Rapid, Regioconvergent, Solvent-Free Alkene Hydrosilylation with a Cobalt Catalyst

Article abstract of DOI:10.1021/jacs.5b08611

Alkene hydrosilylation is typically performed with Pt catalysts, but inexpensive base-metal catalysts would be preferred. We report a Co catalyst for anti-Markovnikov alkene hydrosilylation that can be used without added solvent at low temperatures with l

Full text of DOI:10.1021/jacs.5b08611

Communication
pubs.acs.org/JACS
Rapid, Regioconvergent, Solvent-Free Alkene Hydrosilylation with a
Cobalt Catalyst
Chi Chen,† Maxwell B. Hecht,‡ Aydin Kavara,‡ William W. Brennessel,‡ Brandon Q. Mercado,†
Daniel J. Weix,*,‡ and Patrick L. Holland*,†
†Department of Chemistry, Yale University, New Haven, Connecticut 06511, United States
‡Department of Chemistry, University of Rochester, Rochester, New York 14627, United States
S
* Supporting Information
Cobalt catalysts for alkene hydrosilylation are rare. Brookhart
ABSTRACT: Alkene hydrosilylation is typically per-
formed with Pt catalysts, but inexpensive base-metal
catalysts would be preferred. We report a Co catalyst for
anti-Markovnikov alkene hydrosilylation that can be used
without added solvent at low temperatures with low
loadings, and can be generated in situ from an air-stable
precursor that is simple to synthesize from low-cost,
commercially available materials. In addition, a mixture of
Co catalysts performs a tandem catalytic alkene isomer-
ization/hydrosilylation reaction that converts multiple
isomers of hexene to the same terminal product. This
regioconvergent reaction uses isomerization as a benefit
rather than a hindrance.
reported a cyclopentadienylcobalt complex for selective anti-
Markovnikov hydrosilylation with Et3SiH.9 Deng reported a
NHC-Co system for 1-octene + PhSiH3.17 Hilt reported a Co-
phosphine catalyzed 1,4-hydrosilylation of isoprene.18 In
addition, several recent patents indicate industrial interest in
cobalt catalysts for hydrosilylation.19−21 Here, we show a very
active cobalt catalyst which can use a solvent-free mixture of
equimolar alkene and silane, has excellent functional group
compatibility, uses an air-stable catalyst precursor, and can
convert internal alkenes into terminal silanes under solvent-free
conditions. This regioconvergent reaction could simplify the
hydrosilylation of mixtures of alkenes using an inexpensive, easy
to use catalyst.
The catalysts are β-diketiminate-supported cobalt(I)−arene
complexes 2a−2e (Chart 1), which were synthesized in three
steps from commercial precursors (Scheme 1). The cobalt(II)
chloride precursors 1b−1e came from diketimines, which can
be prepared on large scale from anilines.22 The cobalt(II)
chloride precursors were reduced using Grignard reagents or
using potassium graphite to yield cobalt(I) products that were
crystallographically characterized: in 2b and 2e, cobalt(I) was
lkene hydrosilylation (eq 1) is one of the largest-scale
A
homogeneous catalytic reactions and is used to make
lubricants, adhesives, rubbers, release coatings, and stationary
phases for chromatography.1,2
[M]
R3SiH + R′CHCH2 ⎯⎯→ R′CH2CH2SiR3
(1)
Although platinum complexes such as Speier’s3 and
Karstedt’s4 catalysts are widely used because of their stability
and high turnover frequencies, the precious-metal catalyst is
often not recoverable (in applications such as polymer cross-
linking and silicone release coatings).2 Therefore, catalysts
derived from more abundant metals are an area of active
interest, in both academics and industry.5 Some current
challenges for nonplatinum catalysts are limited catalyst
stability, functional group compatibility, and the ability to
convert mixtures of alkenes into a single, terminal silane
product.
Chart 1. Cobalt(I) β-Diketiminate Complexes Used in This
Study and Their Steric G Parameters
First-row transition metal catalysts for hydrosilylation have
been known for decades,6 but traditional iron7 and cobalt8,9
carbonyl catalysts require high temperatures and suffer from
side reactions such as dehydrogenative silylation and isomer-
ization.10 More recently, iron catalysts reported by Chirik,11,12
Huang,13 Thomas,14,15 and Ritter16 have addressed some of
these limitations, but not all in the same system. Another
limitation is that these reactions use terminal alkenes to give the
desired linear products. Internal alkenes, whether naturally
occurring, resulting from impurities, or resulting from isomer-
ization during hydrosilylation, are generally unreactive for
alkene hydrosilylation.13
Received: August 14, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.5b08611
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
significant amounts of internal alkenes,27 but isomerization was
minor with 2e. These studies again identify 2e as the most
reactive and selective catalyst. Therefore, subsequent studies used
catalyst 2e without added solvent.
Scheme 1. An Example of Catalyst Synthesis
To investigate the limitations of 2e, we examined the
catalytic hydrosilylation of alkenes bearing functional groups
(Scheme 2). Functionalized alkenes bearing silane, silyl ether,
bound to a solvent arene, while, in complexes 2c and 2d,
cobalt(I) was bound to a diketiminate arene to form a dimer.
The supporting ligands span a range of size from 26% to 63% of
the metal overshadowed, as quantified using the G parameter
(Chart 1 and Table S1).23
Scheme 2. Functionalized Hydrosilylation Products from
PhSiH3 Using Catalyst 2e
a
Initial hydrosilylation experiments were conducted with a
equimolar mixture of 1-hexene (1 M) and PhSiH3 (1 M) using
1 mol % loading (10 mM) of catalysts 2a−d in C6D6 solution
1
under an atmosphere of N2. The results were analyzed by H
NMR spectroscopy with mesitylene as an internal standard
(Table 1, entries 1−4). The catalyst supported by the most
Table 1. Hydrosilylation of 1-Hexene with PhSiH3
a
catalyst
reaction conditions
yield (selectivity)
1.0% 2a
1.0% 2b
1.0% 2c
1.0% 2d
0.05% 2e
1 M, 40 °C, 24 h
1 M, 40 °C, 16 h
1 M, 40 °C, 2 h
1 M, 40 °C, 2 h
neat, 25 °C, 1 h
no reaction
90% (>98%)
91% (>98%)
96% (>98%)
97% (>98%)
a
Yields are isolated yields after purification by column chromatog-
a
1
1
raphy. Linear selectivities were determined by H NMR or GC-MS
analysis and appear in parentheses. 2 equiv of PhSiH3 added. Small
amount of 1,1-disilylalkane was detected. 50 μL of THF were added
Yields and selectivities determined by H NMR analysis.
b
c
d
e
bulky ligand (2a) gave no hydrosilylation products, while all
other reactions gave the linear (anti-Markovnikov) silane in
≥90% yield at 40 °C. Higher reactivity was observed under the
same conditions with less bulky supporting ligands.24 Catalyst
2e, with the least bulky supporting ligand, was active at low
loading (0.05 mol %) at room temperature without any solvent:
after 1 h, a 97% yield of hexylsilane was formed with >98%
linear selectivity. 1% of Markovnikov product and <1%
disubstituted silane were detected by GC-MS, and no
dehydrogenated hydrosilylation products were observed. The
rate and the selectivity of hydrosilylation were unchanged when
a drop of Hg was added to the hydrosilylation reactions
catalyzed by 2a−e, suggesting that the reaction is not being
catalyzed by solid or nanoparticulate cobalt.25
to the 1.0 mmol reaction to dissolve solid alkene. 11% internal alkene
f
detected by 1H NMR spectroscopy. The reaction mixture was heated
to 60 °C.
halide, arene, ester, tertiary amine, and amide substituents were
hydrosilylated at room temperature by 2e, as shown by
isolation of 4e−n in 74−94% yield with high linear selectivity.
Some of the products were accompanied by a small amount
(7% (4d), 5% (4m)) of a 1,1-disilylalkane product, and a longer
reaction time was required for bulky substrate 3d. Hydro-
silylation of 1,5-hexadiene successfully gave 4c, but conjugated
dienes (1,3-hexadiene and 1,3-butadiene) gave no conversion.
Hydrosilylation of selected terminal alkenes with (EtO)3SiH
gave a good yield and selectivity under the same conditions
used in Table 2 (Scheme 3), though the rates are slower with
this less-reactive silane (4 h at 60 °C, compared to 1 h at rt
using PhSiH3).
During the studies with the less-active catalyst 2b, we noticed
that it slowly catalyzed hydrosilylation of internal hexenes to
give the terminal silane selectively (Table 3). This has been seen
previously with certain Zr catalysts,28 Pt catalysts,29 and Fe
catalysts.11 We propose that this transformation is a tandem
isomerization−hydrosilylation consisting of two steps: (1) rapid
isomerization of the alkene and (2) selective hydrosilylation of
the terminal alkene. In order to maximize the efficiency of this
tandem process with the Co system, we took advantage of the
fact that 2b can isomerize internal hexenes into an equilibrium
mixture of all isomers,27 and paired it with 2e, which is the most
rapid hydrosilylation catalyst. As shown in Tables 3 and 4, a
mixture of 2 mol % of 2b and 0.5 mol % of 2e was effective for
Consistent with the trend observed with PhSiH3, reactions of
the bulkier, less reactive triethoxysilane12,26 worked best with
catalyst 2e (Table 2). Again, no vinylsilane or allylsilane was
observed. Catalysts 2a−2d with larger supporting ligands gave
Table 2. Hydrosilylation of 1-Hexene with (EtO)3SiH
a
catalyst
internal alkene
yield (linear selectivity)
2.0% 2c
2.0% 2d
2.0% 2e
63%
57%
5%
12% (96%)
23% (>98%)
89% (>98%)
a
Yields and selectivities determined by GC analysis.
B
DOI: 10.1021/jacs.5b08611
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Journal of the American Chemical Society
Communication
(EtO)3SiH, it is particularly likely that the catalysts have
different roles: 2b isomerizes the internal alkene into the
terminal alkene, which is hydrosilylated with 2e.
Scheme 3. Functionalized Hydrosilylation Products from
(EtO)3SiH Using Catalyst 2e
a b
,
Although catalysts 2 must be stored and handled under N2,
the cobalt(II) chloride precursor 1e is stable in dry air for days
(see Supporting Information for details). This allows for the in
situ generation of active catalyst 2e using standard air-free
techniques on the benchtop without the need for a glovebox.
We found that even after standing open in an air-filled
desiccator for 2 days, 1e was analytically pure and could be
activated with nBuLi or EtMgBr in high yield (or using KOtBu
in somewhat lower yield) to give catalytically active mixtures
that gave alkene hydrosilylation in yields similar to those
observed using isolated 2e. In the most convenient protocols
(Table 5), sequential addition of 1 equiv of EtMgBr, 200 equiv
a
b
Isolated yields after distillation under reduced pressure. Linear
Table 5. Comparison of in Situ Formed Catalysts
c
selectivity determined by 1H NMR and GC-MS analysis. 15%
1
ethylidenecyclohexane and 7% 1-ethylcyclohex-1-ene detected by H
d
NMR spectroscopy. 13% internal alkene detected by 1H NMR
spectroscopy.
a
silane
reagent
time
yield (selectivity)
Table 3. Hydrosilylation of Internal Alkenes with PhSiH3
PhSiH3
PhSiH3
PhSiH3
PhSiH3
nBuLi
2 h
2 h
1 h
2 h
85% (>98%)
85% (>98%)
91% (>98%)
KOtBu
EtMgBr
EtMgBr
b
84% (>98%)
a
1
hexene
isomer
2b
2e
alkene
yield
Yields and linear selectivities determined by H NMR spectroscopy
a
(mol %)
(mol %)
remaining
(selectivity)
b
and GC-MS analysis. Gram scale reaction on benchtop, which
provided 1.62 g of 1-hexylphenylsilane.
b
cis-2-
cis-2-
cis-2-
cis-3-
trans-2-
0
0.5
0
83%
4%
9% (ND)
2.0
2.0
2.0
4.0
83% (>98%)
85% (>98%)
86% (>98%)
60% (>98%)
0.5
0.5
0.5
3%
4%
of PhSiH3, and 200 equiv of 1-hexene to 1e under N2 gave a
>90% yield of 1-hexyl-1-phenylsilane with >98% linear
selectivity in 2 h at room temperature. This procedure was
effective on a gram scale using standard Schlenk techniques,
giving a yield that was only slightly lower than a smaller-scale
reaction performed in a glovebox (last entry, Table 5).
Reaction of 1-hexene with PhSiD3 gave the d3 product with
deuteration only at the Si−H and the β-carbon. No deuterium
scambling was observed, suggesting that there is no β-hydride
elimination before reductive elimination of the C−Si bond.
However, we cannot yet distinguish whether the catalysis uses a
standard Chalk−Harrod type pathway or a radical pathway.
Though the rapid rate of catalysis has so far hindered kinetic
studies, mechanistic studies are underway.
33%
a
The yield and linear selectivity was determined by 1H NMR
b
spectroscopy after 4 h. The linear selectivity could not be determined
1
by H NMR and GC-MS due to the low intensity.
Table 4. Hydrosilylation of Internal Alkenes with (EtO)3SiH
hexene
isomer
2b
(mol %)
2e
(mol %)
alkene
remaining
yield
(selectivity)
a
cis-2-
0
2.0
2.0
2.0
2.0
77%
12%
32%
4% (ND)
cis-2-
4.0
4.0
4.0
71% (>98%)
62% (95%)
34% (96%)
cis-3-
Overall, these results show the utility of β-diketiminate-
supported cobalt(I)−arene catalysts for catalytic alkene hydro-
silylation. Future studies will address the mechanism of these
reactions, the identity of the cobalt intermediates, and the
development of improved catalysts.
trans-2-
20%
a
1
Yields and linear selectivities determined by H NMR spectroscopy
and GC-MS analysis after 12 h.
regioselective isomerization/hydrosilylation of cis-2-hexene or
cis-3-hexene by PhSiH3 to form the terminal product, 1-
hexylphenylsilane. The mixture of catalysts was much more
effective for these internal alkenes than catalyst 2e by itself,
supporting the proposed role of 2b for isomerization. The trans
isomers were less reactive in the tandem reaction, perhaps
because they do not coordinate as well to the metal center.27
We note that some silane products can be oxidized to alcohols
through a Tamao−Fleming oxidation, and so this provides a
potential route to linear alcohols from internal alkenes.14,30
Catalytic hydrosilylation of internal hexenes with (EtO)3SiH
was also improved with the mixture of catalysts (Table 4).
Since catalyst 2b is inactive toward hydrosilylation with
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.5b08611.
■
S
Crystallographic data (CIF)
Details of synthesis and characterization (PDF)
AUTHOR INFORMATION
■
Corresponding Authors
*daniel.weix@rochester.edu
*patrick.holland@yale.edu
C
DOI: 10.1021/jacs.5b08611
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
(27) Chen, C.; Dugan, T. R.; Brennessel, W. W.; Weix, D. J.;
Holland, P. L. J. Am. Chem. Soc. 2014, 136, 945.
(28) Takahashi, T.; Suzuki, N.; Saburi, M.; Negishi, E. J. Am. Chem.
Notes
The authors declare no competing financial interest.
Soc. 1991, 113, 8564.
ACKNOWLEDGMENTS
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(30) Related isomerization/hydroboration reactions: (a) Obligacion,
J. V.; Chirik, P. J. J. Am. Chem. Soc. 2013, 135, 19107. (b) Ruddy, A. J.;
Sydora, O. L.; Small, B. L.; Stradiotto, M.; Turculet, L. Chem. - Eur. J.
2014, 20, 13918. (c) Palmer, W. N.; Diao, T.; Pappas, I.; Chirik, P. J.
ACS Catal. 2015, 5, 622.
■
Funding was provided by the U.S. Department of Energy,
Office of Basic Energy Sciences, Grant DE-FG02-09ER16089.
We thank Megan Reesbeck for experimental assistance.
(31) Buslov, I.; Becouse, J.; Mazza, S.; Montandon-Clerc, M.; Hu, X.
Angew. Chem., Int. Ed. 2015, DOI: 10.1002/anie.201507829.
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DOI: 10.1021/jacs.5b08611
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX