversion was attained after 24h at 333 K with 46% selectivity for the β-(E)-vinylsilane.
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However, the H NMR of the yellow oil extracted showed the presence of unreacted alkyne,
but no hydrosilane which suggests that compound 1 could also catalyze the hydrolysis of the
hydrosilane.27 In fact, in an independent experiment, we have found that 1 catalyzes the
hydrolysis of HSiMe2Ph, which confirms that this process competes with the alkyne
hydrosilylation (see Experimental Section for additional details).
Hydrosilylation of 1-alkynes catalyzed by 1: influence of the temperature and catalyst
loading. The outstanding catalytic performance exhibited by [Cp*RhCl{(MeIm)2CHCOO (1)
prompted us to study the influence of the temperature and catalyst loading in the catalytic
activity and selectivity. Therefore, in order to compare with the results obtained at 333 K, the
hydrosilylation reactions were also performed at room temperature (RT). So as to better
understand the catalyst behavior, two hydroslilanes, HSiMe2Ph and HSiEt3, and two substrates,
phenylacetylene and oct-1-yne, were selected. In addition, the influence of the catalyst loading
was studied. For that, several experiments decreasing the catalyst loading from 1 to 0.1 mol%
(HSiR3/RC≡CH/catalyst ratio of 1000/1000/1) were carried out. The results are summarized in
Table 2.
As expected, the hydrosilylation of phenylacetylene and oct-1-yne was slower at RT than at
333 K, especially in the case of HSiEt3. For example, full conversion in the hydrosilylation of
oct-1-yne with HSiMe2Ph and HSiEt3 requires 7 and 48 h at RT, respectively (entries 10 and
14) vs. 0.4 and 6 h at 333 K (entries 9 and 13). Interestingly, in both cases the excellent regio-
and stereoselectivity for the β-(Z)-vinylsilane isomer is maintained at RT. The same trend was
observed in the hydrosilylation of phenylacetylene. Remarkably, an improvement of the β-(Z)
selectivity up to 98% was attained in the hydrosilylation of phenylacetylene at RT using HSiEt3
as hydrosilane (entry 6).
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Table 2. Influence of the temperature and catalyst loading in the hydrosilylation of
terminal alkynes catalyzed by [Cp*RhCl{(MeIm)2CHCOO}] (1).a,b
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Selectivity (%)
T
(K)
conv. S/C
entry
alkyne
hydrosilane
t (h)
(%)
ratio
β-(Z)/β-(E)/α/alkene
1
HSiMe2Ph
HSiMe2Ph
HSiMe2Ph
HSiMe2Ph
HSiEt3
333 0.4
RT
333 48
RT 48
333 9.5
RT 48
333 72
>99
100
>99 / - / - / -
2
5
99
100
100
91
49
43
7
100
>99 / - / - / -
78 / 18 / 3 / 1
>99 / - / - / -
83 / 10 / 4 / 3
98 / 1 / 1 / -
58 / 42 / - / -
86 / 12 / 1 / 1
>99 / - / - / -
>99 / - / - / -
89 / 7 / 3 / 1
95 / 4 / - / 1
94 / - / - / 6
96 / - / - / 4
97 / - / - / 3
>99 / - / - / -
3
1000
1000
100
4
5
6
HSiEt3
100
7
HSiEt3
1000
1000
100
8
HSiEt3
RT
120
9
HSiMe2Ph
HSiMe2Ph
HSiMe2Ph
HSiMe2Ph
HSiEt3
333 0.4
>99
97
82
75
93
100
98
3
10
11
12
13
14
15
16
RT
7
100
333 48
1000
1000
100
RT
333
RT
48
6
HSiEt3
48
100
HSiEt3
333 96
RT 24
1000
1000
HSiEt3
a) Experiments were carried out in CDCl3 using a HSiR3/RC≡CH ratio of 1/1, [1] = 1.54 mM,
S/C = substrates/catalyst ratio. b) Conversion and selectivities determined by 1H NMR using
anisole as internal standard.
When the catalyst loading is reduced up to 0.1 mol%, longer reaction times are needed to
achieve acceptable conversion levels. In accord with the results described in the previous
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section, HSiEt3 is the hydrosilane that shows lowest reaction rates. Under these reaction
conditions, the catalytic activity in the hydrosilylation of both phenylacetylene and oct-1-yne
with HSiEt3 at RT was very low (entries 8 and 16) although increased appreciably at 333 K
(entries 7 and 15). However, the long reaction times at this temperature are at the expense of a
significant decrease in the selectivity in the case of the hydrosilylation of phenylacetylene. This
is likely to be a consequence of the β-(Z)β-(E) vinylsilane isomerization (entry 7).
Interestingly, 98% conversion in 96 h was attained in the hydrosilylation of oct-1-yne with a
97% of selectivity in β-(Z)-vinylsilane (entry 15).
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Remarkably, the hydrosilylation of phenylacetylene and oct-1-yne with HSiMe2Ph at 333 K
with 0.1 mol% catalyst loading resulted in a selectivity decrease (entries 3 and 11). Thus,
although conversions higher than 80% were attained in 48 h, the selectivity for β-(Z)-vinylsilane
dropped from 99% to 78% and 89% (entries 3 and 11) presumably due to β-(Z)β-(E)
vinylsilane isomerization. Noteworthy, the hydrosilylation of phenylacetylene with HSiMe2Ph
at RT using 0.1 mol% catalyst loading was completed in 48 h with full selectivity for β-(Z)-
vinylsilane (entry 4). Under the same reaction conditions, the hydrosilylation of oct-1-yne
yielded a 75% conversion with a selectivity for β-(Z)-vinylsilane as high as 95% (entry 12).
To our delight, the catalytic performance of 1 is well preserved after several consecutive
cycles. The 1-catalyzed (1 mol%) hydrosilylation of phenylacetylene with HSiMe2Ph resulted
in full conversion in less than half an hour at 333 K (entry 1). The further addition of reactants
(phenylacetylene and HSiMe2Ph) after the reaction completion also leaded to the steady
consumption of the reactants in the same reaction time. The monitoring of the reaction by 1H
NMR evidenced that at the end of the six consecutive reaction experiments, the
β-(Z)-vinylsilane isomer was the only obtained product (see Supporting Information for more
details). Thus, catalyst precursor 1 was able to perform at least six consecutive catalytic cycles
without lossing neither activity nor selectivity towards the β-(Z)-vinylsilane isomer, pointing
out the outstanding selectivity induced by 1.
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Hydrosilylation of 1-alkynes catalyzed by 1: substrate scope. Catalyst precursor
[Cp*RhCl{(MeIm)2CHCOO}] (1) allows for the hydrosilylation of a range of aliphatic and
aromatic 1-alkynes using both HSiMe2Ph and HSiEt3. As can be seen in Table 3, excellent
selectivities for the β-(Z)-vinylsilane product were obtained at relatively short reaction times
when using HSiMe2Ph as hydrosilane. The reactions with HSiEt3 required longer reaction times
to achieve reasonable conversion values which resulted, in some cases, in a loss of selectivity
towards the β-(Z)-vinylsilane product.
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Table 3. Hydrosilylation of terminal alkynes with HSiMe2Ph and HSiEt3 catalyzed by
[Cp*RhCl{(MeIm)2CHCOO}] (1).a,b
time conv.
(h) (%)
β-(Z) β-(E)
α
(%)
alkene
(%)
entry
alkyne
silane
(%)
>99
>99
97
(%)
---
---
---
---
3
1
2
3
4
5
6
7
8
9
HSiMe2Ph 0.4 >99
---
---
---
---
4
---
---
3
HSiMePh2 1.5
HSiEt3
95
98
6
HSiMe2Ph 0.75 100
>99
91
---
2
HSiEt3
HSiMe2Ph 24
HSiEt3 24
HSiMe2Ph 24
HSiEt3 48
18
98
91
34
39
33
28
70
19
21
---
---
8
78
13
33
1
>99
76
71
Si
29
1
10
11
HSiMe2Ph 3
HSiMe2Ph 3
>99
82
>99
>99
---
---
---
---
---
---
12
13
14
15
HSiMe2Ph 0.4
HSiMePh2 1.5
>99
95
>99
>99
83
---
---
10
---
---
---
4
---
---
3
HSiEt3
9.5
91
HSiMe2Ph 0.54 >99
>99
---
---
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HSiEt3
4
87
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28
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9
17
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21
22
HSiMe2Ph 1.1
HSiEt3
HSiMe2Ph 0.3
HSiEt3
HSiMe2Ph 1.7
HSiEt3 24
>99
61
98
51
94
19
99
90
2
---
1
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>99
87
---
---
1
---
---
---
4
MeO
F3C
4
81
---
3
98
55
3
a) Experiments were carried out in CDCl3 at 333 K using a HSiR3/RC≡CH/catalyst ratio of
100/100/1, [1]= 1.54 mM. b) Conversion and selectivities determined by 1H NMR using anisole
as internal standard.
The hydrosilylation of oct-1-yne and hex-1-yne with HSiMe2Ph was completed in less than
one hour and provided >99% selectivity to β-(Z)-vinylsilane (entries 1 and 4). The
hydrosilylation of oct-1-yne with the bulkier hydrosilane HSiMePh2 is slightly slower, also
yielding complete selectivity to β-(Z)-vinylsilane (entry 2). Interestingly, an excellent
selectivity for the β-(Z)-vinylsilane product was also attained with HSiEt3 (entries 3 and 5).
However, the hydrosilylation of the bulky t-Bu-C≡CH with HSiMe2Ph and HSiEt3 was
completely unselective and gave significant amounts of all the vinylsilane isomers (entries 6
and 7). In sharp contrast, the hydrosilylation of Et3SiC≡CH was completed in 24 h with an
acceptable selectivity towards the β-(Z)-vinylsilane, up to 71% (entry 8). Remarkably, a reverse
selectivity towards the β-(E)-vinylsilane was observed with HSiEt3 giving a 70% selectivity in
48 h (entry 9). Cycloalkyl acetylenes were also efficiently reduced using HSiMe2Ph. Thus, the
hydrosilylation of cyclohexylacetylene and cyclopentylacetylene gave complete selectivity to
the β-(Z)-vinylsilane product with conversion values up to >99 and 82%, respectively, in 3 h
(entries 10 and 11).
The hydrosilylation of phenylacetylene with HSiMe2Ph showed the best catalytic
performance, with full conversion and selectivity towards the β-(Z)-vinylsilane in 0.4 h (entry
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12). Also, a 95% conversion in 1.5 h with complete selectivity to β-(Z)-vinylsilane was attained
with the bulkier hydrosilane HSiMePh2 (entry 13). The influence of electronic effects on the
hydrosilylation of terminal alkynes was studied in a series of phenylacetylene derivatives with
different substituents at the para position, 4-R-C6H4-C≡CH (R = -CH3, -tBu, -OCH3, -CF3). As
can be observed in the selected reaction profiles of Figure 1, the presence of a -OMe electron-
donating substituent at the para position resulted in an increase in the catalytic activity
compared to phenylacetylene. Thus, the hydrosilylation of (4-methoxyphenyl)acetylene with
HSiMe2Ph gave complete conversion in 0.3 h with 94% selectivity towards the β-(Z)-
vinylsilane (entry 19). In contrast, the hydrosilylation of the phenylacetylene derivative with a
-CF3 electron-withdrawing substituent at the para position is much slower, providing a 98% of
conversion in 1.7 h (entry 21).
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100
80
R = -OCH3
60
R = -H
R = -CH3
40
R = -tBu
R = -CF3
20
0
0
20
40
60
80
100
time (min)
Figure 1. Reaction profile of conversion vs. time for the hydrosilylation of phenylacetylene
derivatives 4-R-C6H4-C≡CH with HSiMe2Ph, catalyzed by 1 in CDCl3 at 333 K.
In general, the presence of weakly electron-donating groups at the para position resulted in
a decrease of the catalytic activity compared to phenylacetylene (Figure 1). In this sense,
although the hydrosilylation of (4-methylphenyl)acetylene with HSiMe2Ph showed a very
similar profile to that of phenylacetylene, the hydrosilylation of (4-tert-butylphenyl)acetylene
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is much slower and only provides a 94% of conversion in 1.1 h (entry 17). This fact can be
ascribed to the steric effect of the bulky substituent. Noteworthy, complete selectivity to the
β-(Z)-vinylsilane derivative was attained in the hydrosilylation of the substituted
phenylacetylene derivatives with R = -CH3 and -CF3, and selectivities as high as 94% and 98%
for the phenylacetylene derivatives with R = -OMe and -tBu (entries 15, 17, 19 and 21).
In full agreement with the previous observations, the hydrosilylation of substituted
phenylacetylene derivatives with HSiEt3 required longer reaction times. The reaction of the
derivatives with R = -Me, -tBu and -OMe (entries 16, 18 and 20) proceeds faster than with
phenylacetylene with conversions in the range of 60-87% in 4 h. As it may be expected, the
derivative with R = -CF3 shows a lower conversion rate, i.e. 55% in 24 h (entry 22). As far as
the selectivity is concerned, the hydrosilylation of the derivative with R = -CF3 gave a 90% of
selectivity for the β-(Z)-vinylsilane isomer. However, moderate selectivity values of 69% and
51% were obtained for the derivatives with R = -Me and -tBu, respectively. In sharp contrast,
the hydrosilylation of the derivative with R = -OMe gave an 87% of selectivity for the opposite
β-(E)-vinylsilane stereoisomer.
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The reaction selectivity is also influenced by the catalyst ability to promote isomerization
reactions once the alkyne substrate has been consumed. Both, the β-(Z)-vinylsilane
isomerization to the more thermodynamically stable β-(E)-vinylsilane isomer, and to the
corresponding allyl-silyl derivatives in the case of linear alkyl chain alkynes, have been
frequently observed.3,16d-f,28
Although catalyst precursor 1 did not show any substantial isomerization activity for aliphatic
alkynes at reasonable reaction times after the consumption of the alkyne, the β-(Z)β-(E)
vinylsilane isomerization process strongly influences the final selectivity in the case of aromatic
alkynes, such as phenylacetylene. The monitoring of the hydrosilylation of phenylacetylene
with HSiEt3 by 1H NMR showed that the reaction was almost completed in 9.5 hours with 83%
of selectivity to β-(Z)-vinylsilane and a β-(Z)/β-(E) ratio of 8.3 (Table 2, entry 15). However,
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we have found that 1 slowly promotes the β-(Z)β-(E) vinylsilane isomerization, since the
selectivity for β-(Z)-vinylsilane dropped to 46% after 48 h and 3% after 96 h as a consequence
of the isomerization process. The extent of this isomerization process can be followed by the
evolution of the β-(Z)/β-(E) ratio along the reaction time (see Figure 2). In contrast with HSiEt3,
when HSiMe2Ph is chosen as the hydrosilane, the β-(Z)β-(E) vinylsilane isomerization
process for the phenylacetylene hydrosilylation did not take place, and the β-(Z)/β-(E) ratio
remains very high at reasonable reaction times after reaction completion.
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60
Figure 2. β-(Z)/ β-(E) ratio for the hydrosilylation of phenylacetylene with HSiEt3 catalyzed
by 1 in CDCl3 at 333 K.
In order to rationalize these results, the hydrosilylation of 4-Me-C6H4-C≡CH and
4-MeO-C6H4-C≡CH with HSiEt3 was monitored by 1H NMR. As can be seen in the reaction
profile of the hydrosilylation of (4-methylphenyl)acetylene with HSiEt3 (Figure 3), the β-
(Z)β-(E) isomerization takes place along the reaction course before the consumption of the
substrates. The amount of β-(Z)-vinylsilane isomer increased up to a maximum of 63% at
approximately 90% alkyne conversion and then, steadily diminishes. The β-(Z)/β-(E) ratio
progressively reduces from 27.0, 10.0 and 2.4 at 67%, 90% and 95% conversion, respectively.
The isomerization of the β-(Z) into the β-(E)-vinylsilane isomer is complete after 48h.
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100
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60
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0
alkyne
ß-E
ß-Z
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60
0
400
800
time (min)
1200
Figure 3. Reaction profile for the hydrosilylation of (4-methylphenyl)acetylene with HSiEt3
catalyzed by 1 in CDCl3 at 333 K.
The reaction profile of the hydrosilylation of (4-methoxyphenyl)acetylene with HSiEt3 also
suggests a fast β-(Z)β-(E) isomerization (see Supporting Information). In this case, in the
early stage of the reaction both vinylsilane isomers are produced at approximately the same
rate. However, after a maximum of 37% (67% alkyne conversion) the amount of β-(Z)-
vinylsilane decreases. The β-(Z)/β-(E) ratio progressively diminishes from 4.5, 1.2 and 0.2 at
67%, 90% and 97% conversion, respectively. As in the precedent case, the isomerization of the
β-(Z) into the β-(E)-vinylsilane isomer is almost complete after 24h.
Hydrosilylation of 1-alkynes catalyzed by 1: the role of the carboxylate function. In order
to ascertain the impact of the carboxylate function on the catalytic activity of
[Cp*RhCl{(MeIm)2CHCOO}] (1), the performance of three related complexes was
investigated (Chart 2). Compound [Cp*RhCl{(MeIm)2CHCOOCH3}]+ (6) has
a
methoxycarbonyl function at the linker of the bis-NHC ligand, whereas the cationic complex
[Cp*RhI{(MeIm)2CH2}]+ (8) features an unfunctionalized bis-NHC ligand. On the other hand,
unlike compound 1 the carboxylate function in the hydrido-iridium(III) complex
[IrH(cod){(MeIm)2CHCOO}]+ (7) is coordinated to the metal center.
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O
OTf
BF4
PF6
O
O
O
N
Rh
N
Rh
H
N
N
N
Cl
I
Ir
H
7
N
N
N
N
N
N
N
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Chart 2. Related catalyst precursors having carboxylate-functionalized and unfunctionalized
bis-NHC ligands.
The results of the different hydrosilylation tests are reported in Table 4. The rhodium(III)-
based compound 6, which features an alkoxycarbonyl functional group at the bis-NHC ligand,
is less active than 1 in the hydrosilylation of both phenylacetylene and oct-1-yne (entries 2 and
6). Namely, the reaction with oct-1-yne provides a 73% selectivity for β-(Z)-vinylsilane
although reverse selectivity was observed with phenylacetylene giving a 62% selectivity for the
β-(E)-vinylsilane isomer. Moreover, the hydrido-iridium(III) compound 7, in addition to be less
active than 1, has been found to be completely non-selective (entries 3 and 7). Unexpectedly,
catalyst precursor 8 did not exhibit any catalytic activity at all in alkyne hydrosilylation under
the same reaction conditions (entries 4 and 8).
The previously explained results have shown that [Cp*RhCl{(MeIm)2CHCOO}] (1) is an
efficient catalyst precursor for the hydrosilylation of a range of terminal alkynes with excellent
regio- and stereoselectivity towards the less thermodynamically stable β-(Z)-vinylsilane
isomer. In contrast, the inactivity of catalyst precursor [Cp*RhI{(MeIm)2CH2}]+ (8) and the
lower catalytic performance of compound [Cp*RhCl{(MeIm)2CHCOOCH3}]+ (6) strongly
suggests that the carboxylate function in 1 plays a key role in the reaction mechanism.
Moreover, the poor selectivity and activity of compound [IrH(cod){(MeIm)2CHCOO}]+ (7),
with the functionalized bis-NHC ligand κ3-C,C’,O-coordinated, also indicates that the
carboxylate function is not likely to be engaged in the coordination to the metal center and that
its action is crucial for the reaction to take place.
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Table 4. Hydrosilylation of terminal alkynes with HSiMe2Ph catalyzed by bis-NHC rhodium,
5
6
iridium and ruthenium complexes.a,b
7
8
9
time conv.
β-(Z)
(%)
β-(E)
(%)
alkene
(%)
entry
alkyne
catalyst
α (%)
(h)
0.4
7
(%)
>99
85
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
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49
50
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58
59
60
1
2
3
4
5
6
7
8
1
6
7
8
1
6
7
8
>99
8
---
62
25
---
---
17
17
---
---
4
---
26
28
---
---
8
24
24
0.4
4
>99
0
29
---
>99
73
5
18
---
---
2
>99
96
24
24
90
21
---
57
---
0
---
a) Experiments were carried out in CDCl3 at 333 K using a HSiMe2Ph/RC≡CH/catalyst
ratio of 100/100/1, [catalyst] = 1.54 mM. b) Conversion and selectivities determined by 1H
NMR using anisole as internal standard.
Mechanistic studies on the hydrosilylation of 1-alkynes catalyzed by 1. In the search for
a deeper understanding of the operating mechanism for the hydrosilylation of terminal alkynes
and the role of the carboxylate group in [Cp*RhCl{(MeIm)2CHCOO}] (1), we have performed
an in-depth study on the basis of theoretical calculations and some specific experiments. This
way, we carried out a complete mechanistic study by using Density Functional Theory (DFT)
at the B3LYP-D3BJ(PCM, chloroform)/def2-TZVP// B3LYP-D3BJ/def2-SVP level of theory,
as explained in the Experimental section. The complete structure of the catalyst precursor 1 was
considered in the calculations, without any structural simplification, due to the presence of the
pentamethylcyclopentadienyl ligand or the carboxylate function which can influence the
catalyst mode of action.14,29 The most reactive reductant, HSiMe2Ph, was selected as the
hydrosilane model, while phenylacetylene was the chosen alkyne. For the sake of clarity, the
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reaction intermediates and transition states are referred to by means of capital letters, starting
from A, which corresponds to structure 1.
7
8
9
In accordance with classical mechanisms, the two possible initial reactions entail prior
coordination of the alkyne to give a vinylidene or the oxidative addition of the silane.30,31
However, both reactions pathways can be excluded as 1 is apparently unreactive towards
alkynes and hydrosilanes, even at high temperatures up to 333 K for 24 hours. Moreover, the
experimental data suggests that the carboxylate fragment in the zwitterionic complex
[Cp*RhCl{(MeIm)2CHCOO}] (1) should actively participate in the hydrosilylation
mechanism. Thus, we envisaged a mechanistic proposal where the carboxylate fragment acts
as a silane carrier, the proposal being shown in Figure 4.
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Cl-
Rh
Cl-
Rh
H
N
N
O
N
N
SiMe2Ph
H
H
H
N
N
N
N
Cl-
Rh
O
SiMe2Ph
O
H
Ph
H
H
Rh
O
SiMe2Ph
O
H
N
N
TS-DA- -(E)

O
N
N
N
N
H
Cl
31.4
O
TS-BC
25.4
N
N
O
TS-AB
17.0
PhMe2Si
HSiMe
2Ph
Ph
H
TS-DA- -(Z)
16.1
Cl-

TS-CD
9.2
B
10.1
Cl-
Ph
D
A
0.0
4.6
Cl-
C
0.0
H
H
H
Rh
Rh
N
O
O
N
N
H
H
H
Cl-
N
N
N
SiMe2Ph
O
N
N
H
O
O
H
SiMe2Ph
Rh
O
Rh
O
Ph
N
N
Cl
H
H
N
N
H
H
Rh
N
N
N
N
O
N
N
O
H
Ph
SiMe
2Ph
N
N
O
SiMe
2Ph
A + -(Z)

Ph
PhMe2Si
-39.4
H
A + -(E)

-41.5
H
SiMe
H
2Ph
Ph
Figure 4. DFT calculated Gibbs free energy profile (in kcal·mol-1) relative to A and the isolated
molecules. Note that for visual purposes the energy diagram is not at a scale.
The first step consists on the ligand exchange between the chlorido ligand and the
hydrosilane, yielding intermediate B, with a relative Gibbs energy of 10.1 kcal·mol-1. This
process requires to overcome an energy barrier of 17.0 kcal·mol-1, dictated by transition
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structure TS-AB, which is feasible under the reaction conditions (RT and 333 K). Remarkably,
within TS-AB the Cp* ligand changes its coordination mode form 5 to 1, as we previously
proposed for the related complex [Cp*IrCl{(MeIm)2CHCOO}],22 and which is enabled by the
flexibility in the coordination modes of the pentamethylcyclopentadiene ligand.32,33,34 The next
step involves the hydrosilane activation through a ligand assisted Si–H bond cleavage process,
in line with other activation proposals for similar Ir-based catalysts.29,35 This step leads to
intermediate C and involves the formation of a Si–O bond in the carboxylate moiety and a Rh–
H bond. This species has a relative energy of 0.0 kcal·mol-1, being 10.1 kcal·mol-1 more stable
than B as a consequence of the high oxophilicity of silicon, which favors the formation of the
Si–O bond. This process bears an energy barrier of 25.4 kcal·mol-1 (with respect to A). At this
point, it is important to mention that we also explored the oxidative addition of the hydrosilane
and the alkyne to the metal center, as it would be the case in a “classical” inner-sphere
mechanism; however, in agreement with the experimental observations, we could not find any
stable reaction product. We attribute this result to the fact that A (catalyst precursor 1) bears a
Rh(III) center, and thus the oxidative addition of the hydrosilane or the alkyne would result into
a Rh(V) intermediate. These kind of intermediates have extensively been reported to be
unstable, as it was the case in the species under study, and there are very few examples in the
literature.36
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The next step consists on the alkyne activation, which was revealed to take place by means
of the direct transference of the Me2PhSi+ moiety from the carboxylate group to the α carbon
of the alkyne, via transition structure TS-CD. This step needs to surmount an energy barrier of
9.2 kcal·mol-1, as dictated by the energy difference between TS-CD and intermediate C. As a
result, a -silyl carbocation intermediate (structure D), with a relative energy of 4.6 kcal·mol-1,
is generated. We attribute its relative high stability to be a consequence of the large
hyperconjugative nature of the -silyl effect.37 Then, as a final step, the -silyl carbocation
reacts selectively with the hydrido complex. Namely, it undergoes a hydride transfer from the
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Rh(III) center and generates the vinylsilane product, thus closing the catalytic cycle and
recovering A. Notice that, along this process, the carboxylate moiety does not have to flip (as
it happened in TS-BC) because its role consists just on transferring the silane to the alkyne for
the generation of the carbocation, without having any direct interaction with the hydride ligand.
This way, the vinylsilane is formed by nucleophilic attack of the hydride over the carbocation.
and the catalytic cycle is closed by coordination of silane to the metal center. Remarkably, there
are two different possibilities for the aforementioned attack, depending on whether the Me2PhSi
moiety points towards the metal complex (TS-DA-(E)) or outside the complex (TS-DA-(Z)),
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see Figure 5. Notice that TS-DA-(E) would lead to the (E)-vinylsilane, while TS-DA-(Z)
would lead to the (Z)-vinylsilane. TS-DA-(Z) is 11.7 kcal·mol-1 more stable than TS-DA-
(E), explaining the outstanding selectivity towards the (Z)-vinylsilane, which is a
consequence of the lower steric repulsion between the Me2PhSi+ moiety and the Cp* ligand in
TS-DA-(Z). The energy barrier for that process (taken form intermediate C) is 19.7 kcal·mol-1
for the (Z)-vinylsilane, making it feasible under the reaction conditions. Also notice that the
final step is very exergonic, as the final (Z)-vinylsilane product plus the original catalyst are
44.0 kcal·mol-1 more stable than D.
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Figure 5. Geometrical representation of DFT calculated TS-DA-(E) (up) and TS-DA-(E)
(down). Key distances in Å.
Overall, the whole process has a G of -39.4 kcal·mol-1,thus being highly thermodynamically
favorable. It features an effective energy span of 25.4 kcal·mol-1 according to the model
proposed by Kozuch et al.38 This barrier is determined by the Gibbs energy difference between
TS-BC and A, which corresponds to the hydrosilane activation (i.e. the ligand assisted Si–H
bond cleavage). It is also remarkable that the original catalyst precursor (structure A, also
referred to as 1) is recovered at the end of the cycle, discarding the need for any pre-activation
process. The proposed reaction mechanism also unravels the key role of the carboxylate
function. In particular, it acts as the shuttle that transfers the silane moiety to the alkyne,
yielding the -silyl carbocation (intermediate D) that will eventually lead to the final product.
Such functionality is also of essential importance in the reaction selectivity, as it enables the
formation of a flat carbocation intermediate (D), which would further evolve to the
(Z)-vinylsilane so as to minimize the steric repulsion in the hydride transfer transition state
(TS-DA). In this regard, we previously reported that the [MI2{κ4-C,C,O,O-(bis-NHC)}]BF4 (M
= Rh, Ir; bis-NHC = methylenebis(N-2-methoxyethyl)imidazole-2-ylidene) complex is
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able to selectively generate the (Z)-vinylsilane thanks to the presence of acetone in the reaction
medium.17 This molecule was revealed by theoretical and experimental studies to be the silane
shuttle that transfers such function to the alkyne. Herein, we propose that this role is played by
the carboxylate group. Moreover, the lack of selectivity and much lower catalytic activity of
compounds 6, 7 and 8 (see Chart 2), in which the uncoordinated carboxylate group has been
eliminated, adds additional evidence to the proposed role of the carboxylate function in the
catalytic activity and selectivity, and thus to the consistency of the proposed reaction
mechanism.
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It is worth mentioning that although the role of carboxylate as an oxygen-nucleophile for
hydrosilane activation is well documented, to the best of our knowledge, the heterolytic
hydrosilane splitting into silyl carboxylate and metal hydride had not been previously
described.39 In this context, silyl formates has been reported as surrogates of hydrosilanes for
transfer hydrosilylation of aldehydes in the presence of a well‐defined ruthenium catalyst. 40
In order to shed light on the feasibility of the catalytic cycle, we performed the hydrosilylation
of deuterated phenylacetylene with HSiMe2Ph and studied it by means of 1H NMR in CDCl3.
The hydrosilylation of PhC≡C-D resulted in the exclusive formation of the (Z)-
dimethyl(phenyl)(2-phenylvinyl-1-d)silane product (see Supporting Information) thereby
confirming the lack of H/D scrambling. This compound was unambiguously identified by the
lack of the vinyl resonance at δ 6.03 ppm and the virtual triplet at δ 7.51 ppm (JH-D = 1.9 Hz),
due to the coupling of the vinyl proton at C2 with the cis-disposed deuterium atom. The
percentage of deuterium in the product is in agreement with the deuteration degree of PhC≡C-
D (97%). This result also agrees with the DFT outcome. Namely, the D atom of
phenylacetylene-d1 would correspond to the hydrogen atom depicted in red in Figure 4, which
remains at the C1 carbon of the final (Z)-vinylsilane product. The lack of deuterium scrambling
in the product rules out the preliminary activation of the alkyne (e.g. vinylidene formation), and
suggests that reaction proceeds through the hydrosilane activation, as shown by the DFT study.
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We also analyzed the mass spectra of the catalytic solution resulting from the hydrosilylation
of phenylacetylene with HSiMe2Ph catalyzed by 1. The ESI-MS mass spectrum of the solid
recovered from the catalytic solution after diethyl ether addition, which could be the resting
state of the catalyst, showed three main peaks at m/z 493.1 (42%), 629.2 (6%) and 763.2 (100%)
(Figure 6).
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Figure 6. ESI+ mass spectrum (CHCl3) of the isolated solid after hydrosilylation of
phenylacetylene with HSiMe2Ph catalyzed by 1: a) full spectrum, b) observed, and c) calculated
isotopic distribution patterns for the main species.
Interestingly, the isotopic distribution of these peaks is in full agreement with the following
rhodium species: the peak at m/z 763.2 (100%) corresponds to the rhodium catalyst precursor
with two SiMe2Ph fragments and one hydrogen atom, whereas the peaks at m/z 629.2 (6%) and
m/z 493.1 (42%) result from the sequential loss of the two SiMe2Ph fragments from the former.
Remarkably, the species at m/z 629.2 may well correspond to intermediate C, which is a
significantly stable reaction intermediate according to the theoretical calculations and has the
same molecular mass. Moreover, the peak at m/z 493.1 is consistent with intermediate D, from
which the vinyl-carbocation moiety has been eliminated, as expected for positively charged
species in the mass spectrometer. We attribute the observation of a doubly silylated species at
m/z 763.2 to be a consequence of the molecular processes that may take place in the mass
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spectrometer environment, which are in many cases different from those observed in solution
as a consequence of the very different conditions.41
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CONCLUSIONS
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The catalytic activity towards alkyne hydrosilylation of a series of zwitterionic rhodium,
iridium and ruthenium [Cp*MCl{(MeIm)2CHCOO}], [M(cod){(MeIm)2CHCOO}] (M = Rh,
Ir) and [(p-cymene)RuCl{(MeIm)2CHCOO}] compounds has been investigated using the
hydrosilylation of phenylacetylene and oct-1-yne with HSiMe2Ph as model reaction under mild
conditions. The rhodium(III) compound [Cp*RhCl{(MeIm)2CHCOO}] is by far the most
active one giving full conversion to the β-(Z)-vinylsilane product. The outstanding
stereoselectivity is maintained both at room temperature and low catalyst loading.
Compound [Cp*RhCl{(MeIm)2CHCOO}] has proven to be an efficient catalyst for the
hydrosilylation of a range of aliphatic and aromatic terminal alkynes. Hydrosilylation of linear
1-alkynes, cycloalkyl acetylenes and aromatic alkynes with HSiMe2Ph selectively affords the
β-(Z)-vinylsilane product in very short reaction times. The reaction of aliphatic alkynes with
HSiEt3 is slower, which results in a slight decrease of selectivity. The efficiency of the
hydrosilylation of aromatic alkynes is influenced by the electronic and steric effects of the
substituent at the phenyl ring. A clear cut off is found in the hydrosilylation of aromatic alkynes
with HSiEt3, electron-donating substituents at the para position increase the catalytic activity
whereas the opposite effect is observed for electron-withdrawing substituents. However, in this
case, the selectivity for the β-(Z)-vinylsilane products is lost due to the β-(Z)β-(E)
isomerization process that takes places upon the vinylsilane formation. This is especially
remarkable in the hydrosilylation of derivatives bearing electron-donating substituents. On the
other hand, the hydrosilylation of bulky alkynes such as t-Bu-C≡CH and Et3SiC≡CH with either
HSiMe2Ph or HSiEt3 was unselective.
The outstanding catalytic performance of [Cp*RhCl{(MeIm)2CHCOO}] contrasts with that
of [Cp*RhI{(MeIm)2CH2}]+, a related cationic compound featuring an unfunctionalized bis-
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NHC ligand, which is completely inactive under the same reaction conditions. This suggests
that the carboxylate function plays a key role of in the reaction mechanism. In this regard, the
application of DFT calculations has allowed to propose a plausible ionic outer-sphere
mechanism pathway. According to our proposal, the carboxylate fragment acts as a silane
carrier and explains the excellent β-(Z) selectivity outcome of the hydrosilylation reactions. The
first step involves the heterolytic activation of the hydrosilane assisted by the carboxylate
function to give the hydrido intermediate [Cp*RhH{(MeIm)2CHCOO-SiR3}]+, which features
a silyl-carboxylate functionalized bis-NHC ligand. Then, the direct transference of the R3Si+
moiety from the carboxylate group to the alkyne results in the formation of a flat -silyl
carbocation intermediate that undergoes a hydride transfer from the Rh(III) center to generate
the vinylsilane product. The minimization of the steric repulsions in the hydride transfer
transition state explains the observed selectivity for the (Z)-vinylsilane product. This proposal
is substantiated by labelling studies involving PhC≡C-D, which rules out a possible alkyne
activation pathway. Finally, ESI+ mass spectroscopy has allowed to identify some of the
species that are in agreement with those proposed in the catalytic cycle, such as the key hydrido
intermediate.
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EXPERIMENTAL
General Considerations. All the experimental procedures were performed under argon
atmosphere by using Schlenk or glovebox techniques. Solvents were taken under argon
atmosphere from an Innovative Technologies solvent purification system (SPS) or dried
following standard procedures and distilled under argon prior to use. CDCl3 and CDCl2 (Euriso-
top) were dried using activated molecular sieves and degassed by three freeze-pump-thaw
cycles. The zwitterionic compounds [Cp*MCl{(MeIm)2CHCOO}] (M = Rh, 1; Ir, 2),
[Cp*RuCl{(MeIm)2CHCOO}] (3) and [M(cod){(MeIm)2CHCOO}] (M = Rh, 4; Ir, 5), and
complexes [Cp*RhCl{(MeIm)2CHCOOCH3}]OTf (6) and [IrH(cod){(MeIm)2CHCOO}]BF4
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(7) were prepared following the procedures recently reported by us.19,21 Deuterated
phenylacetylene, PhCCD, was prepared according to the literature procedure.42 All other
reagents were commercially available and used as received.
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1
Scientific Equipment. H and 13C{1H} NMR spectra were recorded on Bruker Avance
spectrometers (300, 400 or 500). Chemical shifts are reported in ppm relative to
tetramethylsilane and referenced to partially deuterated solvent resonances. Coupling constants
(J) are given in Hertz (Hz). Electrospray ionization (ESI) mass spectra were recorded using a
Bruker Esquire3000 plus™ ion-trap mass spectrometer equipped with a standard ESI source.
Gas chromatography/mass spectrometry (GC/MS) analyses were performed on an Agilent 7673
GC autosampler system with an Agilent 5973 MS detector operating in EI ionization method
at 70 eV, equipped with an Phenomenex ZB-5HT apolar capillary column (0.25 um film
thickness, 30 m × 0.25 mm i.d.).
Synthesis of [Cp*RhI{(MeIm)2CH2}]PF6 (8).43 The compound was prepared following an
one-pot procedure according to the synthetic method described by Heinekey and co-workers
for the analogous iridium complex.44 AgPF6 (334.8 mg, 1.324 mmol) was added to a solution
of [Cp*RhCl2]2 (204.6 mg, 0.331 mmol) in acetonitrile (5 mL) and the mixture stirred for 4
hours at room temperature. The resulting yellow suspension was filtered through Celite to
remove the silver salt formed to give a yellow solution. The addition of [(MeIm)2CH2]I2 (286.2
mg, 0.662 mmol) gave an orange suspension which turned into red whereupon the addition of
NE3 (185.5 μL,  = 0.726 g·mL-1, 99.5%, 1.324 mmol). The red suspension was heated at 333
K for 48 h to give a dark orange solution. The solution was cooled to room temperature and the
solvent was removed under vacuum. The addition of CHCl3 (5 mL) resulted in a yellow
suspension which was decanted and then filtered. The yellow solid was washed with methanol
(2 x 5 mL) and diethyl ether (3 x 3 mL), and dried in vacuo. Yield: 308.9 mg, 68%. Anal. Calc.
for C19H27F6IN4PRh: C, 33.25; H, 3.97; N, 8.16. Found: C, 32.91; H, 3.96; N, 8.11. 1H NMR
(298 K, 400 MHz, CD2Cl2): δ 7.35 (d, JH-H = 1.3, 2H, CH), 7.07 (d, JH-H = 1.2, 2H, CH), 6.13
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