Cobalt-Catalyzed Regiodivergent Hydrosilylation of Vinylarenes and Aliphatic Alkenes: Ligand- and Silane-Dependent Regioselectivities

Article abstract of DOI:10.1021/acscatal.6b02518

We report a regiodivergent hydrosilylation of alkenes catalyzed by catalysts generated in situ from bench-stable Co(acac)₂ and phosphine- or nitrogen-based ligands. A wide range of vinylarenes and aliphatic alkenes reacted to afford either branched (45 examples) or linear (37 examples) organosilanes in high isolated yields (average: 84%) and high regioselectivities (from 91:9 to >99:1). This transformation tolerates a variety of functional groups including ether, silyloxy, thioether, epoxide, halogen, amine, ester, boronic ester, acetal, cyano, and ketone moieties. Mechanistic studies suggested that the hydrosilylation of alkenes catalyzed by the cobalt/bisphosphine system follows the Chalk-Harrod mechanism (with a Co-H intermediate), and the hydrosilylation of alkenes catalyzed by the cobalt/pyridine-2,6-diimine system follows the modified Chalk-Harrod mechanism (with a Co-Si intermediate). Systematic studies with sterically varied silanes revealed that the steric properties of silanes play a pivotal role in controlling the regioselectivity of vinylarene hydrosilylation and the chemoselectivity of the reactions of aliphatic alkenes and silanes catalyzed by the cobalt/pyridine-2,6-diimine system.

Full text of DOI:10.1021/acscatal.6b02518

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Article
Cobalt-Catalyzed Regiodivergent Hydrosilylation of Vinylarenes and
Aliphatic Alkenes: Ligand- and Silane-Dependent Regioselectivities
Chao Wang, Wei Jie Teo, and Shaozhong Ge
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02518 • Publication Date (Web): 28 Nov 2016
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ACS Catalysis
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Cobalt-Catalyzed Regiodivergent Hydrosilylation of Vi-
nylarenes and Aliphatic Alkenes: Ligand- and Silane-
Dependent Regioselectivities
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Chao Wang, Wei Jie Teo, and Shaozhong Ge*
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore
KEYWORDS: cobalt, hdyrosilylation, vinylarenes, aliphatic alkenes, regiodivergent, homogeneous catalysis
ABSTRACT: We report a regiodivergent hydrosilylation of alkenes catalyzed by catalysts generated in situ from benchꢀstable
Co(acac)2 and phosphine or nitrogenꢀbased ligands. A wide range of vinylarenes and aliphatic alkenes reacted to afford either
branched (45 examples) or linear (37 examples) organosilanes in high isolated yields (average: 84%) and high regioselectivities
(from 91:9 to >99:1). This transformation tolerates a variety of functional groups including ether, silyloxy, thioether, epoxide, haloꢀ
gen, tertiary amine, ester, boronic ester, acetal, cyano, and ketone moieties. Mechanistic studies suggested that the hydrosilylation
of alkenes catalyzed by the cobalt/bisphosphine system follows the ChalkꢀHarrod mechanism (with a CoꢀH intermediate) and the
hydrosilylation of alkenes catalyzed by the cobalt/pyridineꢀ2,6ꢀdiimine system follows the modified ChalkꢀHarrod mechanism
(with a CoꢀSi intermediate). Systematic studies with sterically varied silanes revealed that the steric properties of silanes play a
pivotal role in controlling the regioselectivity of vinylarene hydrosilylation and the chemoselectivity of the reactions of aliphatic
alkenes and silanes catalyzed by the cobalt/pyridineꢀ2,6ꢀdiimine system.
INTRODUCTION
oselectivity and afforded a 1:1 mixture of linear and branched
organosilane products.12 Here we report effective cobalt cataꢀ
lysts that can selectively catalyze either Markovnikov or antiꢀ
Markovnikov hydrosilylation of both vinylarenes and aliphatic
alkenes. Mechanistic studies reveal a pronounced ligandꢀ and
silaneꢀdependent regioselectivity.
Hydrosilylation of alkenes catalyzed by transition metals is
the most straightforward and atomꢀeconomic approach to synꢀ
thesize organosilanes, which can be converted to alcohols or
used as monomers for the production of siliconꢀbased polyꢀ
mers.1 Platinumꢀcatalyzed hydrosilylation of alkenes is one of
the most frequently conducted catalytic processes in industry
due to its effectiveness and high selectivity.2 A number of
other metals, such as lanthanides,3 early transition metals,4
iron,5 cobalt,6 nickel,7 rhodium,8 and iridium,9 have also been
used for this transformation. The majority of these catalysts
show high selectivity toward antiꢀMarkovnikov hydrosilylaꢀ
tion.
In contrast, the Markovnikov selective hydrosilylation of
alkenes has been barely studied and only a limited number of
transition metals (lanthanides,3a,3e Zr,10 Ni7b and Pd11) have
been used to catalyze this transformation, but with limited
substrate scope or with modest regioselectivities. For example,
hydrosilylation of vinylarenes can be achieved in high regioseꢀ
lectivity with lanthanide catalysts, but this transformation
showed very limited functional group tolerance.3a It is difficult
to find catalysts that can combine high catalyst activity, high
selectivity and broad substrate scope for Markovnikov hydrosꢀ
ilylation of alkenes. During the preparation of this manuscript,
Huang’s group reported a cobaltꢀcatalyzed Markovnikov hyꢀ
drosilylation of aliphatic alkenes,12 but the reported cobalt
catalyst was not selective for hydrosilylation of vinylarenes.
For example, the hydrosilylation of styrene with PhSiH3 cataꢀ
lyzed by the reported cobalt catalyst proceeded with low regiꢀ
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Scheme 1. Expected Pathways for Co-Catalyzed Re-
giodivergent Hydrosilylation of Vinylarenes (A and B) and
Aliphatic Alkenes (C and D)
The reactions of styrene with secondary silane Ph2SiH2 were
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conducted with 2 mol % cobalt catalysts at room temperature
or 50 °C (entries 6−11). The reaction conducted with 2 mol %
of Co(acac)2 and xantphos at room temperature did not proꢀ
duce any hydrosilylation products (entry 6). The same reaction
conducted at 50 °C proceeded to full conversion and afforded
a mixture of branched and linear products with the ratio of
36:64 (entry 7). The catalyst generated in situ from Co(acac)2
and dpephos showed very low reactivity (entry 8). Improved
results were obtained for the reaction conducted with
Co(acac)2/dppf and these reactions occurred in full conversion
with 98% selectivity for the linear product at room temperaꢀ
ture or 50 °C (entries 9 and 10). In addition, the combination
of Co(acac)2 and mesPDI did not catalyze styrene hydrosilylaꢀ
tion with Ph2SiH2 at 50 °C (entry 11).
Our rationale to develop the Coꢀcatalyzed regiodivergent
hydrosilylation of alkenes is depicted in Scheme 1. Phosphineꢀ
ligated cobalt hydride complexes have been well studied for
hydroformylation of alkenes.13 Migratory insertion of alkenes
into metalꢀhydride bonds has been shown to provide a versaꢀ
tile foundation for hydrofunctionalization of alkenes.14 As for
migratory insertion, vinylarenes have an electronic preference
for 2,1ꢀinsertion to form a secondary benzylcobalt species (I)
over 1,2ꢀinsertion to form a primary alkylcobalt species
(II).3a,15 Due to steric influence of the alkyl group on the cobalt
center, the major species (I) is less reactive than the minor
species (II) toward σꢀbond metathesis,16 but these two interꢀ
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Table 1. Evaluation of Cobalt Precursors, Ligands, and
Silanes for Hydrosilylation of Styrenea
mediates are interconverted through βꢀH elimination and reꢀ
insertion of vinylarene into the CoꢀH bond. We envision that a
Coꢀalkyl species coordinated with an appropriate ligand can
selectively react with sterically differentiated hydrosilanes to
produce either branched (Markovnikov selective, pathway A)
or linear (antiꢀMarkovnikov selective, pathway B) orgaꢀ
nosilanes.
In contrast, aliphatic alkenes have a steric preference for
1,2ꢀinsertion to generate a primary alkylcobalt species (III)
over 2,1ꢀinsertion to generate a secondary alkylcobalt species
(IV).3a,17 Since the major intermediate (III) is more reactive
than the minor intermediate (IV) toward σꢀbond metathesis,
the pathway involving “CoꢀH” will preferentially produce
linear alkylsilanes (pathway C) regardless of the hydrosilanes
used. To develop a Coꢀcatalyzed Markovnikov hydrosilylation
of aliphatic alkenes, we envision that a pathway involving a
CoꢀSi intermediate (bearing a less congested silyl group) will
operate, as aliphatic alkenes have a steric preference for 1,2ꢀ
insertion over 2,1ꢀinsertion. This pathway is designated as D
in Scheme 1. A pathway involving a CoꢀSi species was previꢀ
ously proposed by Chirik for the Coꢀcatalyzed dehydrogenaꢀ
tive silylation of alkenes.18
RESULTS AND DISCUSSION
Evaluation of Ligands and Silanes for the Co-Catalyzed
Regiodivergent Hydrosilylation of Styrene. We initiated our
studies on the Coꢀcatalyzed regiodivergent hydrosilylation of
vinylarenes by evaluating the reactions of styrene with steriꢀ
cally varied hydrosilanes, such as PhSiH3, Ph2SiH2, PhMeꢀ
SiH2, Et3SiH, (EtO)2MeSiH, (EtO)3SiH, and (Me3SiO)2MeSiH
(MD'M). We examined a variety of cobalt precursors, ligands,
and temperatures for these model reactions. The selected exꢀ
amples of these experiments are summarized in Table 1 (see
the Supporting Information for the detailed evaluation).
aConditions: styrene (0.500 mmol), silane (0.550 mmol), cobalt
precursor (5.0−10.0 ꢀmol), ligand (5.0−10.0 ꢀmol), THF (1 mL),
conversions and product ratios are determined by GC analysis
using dodecane as internal standard; b24 h.
The reactions of styrene with tertiary silanes (Ph3SiH,
PhMe2SiH, Et3SiH, and (EtO)2MeSiH) did not occur in the
presence of 2 mol % of Co(acac)2 and xantphos at 50 °C. The
lack of reactivity for this Co(acac)2/xantphosꢀcatalyzed hyꢀ
drosilylation with tertiary silanes implies two possibilities: (1)
Co(acac)2 can not be activated by these tertiary silanes to genꢀ
erate active cobalt species; (2) the combination of Co(acac)2
and xantphos does not catalyze the hydrosilylation of styrene
with tertiary silanes under the applied conditions. To distinꢀ
guish these two cases, we conducted hydrosilylation of styrene
with tertiary silanes at 50 °C in the presence of 3 mol % of
Co(acac)2/xantphos and 30 mol % of PhSiH3 (to activate the
cobalt catalyst precursor). These reactions exclusively affordꢀ
ed the branched benzylic silane (from the reaction with
The reactions of styrene with primary silane PhSiH3 were
carried out with 1 mol % of cobalt catalyst at room temperaꢀ
ture or 50 °C (entries 1−5). Reactions catalyzed by the combiꢀ
nation of Co(acac)2 and dppf or dpephos occurred in low conꢀ
versions (entries 1 and 2). Interestingly, the reaction conducted
with the catalyst generated from Co(acac)2 and xantphos at
room temperature proceeded to full conversion with excellent
selectivity (97%) for Markovnikov hydrosilylation (entry 3).
The same reactions conducted at 50 °C proceeded to full conꢀ
version, but with slightly lower regioselectivity (entry 4). We
also tested anitrogenꢀbased ligand mesPDI (entry 5) and the
reaction afforded a mixture of the branched and linear prodꢀ
ucts with b/l of 69:31.
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aConditions: 1ꢀoctene (0.500 mmol), silane (0.550 mmol), cobalt
PhSiH3), without the formation of any hydrosilylation prodꢀ
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precursor (10.0−15.0 ꢀmol), ligand (10.0−15.0 ꢀmol), THF (1
mL), yields are determined by GC analysis using dodecane as
internal standard; bhexane (1 mL); ctoluene (1 mL).
ucts from reactions with tertiary silanes. These results confirm
the possibility (2), but can not rule out the possibility (1).
The data in Table 1 indicate that silanes have significant efꢀ
fects on the regioselectivity of styrene hydrosilylation. For the
same catalyst system (entries 2 and 8; 3 or 4 and 7), the reacꢀ
tion with PhSiH3 afforded the branched benzylic silane as the
major product (entries 2, 3, and 4), but the corresponding reacꢀ
tion with Ph2SiH2 gave the linear silane as the major product
(entries 7 and 8). This inverted regioselectivities were exꢀ
pected from the proposed pathways A and B in Scheme 1, as
Ph2SiH2 is more congested than PhSiH3. Reactions with more
sterically varied silanes to illustrate this observed silane effects
will be discussed in detail in the mechanistic consideration
section.
To develop the Coꢀcatalyzed Markovnikov hydrosilylation
of aliphatic alkenes, we studied the hydrosilylation of 1ꢀoctene
with Co(acac)2 and nitrogenꢀbased pyridineꢀ2,6ꢀdiimine (PDI)
ligands, a class of ligand used for the Coꢀcatalyzed dehydroꢀ
genative silylation of aliphatic alkenes.6i, 18 The reaction of 1ꢀ
octene with PhSiH3 catalyzed by 3 mol % of Co(acac)2 and
mesPDI in THF at 60 °C proceeded sluggishly and afforded the
branched product in 55% yield (entry 8). The use of more
congested iPrPDI ligand led to a decreased yield (entry 9).
However, the reactions conducted in hexane and toluene ocꢀ
curred smoothly and afforded the branched product in high
yields and high selectivity (entries 10 and 11). This observed
solvent effect might stem from the coordinating ability of solꢀ
vents (THF > toluene > hexane) and the solubility of the cataꢀ
lyst in these solvents (THF ~ toluene > hexane).19
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Evaluation of Ligands and Silanes for the Co-Catalyzed
Regiodivergent Hydrosilylation of 1-Octene. After identifyꢀ
ing the conditions for regiodivergent hydrosilylation of styꢀ
rene, we evaluated the conditions for hydrosilylation of 1ꢀ
octene and the selected experiments are summarized in Table
2. The reactions of 1ꢀoctene with PhSiH3 conducted with cataꢀ
lysts generated in situ from Co(acac)2 and xantphos, dppf or
dppe occurred in full conversion at room temperature in 3 h
and afforded the linear product in high yields with only trace
amounts of the branched product (entries 1−3). The same reacꢀ
tions with more congested Ph2SiH2 proceeded sluggishly and
gave the linear product in low to modest yields (entries 4−6).
In addition, we also conducted the 1ꢀoctene hydrosilylation at
60 °C (entry 7) and the similarly high regioselectivity was
obtained compared to the reaction at room temperature (entry
1). The same regioselectivity observed for 1ꢀoctene hydrosiꢀ
lylation with sterically differentiated silanes PhSiH3 and
Ph2SiH2 can be explained by the preferential formation of
more reactive primary alkyl cobalt species by the 1,2ꢀinsertion
of 1ꢀoctene to the CoꢀH bond (pathway C in Scheme 1). Under
the conditions for entries 1 and 7, 1ꢀoctene did not undergo
hydrosilylation with tertiary silanes (Ph3SiH, PhMe2SiH,
Et3SiH, and (EtO)2MeSiH).
The reaction of 1ꢀoctene with Ph2SiH2 in the presence of 3
mol % of Co(acac)2 and mesPDI at 60 °C proceeded very slugꢀ
gishly and afforded a mixture of branched/linear alkylsilane
and E/Zꢀallylsilane products in a low yield (the total yield for
the four products <10%). The detailed studies with tertiary
silanes (PhMe2SiH, Et3SiH, and (EtO)3SiH) to reveal the
silane effects on selectivity for the Co/PDI catalysts will be
discussed in the mechanistic study section.
Scope of the Co-Catalyzed Markovnikov Hydrosilylation
of Vinylarenes. With an active catalyst and reliable conditions
identified for the Markovnikov hydrosilylation of styrene (enꢀ
try 3 in Table 1), we studied the scope of vinylarenes that unꢀ
dergo this transformation. The results are summarized in Table
3. In general, a wide range of vinylarenes containing electronꢀ
ically and sterically varied aryl groups reacted with PhSiH3
smoothly at room temperature in the presence of 1 mol %
Co(acac)2 and 1 mol % xantphos, yielding the corresponding
secondary benzylic silanes (1a−1w) in modest to excellent
isolated yields (51%−96%, average: 86%) with high to excelꢀ
lent regioselectivities (b/l ratio: 97:3 to >99:1). The GCꢀMS
analysis on the crude mixtures of these reactions indicated that
vinylsilanes, products derived from catalytic dehydrogenative
silylation of vinylarenes, were not formed during this catalytic
hydrosilylation reaction.20
Table 2. Evaluation of Cobalt Precursors, Ligands, and
Silanes for Hydrosilylation of 1-octenea
The data in Table 3 show that the electronic properties of
the aryl groups do not have significant influence on this Coꢀ
catalyzed hydrosilylation. Products from reactions of viꢀ
nylarenes containing electronꢀdeficient (1a−1f) and electronꢀ
rich (1h−1l) aryl groups were isolated in similarly high yields
as the product (1g) from the reaction of styrene.
The steric hindrance at the metaꢀposition (1a, 1n, and 1u) or
the paraꢀposition (1b, 1e, 1h−1m, 1o−1s, 1v, and 1w) of aryl
groups has little influence on the regioselectivities. However,
the steric hindrance at the orthoꢀposition has significant influꢀ
ence on the regioselectivities. For example, the hydrosilylation
of 2ꢀfluorostyrene afforded the branched silane as the major
product with b/l of 70:30 (1x), but the reaction of 2,4ꢀ
dimethylstryrene gave the linear silane as the major product
with b/l of 20:80 (1y). In particular, the sterically demanding
2,4,6ꢀtrimethylstyrene reacted to afford linear products excluꢀ
sively (1z).
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This Co(acac)2/xantphosꢀcatalyzed Markovnikov hydrosiꢀ excellent regioselectivities (l/b ratio: >99:1). These reactions
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lylation of vinylarenes tolerates a wide range of functionaliꢀ
ties, including fluoro (1f), ether (1j), tertiary amine (1k), siꢀ
loxy (1l), chloro (1m and 1n), boronic ester (1o), cyano (1p),
ester (1q and 1r), free primary amine (1s), epoxide (1v), and
acetal (1w) moieties. Vinylarenes containing ketone, nitro,
bromo, or free hydroxyl groups are not compatible with the
reaction conditions. In addition, 2ꢀvinylthiophene reacted to
provide the branched product (1t) in 82% isolated yield with
b/l >99:1, but vinylpyridines did not undergo this Coꢀ
catalyzed hydrosilylation reaction.
did not form any vinylsilanes, as indicated by the GCꢀMS
analysis on the crude reaction mixtures.
This Co(acac)2/dppfꢀcatalyzed antiꢀMarkovnikov hydrosiꢀ
lylation of vinylarenes tolerates a range of functionalities,
including fluoro (2e and 2l), ether (2i), tertiary amine (2j),
siloxy (2k), and acetal (2o) moieties. Vinylarenes containing
ketone, ester, cyano, nitro, chloro, bromo, or free hydroxyl
groups are not compatible with the reaction conditions. This
relatively narrow scope of vinylarenes, compared with the
scope of vinylarene for Markovnikov hydrosilylation (Table
3), might be due to the presence of a CoꢀH intermediate at a
higher temperature (50 °C).
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Table 3. The Scope of Vinylarenes for the Co-Catalyzed
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Markovnikov Hydrosilylation with PhSiH3
Table 4. The Scope of Vinylarenes for the Co-Catalyzed
a
anti-Markovnikov Hydrosilylation with Ph2SiH2
aConditions: vinylarene (0.500 mmol), Ph2SiH2 (0.550 mmol),
Co(acac)2 (10.0 ꢀmol), dppf (10.0 ꢀmol), THF (1 mL), yields of
isolated products; b Co(acac)2 (15.0 ꢀmol) and dppf (15.0 ꢀmol).
The Scope of Aliphatic Alkenes for the Co-Catalyzed
Markovnikov Hydrosilylation. The scope of aliphatic alꢀ
kenes that undergo this Coꢀcatalyzed Markovnikov hydrosiꢀ
lylation is summarized Table 5. In general, a wide range of
aliphatic alkenes reacted with PhSiH3 in the presence of 3 mol
% of Co(acac)2/mesPDI in toluene at 60 °C, affording the
branched organosilanes (3a−3r) in high yields (50%−98%,
average: 83%) with high to excellent regioselectivities (b/l
ratio: 91:9 to >99:1). However, alkenes containing allylether
moiety (3s−3u) reacted with modest regioselectivities (b/l
ratio: 68:32 to 71:29). As these allylether substrates are not
expected to have an electronic preference for 2,1ꢀinsertion into
a CoꢀSi intermediate (a CoꢀSilyl species is proposed to be
active for the Co/mesPDIꢀcatalyzed reaction of alkenes with
silanes), the increased amount of linear products is likely to
aConditions: vinylarene (0.500 mmol), PhSiH3 (0.550
mmol), Co(acac)2 (5.0 ꢀmol), xantphos (5.0 ꢀmol), THF (1
mL), yields of isolated products; b2 mol % catalyst. Cl?
Scope of the Co-Catalyzed anti-Markovnikov Selective
Hydrosilylation of Vinylarenes. Table 4 summarizes the
scope of vinylarenes for the Coꢀcatalyzed antiꢀMarkovnikov
hydrosilylation of vinylarenes. These reactions were conductꢀ
ed with Ph2SiH2 in the presence of 2 mol % of Co(acac)2 and 2
mol % of dppf at 50 °C for 6 h. In general, a range of electronꢀ
ically and sterically varied vinylarenes reacted under these
conditions to produce the corresponding linear organosilanes
(2a−2o) in high isolated yields (70%−97%, average: 85%) and
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stem from the interaction of the substrate ether groups with the with the reaction conditions. However, internal alkenes and
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cobalt center. The GCꢀMS analysis on the crude mixtures of
these reactions (3a−3u) indicated that vinylsilanes or alꢀ
lylsilanes, products derived from the Coꢀcatalyzed dehydroꢀ
genative silylation of alkenes, were not formed.18
alkenes containing aldehyde, ketone, nitro, free hydroxyl, or
free amine group did not react under these identified condiꢀ
tions.
10 mmol-Scale Reaction. As both Co(acac)2 and phosphine
ligands used for this transformation are benchꢀstable, we testꢀ
ed the hydrosilylation of 4ꢀtertꢀbutylstyrene with PhSiH3 on a
This transformation tolerates a wide range of functional
groups, such as ether (3g, 3s, 3t), thioether (3h), siloxy (3i),
chloro (3j), bromo (3k), ester (3l−3n), amide (3o), acetal (3p),
ketone (3q), pyridine (3r), and epoxide (3u) moieties. Howevꢀ
er, internal alkenes and alkenes containing aldehyde, nitro,
unprotected hydroxyl, or unprotected amine group did not
react under these conditions.
10 mmol scale in the presence of 0.2 mol % of
Co(acac)2/xantphos weighed on the benchtop without using a
dry box (See the Supporting Information for the 10 mmolꢀ
scale reactions of other three procedures). This reaction afꢀ
forded 1i in 93% isolated yield. Benzylic silane 1i was readily
oxidized by H2O2 in the presence of KHCO3, yielding the corꢀ
responding benzylic alcohol in 96% isolated yield (for proceꢀ
dures, see the Supporting Information). Thus, this Coꢀ
catalyzed Markovnikov hydrosilylation can be conducted on a
scale that would allow practical applications in synthesis.
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Table 5. The Scope of Aliphatic Alkenes for the Co-
a
Catalyzed Markovnikov Hydrosilylation with PhSiH3
Table 6. The Scope of Aliphatic Alkenes for the Co-
a
Catalyzed anti-Markovnikov Hydrosilylation with PhSiH3
aConditions: aliphatic alkene (0.500 mmol), PhSiH3 (0.550
mmol), Co(acac)2 (10.0 ꢀmol), xantphos (10.0 ꢀmol), THF (1
mL), yields of isolated products.
aConditions: aliphatic alkene (0.500 mmol), PhSiH3 (0.550
mmol), Co(acac)2 (15.0 ꢀmol), mesPDI (15.0 ꢀmol), toluene (1
mL), yields of isolated products; b5 mol % catalyst.
Mechanistic Consideration. The mechanism of the Coꢀ
catalyzed alkene hydrosilylation with antiꢀMarkovnikov selecꢀ
tivity has been studied in detail. In general, these reactions
follow either the Chalk−Harrod mechanism or the Modified
Chalk−Harrod mechanism.1e In the Chalk−Harrod mechanism,
the migratory insertion of alkene into a CoꢀH bond (hydroꢀ
metallation) is a key step.6b However, in the modified
Chalk−Harrod mechanism, the migratory insertion of alkene
into a CoꢀSi bond (silylmetallation) is a key step.21 To rationꢀ
alize the obtained regioselectivity for these Coꢀcatalyzed hyꢀ
drosilylation of vinylarenes and aliphatic alkenes, we conductꢀ
ed the following mechanistic studies.
Scope of the Co-Catalyzed anti-Markovnikov Hydrosi-
lylation of Aliphatic Alkenes. Table 6 summarizes the scope
of alkenes for the Coꢀcatalyzed antiꢀMarkovnikov hydrosilylaꢀ
tion. A wide range of functionalized aliphatic alkenes readily
reacted with PhSiH3 in the presence of 2 mol % of Co(acac)2
and xantphos in THF at room temperature, providing the corꢀ
responding linear products (4a−4s) in high isolated yields
(57%−93%, average: 80%) with excellent regioselectivities
(l/b ratio >99:1). These reactions did not produce any viꢀ
nylsilanes or allylsilanes, as indicated by the GCꢀMS analysis
on the crude mixtures of these reactions.
Cobalt/Phosphine Catalyst System. To study the activation
of Co(acac)2/bisphosphine in the presence of silane, we tested
the reaction of Co(acac)2 with 8 equivalents of PhSiH3 in the
presence of 2 equivalents of dppe in THFꢀd8 on an NMR scale
(see the Supporting Information). A diagnostic H NMR resoꢀ
nance (quintet, JPH = 23.6 Hz) at δ ꢀ15.1 ppm confirmed the
This Co(acac)2/xantphosꢀcatalyzed antiꢀMarkovnikov hyꢀ
drosilylation of aliphatic alkenes shows high functional group
tolerance. A wide range of functional groups, including ether
(4i), epoxide (4j), siloxy (4k−4m), chloro (4n), cyano (4o),
ester (4p, 4r, and 4s), and acetal (4q) moieties, are compatible
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from reactions of 4ꢀtertꢀbutylstyreneꢀβ,βꢀd2 (eqs 1 and 2), the
reaction of 1ꢀdodeceneꢀ1,1ꢀd2 afforded the product with deuꢀ
terium atoms mainly localized on the methylene carbon next
to the silyl group, with only a detectable amount of deuterium
incorporation onto the βꢀcarbon and silicon atom (eq 4). This
observation implies that 1ꢀdodeceneꢀ1,1ꢀd2 predominantly
undergoes 1,2ꢀinsertion into the CoꢀH intermediate to generate
formation of (dppe)2CoH, a known CoꢀH compound.22 Subseꢀ
1
2
3
4
5
6
7
8
quently, we conducted the reaction of Co(acac)2 with 8
equivalents of PhSiH3 in the presence of 1 equivalent of
xantphos ligand. HRꢀMS (ESI) analysis (m/z calc’d for
[(xantphos)Co]+: 637.1260; found: 637.1248) together with a
1H NMR resonance at ꢀ11.37 ppm indicated the formation of
(xantphos)CoH. However, attempts to isolate this CoꢀH comꢀ
plex resulted in the formation of illꢀdefined species.
a Coꢀalkyl species with deuterium on the
αꢀcarbon of the alkyl
group (eq 5). Subsequent ꢀhydrogen elimination from the
β
As proposed in Scheme 1 (pathways A and B), cobalt alkyl
species generated from 1,2ꢀinsertion and 2,1ꢀinsertion of viꢀ
nylarene into the CoꢀH bond can be interconverted through βꢀ
resulting Coꢀalkyl species forms the CoꢀH intermediate.
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H elimination and reꢀinsertion of vinylarene into the CoꢀH
bond. To test this hypothesis, we conducted the hydrosilylaꢀ
tion of 4ꢀtertꢀbutylstyreneꢀ
incorporation on the ꢀcarbon (benzylic position) and silicon
atom of the products supports the formation of a CoꢀH interꢀ
mediate. The deuterium scrambling between ꢀ and ꢀcarbon
occurred through ꢀhydrogen elimination and reꢀinsertion of
β,βꢀd2 (eqs 1 and 2). The deuterium
α
β
α
β
olefin into the resulting CoꢀD bond (Scheme 2). The incorpoꢀ
ration of deuterium on the silicon atom of the products resultꢀ
ed from CoꢀD/SiꢀH hydride exchange. To provide evidence
for this CoꢀD/SiꢀH hydride exchange, we analyzed the isotope
pattern (GCꢀMS) of the unreacted Ph2SiH2 for the reaction (eq
2) and found the incorporation of deuterium into Ph2SiH2 (see
the Supporting Information). Furthermore, we conducted the
Markovnikov hydrosilylation of 4ꢀtertꢀbutylstyrene with
PhSiD3 (eq 3) and we found similar deuterium incorporation
for the product of this reaction as for the product of the reacꢀ
tion depicted in eq 1.
For comparison with antiꢀMarkovnikov hydrosilylation of
1ꢀdodeceneꢀ1,1ꢀd2 catalyzed by the Co/xantphos system, we
discuss the Markovnikov hydrosilylation of this alkene with
PhSiH3 here (eq 6). This reaction afforded the branched prodꢀ
uct with deuterium localized exclusively on the
βꢀcarbon.
Deuterium was not detected for the αꢀcarbon and silicon posiꢀ
tions by 2H NMR analysis. In addition, we tested the Markovꢀ
nikov hydrosilylation of 1ꢀdodecene with PhSiD3 (eq 7). This
reaction afforded the product with deuterium localized excluꢀ
2
sively on the
β
ꢀcarbon and silicon atoms, as indicated by H
1
NMR analysis. Furthermore, H NMR analysis showed that
hydrogen was not incorporated onto the silicon of the product.
These results stand against the formation of a CoꢀH species
during the Co(acac)2/mesPDIꢀcatalyzed Markovnikov hydrosiꢀ
lylation.
As proposed in Scheme 1 (pathway A and B), the steric
properties of silanes have significant effect on the regioselecꢀ
tivity of vinylarene hydrosilylation. Indeed, the data in Table 1
(entries 7 and 13) showed this profound silane effect on the
regioselectivity. However, reactions with more silanes are
needed to consolidate the observed silane effect. Accordingly,
we tested a variety of sterically varied silanes for hydrosilylaꢀ
tion of styrene and the results are summarized in Table 7. The
results indicated that increasing the steric hindrance around Siꢀ
H (from entry 1 to entry 5) favors the formation of linear
products (antiꢀMarkovnikov selectivity). These data support
the proposed pathways A and B in Scheme 1. Compared with
the b/l ratio of 97:3 for the reaction at room temperature (entry
3 in Table 1), the relatively lower b/l ratio of 85:15 (entry 1 in
Table 7) is likely due to the elevated reaction temperature.
Scheme 2. Deuterium Scrambling between
Carbon of Vinylarene
β- and α-
We also tested the hydrosilylation of 1ꢀdodeceneꢀ1,1ꢀd2
with PhSiH3 catalyzed by Co(acac)2/xantphos (eq 4). Different
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reacted to give a mixture of branched/linear alkylsilanes and
1
2
3
4
5
E/Zꢀallylsilanes (entries 2 and 3). In particular, tertiary silanes
PhMe2SiH and Et3SiH reacted with complete selectivity for
dehydrogenative silylation (entries 4 and 5).
Table 7. Hydrosilylation of Styrene with Sterically Varied
Table 8. Hydrosilylation of 1-Octene with Primary, Sec-
ondary, and Tertiary Silanesa
Silanes Catalyzed by Co(acac)2/xantphosa
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aConditions: 1ꢀoctene (0.250 mmol), silane (0.275 mmol), cobalt
catalyst (5.0 ꢀmol), toluene (0.5 mL), conversion of 1ꢀoctene and
the ratios of product distribution were determined by GC analysis
with dodecane as internal standard.
aConditions: styrene (0.250 mmol), silane (0.275 mmol),
Co(acac)2 (7.5 ꢀmol), xantphos (7.50 ꢀmol), conversion of styꢀ
rene and b/l ratios are determined by GC analysis with dodecane
as internal standard.
Cobalt/mesPDI Catalyst System. Pyridineꢀ2,6ꢀdiimine (PDI)
cobalt complexes have been studied in detail by Chirik’s
group for the Coꢀcatalyzed dehydrogenative silylation of terꢀ
minal alkenes.18 They found that reactions of a wide range of
hydrosilanes with 2 equivalents of terminal alkenes in the
presence of (mesPDI)CoMe (0.5 mol %) in neat afforded alꢀ
lylsilanes (E/Z mixture) and/or Eꢀvinylsilanes with the conꢀ
comitant formation of an equimolar amount of alkanes. Mechꢀ
The data in Table 8 indicate a pronounced silane effect on
the regioꢀ and chemoselectivity for the Co/mesPDIꢀcatalyzed
reaction of 1ꢀoctene with silanes. It is well established that
these reactions proceed through a cobalt silyl intermediate.18
As the migratory insertion of aliphatic alkenes is mainly conꢀ
trolled by sterics, the steric interaction between alkene and
cobalt silyl species could account for the observed selectivity.
Three pathways are shown in Scheme 3 for catalytic reactions
of aliphatic alkenes and silanes. (1) For the cobalt species conꢀ
taining a small silyl group, the alkene has a preference for 1,2ꢀ
insertion due to the weak steric repulsion between alkene and
silyl group. The resulting Coꢀalkyl species (with a primary
alkyl group) reacts with silane to release the branched hydrosiꢀ
lylation product (MHS pathway). (2) When the cobalt interꢀ
mediate contains a sterically demanding silyl group, the strong
steric repulsion between alkene and the silyl group disfavors
1,2ꢀinsertion. Instead, 2,1ꢀinsertion occurs to generate a Coꢀ
alkyl species (with a secondary alkyl group). The subsequent
reaction of this Coꢀalkyl species with hydrosilane will afford
the linear hydrosilylation product (antiꢀMHS pathway). (3) If
the Coꢀalkyl species resulted from 2,1ꢀinsertion does not react
with the sterically congested silane or the reaction is much
anistic studies revealed rapid βꢀhydride elimination from a Coꢀ
alkyl intermediate (formed by migratory 2,1ꢀinsertion of alꢀ
kene into a CoꢀSi species) prior to turnoverꢀlimiting reaction
with silanes as the origin of the obtained selectivity (the DHS
pathway in Scheme 3).
In this study, we found that Co(acac)2/mesPDIꢀcatalyzed hyꢀ
drosilylation reactions of aliphatic alkenes with primary silane
PhSiH3 readily afforded branched products in high yields with
excellent selectivity when conducted with alkenes as limiting
reagents in toluene (Table 5). To rationalize this obtained
Markovnikov selectivity, which is different from the reported
dehydrogenative silylation for the Co/mesPDI system,18 we
tested Co(acac)2/mesPDI catalyst for the reactions of 1ꢀoctene
with a variety of silanes, such as Ph2SiH2, PhMe2SiH, Et3SiH,
and (EtO)3SiH. However, only the reaction with Ph2SiH2 ocꢀ
curred, but with very low conversions (<5%). These results
indicated that these silanes are not reactive enough to activate
Co(acac)2 in the presence of mesPDI.
slower compared with
βꢀH elimination, the Coꢀalkyl complex
will undergo ꢀH elimination to release allyl or vinyl silane
β
product (DHS pathway). Subsequently, alkene undergoes miꢀ
gratory insertion into the resulting CoꢀH bond to form a coꢀ
baltꢀalkyl species, which reacts with hydrosilane to regenerate
the Coꢀsilyl intermediate with the concomitant formation of an
alkane byproduct.18
Subsequently, we conducted the hydrosilylation reaction of
1ꢀoctene with selected silanes using wellꢀdefined cobalt alkyl
complex (mesPDI)CoMe23 and the results are summarized in
Table 8. The reaction with primary silane PhSiH3 produced the
branched silane as the major product (entry 1), which is comꢀ
parable to the results of the reaction catalyzed by
Co(acac)2/mesPDI (entry 11 in Table 2). Ph2SiH2 and (EtO)3SiH
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cobaltꢀcatalyzed asymmetric Markovnikov hydrofunctionaliꢀ
zation of alkenes will be the subject of future work.
1
2
3
4
5
6
7
8
Scheme 3. Pathways for the Co-Catalyzed Reactions of
Alkenes with Sterically Varied Silanes
ASSOCIATED CONTENT
Supporting Information
Supporting Information available: Detailed experimental proceꢀ
dures and compound characterization data (pdf). This material is
available via the Internet at http://pubs.acs.org
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AUTHOR INFORMATION
Corresponding Author
* chmgsh@nus.edu.sg
Note
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This work was supported by National University of Singapore
(No. Rꢀ143ꢀ000ꢀ614ꢀ133) and the Ministry of Education (MOE)
of Singapore (No. Rꢀ143ꢀ000ꢀ615ꢀ112). The authors thank Dr.
Rowan D. Young and Dr. Simon Sung for conducting the HRꢀMS
(ESI) analysis.
The pathways depicted in Scheme 3 show that the branched
hydrosilylation product results from 1,2ꢀinsertion of alkenes
and 2,1ꢀinsertion can lead to both linear hydrosilylation and
dehydrogenative silylation products. Based on this, the silane
effect on the migratory 1,2ꢀ/2,1ꢀinsertion of 1ꢀoctene for the
Co/mesPDI system is quantified and the data are listed in Table
8. The reaction of 1ꢀoctene with the sterically least congested
primary silane PhSiH3 primarily follows the MHS pathway in
Scheme 3, affording the branched silane as the major product
(entry 1 in Table 8). The reactions of 1ꢀoctene with sterically
most congested tertiary silanes (PhMe2SiH and Et3SiH) proꢀ
ceed through the DHS pathway and afford allyl or vinylsilanes
exclusively (entries 4 and 5 in Table 8). All three pathways in
Scheme 3 operate for reactions of 1ꢀoctene with hydrosilanes
of medium steric hindrance (Ph2SiH2 and (EtO)3SiH) and
products from both hydrosilylation and dehydrogenative siꢀ
lylation were obtained (entries 2 and 3).
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We have developed convenient and efficient protocols for
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The cobalt catalysts are generated from benchꢀstable Co(acac)2
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