Bis(imino)pyridine cobalt-catalyzed dehydrogenative silylation of alkenes: Scope, mechanism, and origins of selective allylsilane formation

Article abstract of DOI:10.1021/ja5060884

The aryl-substituted bis(imino)pyridine cobalt methyl complex, ( ^MesPDI)CoCH₃ (^MesPDI = 2,6-(2,4,6-Me ₃C₆H₂-N=CMe)₂C₅H ₃N), promotes the catalytic dehydrogenative silylation of linear α-olefins to selectively form the corresponding allylsilanes with commercially relevant tertiary silanes such as (Me₃SiO) ₂MeSiH and (EtO)₃SiH. Dehydrogenative silylation of internal olefins such as cis- and trans-4-octene also exclusively produces the allylsilane with the silicon located at the terminus of the hydrocarbon chain, resulting in a highly selective base-metal-catalyzed method for the remote functionalization of C-H bonds with retention of unsaturation. The cobalt-catalyzed reactions also enable inexpensive α-olefins to serve as functional equivalents of the more valuable α, ω-dienes and offer a unique method for the cross-linking of silicone fluids with well-defined carbon spacers. Stoichiometric experiments and deuterium labeling studies support activation of the cobalt alkyl precursor to form a putative cobalt silyl, which undergoes 2,1-insertion of the alkene followed by selective β-hydrogen elimination from the carbon distal from the large tertiary silyl group and accounts for the observed selectivity for allylsilane formation.

Full text of DOI:10.1021/ja5060884

Article
pubs.acs.org/JACS
Bis(imino)pyridine Cobalt-Catalyzed Dehydrogenative Silylation of
Alkenes: Scope, Mechanism, and Origins of Selective Allylsilane
Formation
Crisita Carmen Hojilla Atienza,† Tianning Diao,† Keith J. Weller,‡ Susan A. Nye,‡ Kenrick M. Lewis,§
Johannes G. P. Delis,∥ Julie L. Boyer,‡ Aroop K. Roy,‡ and Paul J. Chirik*,†
†Department of Chemistry, Frick Laboratory, Princeton University, Princeton, New Jersey 08544, United States
‡Momentive Performance Materials, Inc., 260 Hudson River Road, Waterford, New York 12188, United States
§Momentive Performance Materials, Inc., 769 Old Saw Mill River Road, Tarrytown, New York 10591, United States
∥Momentive Performance Materials BV, Plasticslaan 1, 461PX Bergen op Zoom, The Netherlands
S
* Supporting Information
ABSTRACT: The aryl-substituted bis(imino)pyridine cobalt
methyl complex, (MesPDI)CoCH3
(
MesPDI = 2,6-(2,4,6-
Me3C6H2-NCMe)2C5H3N), promotes the catalytic dehy-
drogenative silylation of linear α-olefins to selectively form the
corresponding allylsilanes with commercially relevant tertiary
silanes such as (Me3SiO)2MeSiH and (EtO)3SiH. Dehydro-
genative silylation of internal olefins such as cis- and trans-4-
octene also exclusively produces the allylsilane with the silicon located at the terminus of the hydrocarbon chain, resulting in a
highly selective base-metal-catalyzed method for the remote functionalization of C−H bonds with retention of unsaturation. The
cobalt-catalyzed reactions also enable inexpensive α-olefins to serve as functional equivalents of the more valuable α, ω-dienes
and offer a unique method for the cross-linking of silicone fluids with well-defined carbon spacers. Stoichiometric experiments
and deuterium labeling studies support activation of the cobalt alkyl precursor to form a putative cobalt silyl, which undergoes
2,1-insertion of the alkene followed by selective β-hydrogen elimination from the carbon distal from the large tertiary silyl group
and accounts for the observed selectivity for allylsilane formation.
Scheme 1. Metal-Catalyzed Hydrosilylation and
Dehydrogenative Silylation
INTRODUCTION
■
The anti-Markovnikov hydrosilylation of terminal alkenes with
precious metal catalysts is well-established and widely
practiced for the commercial manufacture of adhesives,
surfactants, fluids, and molding products.1 Base-metal
alternatives, particularly iron catalysts with higher activity,
have been reported and often operate without solvent with
high selectivity for anti-Markovnikov addition.2−6 An alter-
native and related catalytic process is dehydrogenative
silylation where carbon−silicon bond formation occurs but
the unsaturation of the alkene substrate is retained (Scheme
1). Formal loss of dihydrogen accompanies C−Si bond
formation, and both vinyl- and allylsilanes are possible
products.
Dehydrogenative silylation was first identified as an
undesired side pathway that is competitive with metal-
catalyzed hydrosilylation, and catalysts that selectively furnish
either vinyl- or allylsilanes remain rare.1,7−9 Most catalysts for
alkene dehydrogenative silylation rely on precious metals such
as rhodium,10−12 iridium,13,14 ruthenium,15 platinum,16 and
palladium.17 Examples with alternative metals including
titanium,18 nickel,19 rhenium,20 and iron21 have been reported,
although regioselectivity (vinyl- versus allylsilane) and stereo-
selectivity are often poor. Nakazawa and co-workers recently
described an iron-catalyzed variant of this reaction whereby
diene substrates lacking allylic hydrogens undergo C−Si bond
formation to yield a vinylsilane with the other olefin serving as
an internal H2 acceptor.22 Given the utility of allylsilanes in
synthesis,23 base-metal-catalyzed dehydrogenative silylation is
a potentially attractive route to these valuable products. Other
precious-metal-catalyzed routes to allyl- and vinylsilanes
include silyl-Heck-type reactions24−26 and selective alkyne
hydrosilylations.27,28
Received: June 23, 2014
© XXXX American Chemical Society
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Unlike with iron, cobalt-catalyzed carbon−silicon bond
forming reactions remain rare. Seminal examples include
cobalt carbonyl and phosphine complexes that produced low
readily available olefins to function as synthetic equivalents of
more valuable α,ω-dienes and provides a unique method of
tethering polysiloxane fluids with defined hydrocarbon chain
lengths. The origin of the high selectivity of these reactions
and activation modes of the base-metal catalyst has been
elucidated.
Chart 1. (MesPDI)CoX Complexes Evaluated for the
Dehydrogenative Silylation of Alkenes
RESULTS AND DISCUSSION
■
Evaluation of Various Bis(imino)pyridine Cobalt
Precatalysts: Dehydrogenative Silylation of 1-Octene.
Aryl-substituted bis(imino)pyridine cobalt alkyl complexes
were studied for catalytic C−Si bond forming chemistry given
their relative ease of preparation,38 synthetic modularity, and
established performance in catalytic alkene hydrogenation39,40
and tandem isomerization−hydroboration reactions.41 The
readily available mesityl-substituted variant, (MesPDI)CoCH3
(Chart 1), was chosen for initial evaluation using 1-octene and
the commercially relevant tertiary silane, (Me3SiO)2MeSiH. In
neat substrate and in the presence of 2 mol % of the cobalt
complex, selective formation of the anti-Markovnikov allyl-
silane was observed as a 3:1 ratio of E/Z isomers. An
equivalent of octane was also formed and accounts for the
balance of the dihydrogen (Table 1). The reported
conversions are based on consumption of silane to the
allylsilane product.
Other bis(imino)pyridine cobalt precursors were also
evaluated, and the results of these studies are presented in
Table 1. Both cobalt dinitrogen, (ArPDI)CoN2,42 and methyl
complexes are effective precursors, and as is often observed,2
the catalytic activity increases with smaller 2,6-aryl sub-
stituents. Notably, both (MesPDI)CoOH and (MesPDI)CoCl
are also effective precatalysts, suggesting that catalytic
dehydrogenative silylation reactions may be tolerant to small
quantities of water or halide impurities. Because of the ease of
preparing (MesPDI)CoCH3 and its relatively high activity in
catalytic dehydrogenative silylation, subsequent experiments
were conducted using this cobalt precursor. We also note that
similar catalytic performance was obtained by in situ activation
of the more readily accessible cobalt dihalide, (MesPDI)CoCl2,
with 2 equiv of NaBEt3H, LiCH3, DIBAL-H (diisobutyl-
aluminum hydride) or LiN(SiMe3)2.43
hydrosilylation activity for terminal alkenes with olefin
isomerization and nonselective dehydrogenative silylation as
significant side reactions.29−33 Grant and Brookhart have
reported a cyclopentadienyl cobalt complex for the selective
anti-Markovnikov hydrosilylation of 1-hexene with Et3SiH.34
Isotopic labeling studies established alkene insertion into a
cobalt silyl followed by chain running and ligand substitution
as key steps. Experimental35 and computational studies36 on
various cyclopentadienyl cobalt complexes with neutral ligands
(L = CO, ethylene, phosphine) have been conducted to gain
fundamental insight into the transformations such as Si−H
oxidative addition that are likely relevant to catalytic turnover.
Deng and co-workers have since described cobalt complexes
supported by N-heterocyclic carbene with pendant silyl
donors that are active for the hydrosilylation of 1-octene
with PhSiH3.37 Although high turnover numbers were
observed, the scope of the cobalt-catalyzed reaction beyond
these two silane substrates has not been reported. Here we
describe bis(imino)pyridine cobalt-catalyzed methods for the
dehydrogenative silylation of alkenes to selectively form
allylsilanes with the silyl substituent located exclusively at
the terminus of the hydrocarbon chain. The method has been
extended to internal olefins and provides a functionalization
sequence for remote C−H bonds where unsaturation is
maintained in the final product. This methodology allows
Table 1. Evaluation of Bis(imino)pyridine Cobalt Compounds for Catalytic Carbon−Silicon Bond Formation
a
a
entry
cobalt precursor
iPrPDI)CoN2
(EtPDI)CoN2
% conversion (1 h)
% conversion (24 h)
1
2
3
4
5
6
7
8
(
<2
39 (10:1)
>98 (5:1)
87
>98 (3:1)
<2
(
(
(
(
(
MesPDI)CoN2
iPrPDI)CoCH3
MesPDI)CoCH3
MesPDI)CoOH
MesPDI)CoCl
38
>98 (3:1)
65
>98 (3:1)
>98 (3:1)
32
18
b
[(MesPDI)CoCH3][BArF ]
<2
4
a
Determined by GC-FID. In all cases, the % conversion was based on consumption of silane. In all cases, a 1:1 molar ratio of octane to allylsilane was
b
observed. Values in parentheses are E/Z ratios. BArF = B(3,5-(CF3)2-C6H3)4. The cobalt dinitrogen complexes were prepared as described
4
previously.42
B
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Table 2. Evaluation of Silanes in the Bis(imino)pyridine-Catalyzed Dehydrogenative Silylation of 1-Octene
silane
time
% conversion
E/Z ratio
(Me3SiO)2MeSiH
(EtO)3SiH
Et3SiH
15 min
15 min
24 h
>98
>98
>98
3:1
3:1
3:1
2:1
a
Ph2SiH2
15 min
15 min
>98
b
c
PhSiH3
>98
2:1
a
b
c
With 45% of the Markovnikov hydrosilylation product observed. The stoichiometry was 1:4 PhSiH3/1-octene. Overall E/Z ratio of the two octyl
substituents.
Table 3. Silylation of Alkenes Using (Me3SiO)2MeSiH in the Presence of 0.5 mol % of (MesPDI)CoCH3
a
Based on consumption of the silane and determined by GC-FID. Unless stated otherwise, all product mixtures contain an equimolar quantity of
b
c
d
alkane. Percentage in parenthesis corresponds to propylsilane. Reaction performed at 65 °C. Cyclohexene (5 equiv) was used relative to silane,
and 86% of the hydrosilylated product was observed. E isomer only.
e
The silane substrate scope of the cobalt-catalyzed
dehydrogenative silylation was also explored. Initial emphasis
was on tertiary silanes due to their commercial relevance.
Because of the high activity and selectivity observed with 1-
octene and (Me3SiO)2MeSiH, each of the subsequent silane
evaluation experiments was conducted with 0.5 mol % of
yielded PhSi(2-octenyl)2H as a 2:1 mixture of E and Z
isomers.
The alkene scope of the bis(imino)pyridine cobalt-catalyzed
dehydrogenative silylation was also explored and is presented
in Table 3. Entries 1−6 demonstrate that simple, unactivated
α-olefins underwent rapid and selective conversion to the
corresponding anti-Markovnikov allylsilanes. In all cases, 1
equiv of the corresponding alkane accompanied the catalytic
dehydrogenative silylation reactions. The reported yield of
products is based on silane. The functional group tolerance of
the reaction was briefly evaluated, and introduction of an
allylic N,N-dimethylamino substituent (entry 10) or an allyl
polyether (entry 11) had little impact on the activity or
selectivity of catalytic turnover as complete conversion of
silane to allylsilane was observed in less than 1 h at 23 °C. In
cases where mixtures of E and Z isomers are observed, the
ratios remained constant throughout the reaction and in
(
MesPDI)CoCH3. As reported in Table 2, (EtO)3SiH was also
effective for dehydrogenative silylation, reaching complete
conversion in approximately 15 min at 23 °C. Reduced
activity was observed with Et3SiH requiring 24 h for complete
formation of the allylsilane product. In both cases, a 3:1 ratio
of E/Z isomers of terminally functionalized allylsilanes were
observed. Primary and secondary silanes also participated in
the cobalt-catalyzed dehydrogenative silylation of 1-octene
(Table 2). Ph2SiH2 underwent silylation to afford a mixture of
2-octyldiphenylsilane and 2-octenyldiphenylsilane with the E
and Z products in a 2:1 ratio. With PhSiH3, multiple
dehydrogenative silylations occurred per silane and principally
C
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Scheme 2. Cobalt-Catalyzed Dehydrogenative Silylation of Dienes
Scheme 3. Application of 1-(TMSO)2MeSi-2-octene to Synthetic Transformations
subsequent chemistry. As such, we are unable to distinguish
whether the mixture is kinetic or thermodynamic in origin.
Substrates lacking allylic hydrogens were examined to probe
the possibility of alternative outcomes of the dehydrogenative
silylation process. For 3,3-dimethylbutene (entry 7), the E
isomer of the vinylsilane was observed exclusively along with
the requisite quantity of 2,2-dimethylbutane. The geminal
olefin isobutene (entry 8) produced poor selectivity for
allylsilane, yielding only 15% of the product after 24 h.
Conversion, however, was complete with the remainder of the
product being identified as vinylsilane. Reduced selectivity was
obtained with cyclohexene (entry 9), as significant quantities
(86%) of the hydrosilylation product were observed.
Because the catalytic reactions consume 1 equiv of the
alkene substrate, performing the dehydrogenative silylation in
the presence of sacrificial olefins as hydrogen acceptors was
explored. One strategy, pioneered by Nakazawa and co-
workers using cyclopentadienyl iron carbonyl precatalysts,22
relies on diene substrates where one olefin undergoes
dehydrogenative silylation and the other undergoes hydro-
genation. With a neat mixture of 1,5-hexadiene and
(Me3SiO)2MeSiH in the presence of 1 mol % of (MesPDI)-
CoCH3, a complex mixture of silylated products was observed.
The desired 1-silylhex-2-ene product was observed along with
1-silyl-2,5-hexadiene, 1,6-disilyl-2,4-hexadiene, 1-hexene, and
hexane, suggesting that an internal hydrogen acceptor is not
selective. Likewise, stirring 2:1 mixtures of either 4-vinylcyclo-
hexene or 5-vinyl-2-norbornene with (Me3SiO)2MeSiH and 1
mol % of (MesPDI)CoCH3 resulted in complete (>98%)
conversion of the silane to silylated products over the course
1 h at 23 °C (Scheme 2). With 4-vinylcyclohexene, a 3:1
mixture of the allyl- and vinylsilanes was obtained. For the
vinylsilanes, the E isomer was formed, whereas no preference
for either isomer was observed with the allylsilane. In the case
of 5-vinyl-2-norbornene, the allylsilane product was obtained
as a 1:1 E/Z mixture in 90% yield. Importantly, no products
featured hydrogenation of the internal alkene.
Addition of a sacrificial alkene as the hydrogen acceptor was
also studied. The dehydrogenative silylation of 1-octene with
(Me3SiO)2MeSiH was conducted with 1 mol % of (MesPDI)-
CoCH3 in the presence of 10 equiv of cyclohexene, cyclo-
octene, 1,5-cyclo-octadiene, or norbornene and produced no
perturbation to the product mixture. By contrast, with the
addition of 10 equiv of tert-butyl ethylene (TBE) as the H2
acceptor, 63% of the 1-octene was converted to a 3:1 mixture
of allylsilane to octane (eq 1). Small amounts, approximately
5% of the vinylsilane, were observed from dehydrogenative
silylation of the TBE. Attempts to perform “acceptorless”
dehydrogenative silylation by increasing the amount of silane
relative to olefin were conducted with the goal of intercepting
the putative cobalt hydride, (MesPDI)CoH, to liberate H2. No
perturbation to the product mixture of octane and silyloctene
was observed when performing the catalytic dehydrogenative
silylation with a 10:1 ratio of (Me3SiO)2MeSiH to 1-octene.
D
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Scheme 4. Cobalt-Catalyzed Dehydrogenative Silylation of Internal Olefins To Yield Terminally Functionalized Allylsilanes
a
Scheme 5. Cobalt-Catalyzed Tandem Isomerization−Dehydrogenative Silylation of Allylsilanes
a
Alkane byproduct denotes alkyl silane.
1
product by H NMR spectroscopy established a 79:21 mixture
of diastereomers.
Cobalt-Catalyzed Dehydrogenative Silylation of In-
ternal Olefins: Application to Remote C−H Functional-
ization. The dehydrogenative silylation of linear, internal
alkenes was also studied with (MesPDI)CoCH3 to explore the
selectivity of the process and potential for remote C−H bond
functionalization. Our laboratory recently reported that
bis(imino)pyridine iron45 and cobalt41 precatalysts are
effective for the tandem isomerization−hydroboration of
internal alkenes, providing a convenient method for remote
hydrofunctionalization of terminal methyl groups. If success-
ful, cobalt-catalyzed dehydrogenative silylation would provide
a similar strategy for remote terminal functionalization but
maintain the unsaturation of the substrate. Controlling the
selectivity of the reaction is potentially more challenging given
the number of possible regioisomers of the alkene and the
silyl group.
With a base-metal-catalyzed method for the synthesis of
allylsilanes in hand, the utility of the products in additional
synthetic transformations was briefly assayed. Using 1-
(TMSO)2MeSi-2-octene as a representative example, Tamao-
Fleming oxidation (Scheme 3)44 yielded the corresponding
alcohol in 82% isolated yield while treatment with meta-
chloroperbenzoic acid (m-CPBA) followed by tetrabutyl-
ammonium fluoride (TBAF) furnished 1-octen-3-ol in 65%
yield (Scheme 3). An allyl transfer reaction was also explored
as addition of benzaldehyde dimethylacetal to 1-
(TMSO)2MeSi-2-octene in the presence of BF3·Et2O as
Lewis acid generated the methyl ether, 3-((methoxyphenyl-
methyl)-1-octene, in 76% yield (Scheme 3). Analysis of the
Stirring a 2:1 mixture of cis-4-octene and (Me3SiO)2MeSiH
in the presence of 1 mol % of (MesPDI)CoCH3 resulted in
E
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Table 4. Cross-Linking of Polymeric Silylhydrides with α-Olefins in the Presence of 2000 ppm (MePDI)CoCH3
siloxane
α-olefin
product label
yield of polymer
Si-H A (0.500 g)
Si-H A (0.285 g)
Si-H B (0.825 g)
Si-H B (0.580 g)
1-octene (0.750 g)
Si-H A/C8
Si-H A/C18
Si-H B/C8
Si-H B/C18
0.860 g (98%)
0.733 g (96%)
0.985 g (95%)
0.868 g (95%)
1-octadecene (0.965 g)
1-octene (0.485 g)
1- octadecene (0.672 g)
quantitative conversion to the terminal allylsilane (Scheme 4).
The same outcome was observed with trans-4-octene. With
both alkene isomers, the dehydrogenative silylation was slower
than that with terminal alkenes requiring 24 h to reach
complete conversion. Monitoring the course of the reaction
by 13C NMR spectroscopy established the intermediacy of 2-
and 3-octene isomers. Performing the dehydrogenative
silylation with (EtO)3SiH produced a mixture of silane
redistribution products as well as octyl and octenylsilanes,
suggesting that the more reactive silane intercepts cobalt
internal alkyl intermediates. With Et3SiH, however, the
allylsilane was obtained in good yields (70−75%).46 Excess
olefin was used to push the reaction to completion.
the product as the vinylsilane, D, arising from Markovnikov
Si−H addition and olefin isomerization. Such products may
prove valuable to exploit the differential reactivity of the
silanes. In all dehydrogenative silylation reactions, regardless
of the identity of the silane, terminal olefins such as 1-octene
and 1-butene have proven to be much more reactive than the
corresponding allylsilane product.
Application to Silicone Cross-Linking. The cobalt-
catalyzed dehydrogenative silylation method allows readily
available and inexpensive α-olefins to serve as functional
equivalents of α,ω-dienes. In most cases, the dienes are
expensive and, for many longer chain lengths, not
commercially available. We sought to apply the remote C−
H functionalization chemistry to the cross-linking of silicone
fluids as tethering two poly(dimethyl)−poly(methylhydrogen)
siloxanes by a defined carbon tether would provide access to
new materials with potentially unique physical properties. In a
typical experiment, a mixture of a poly(dimethyl)−poly-
(methylhydrogen) siloxane containing [Si−H] functionality
was stirred with an α-olefin in the presence of 2000 ppm (0.5
mol %) of (MesPDI)CoCH3 (mg Co/kg reaction mixture) at
65 °C for 4 h. Two poly(dimethyl)−poly(methylhydrogen)
siloxanes, Si-H A and Si-H B, that differ by the amount of
[Si−H] content were examined, while both 1-octene and 1-
octadecene were evaluated to determine the influence of
hydrocarbon tether length on the resulting polymer proper-
ties. The results of these experiments are reported in Table 4.
Each reaction initially yielded a hard gel which, following
grinding and washing with hexane to remove the alkane
byproduct, furnished white powders.
The degree of double silylation in the silicone polymers was
determined to be between 46 and 92%, depending on the
starting silane. These determinations were made using a
combination of IR-ATR spectroscopy, solid-state NMR
spectroscopy, and chemical degradation experiments. Repre-
sentative spectra and results of Toepler pump experiments are
reported in the Supporting Information. In general, use of
longer chain α-olefins resulted in higher degrees of double
silylation, likely due to the lower volatility of the alkene and
the resulting alkane product that maintains a fluid reaction
mixture. Higher degrees of double silylation were observed
with decreasing amount of [Si−H] groups in the polysilox-
anes. This again is likely a result of the lower viscosity of the
partially silylated material, allowing the catalyst to mix with
the polymer and promote additional reactivity.
The facile and selective dehydrogenative silylation of
internal olefins prompted the study of allylsilanes as a
means of introducing a second silyl group and accomplishing
a net, remote C−H functionalization reaction. Stirring a neat
2:1 molar mixture of 1-(Me3SiO)2MeSi-2-octene and
(Me3SiO)2MeSiH at 23 °C in the presence of 0.5 mol % of
(
MesPDI)CoCH3 produced an equimolar mixture of the
desired doubly silylated product along with the corresponding
alkane (Scheme 5). Notably, the silyl groups were located
exclusively at the termini of the hydrocarbon chain,
establishing a useful method for the selective, remote
hydrofunctionalization of sp3 C−H bonds with retention of
the allylsilane functionality in the starting material. The major
isomer of the doubly silylated product, A, was identified and
constituted 85% (relative to silane) of the product mixture as
a 3:1 mixture of E/Z isomers, the same ratio found in starting
allylsilanes. The remaining 15% of the material was also
doubly silylated and is a mixture of other regioisomers. The
doubly silylated product, A, was also obtained directly from 1-
octene. Complete conversion was achieved in 24 h using 0.3
mol % of (MesPDI)CoCH3 (Scheme 5). In all cases, removal
of the alkane byproduct was accomplished by fractional
distillation.
The double silylation method was also studied with 1-
butene as the shorter carbon chain facilitates product
characterization by NMR spectroscopy and GC-MS methods.
Addition of (Me3SiO)2MeSiH to neat 1-(Me3SiO)2MeSi-2-
butene in the presence of 1 mol % of (MesPDI)CoCH3
resulted in complete consumption of the silane over the
course of 24 h at 23 °C and furnished the 1,4-disilylated
compound, B, as a 1.8:1 ratio of E and Z isomers in 92%
yield. The remainder of the material was identified as a 1:1
mixture of linear and branched vinylsilanes. Using (EtO)3SiH
as the silane allowed introduction of two different silyl groups
into the hydrocarbon chain and furnished the expected disilyl-
2-butene, C, in 77% yield (E/Z = 2:1) with the remainder of
A final set of control experiments was conducted to
definitively confirm formation of cross-linked material derived
from cobalt-catalyzed dehydrogenative silylation. The cross-
linking of Si-H A was conducted with 1,7-octadiene in the
F
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Scheme 6. Deuterium Labeling Studies in Bis(imino)pyridine Cobalt-Catalyzed Dehydrogenative Silylation of 1-Butene and
1-Octene
presence of 2000 ppm Karstedt’s catalyst, a known hydro-
silylation catalyst. Immediate gel formation and an exothermic
reaction were observed. Analysis of the product using the
ethanolic KOH degradation experiments established 65 4%
double silylation. It should be noted that in this material the
hydrocarbon tether is completely unsaturated. The morphol-
ogy of this material and that prepared from the cobalt-
catalyzed method were indistinguishable.
deuterium incorporation into the 2,6-methyl groups of the
aryl ring, a result of reversible C−H cyclometalation. This
process likely accounts for formation of CH4 and the
instability of the putative cobalt silyl.
The synthesis of another likely catalytic intermediate,
(
MesPDI)CoH, was also explored. Attempts to synthesize
this compound by hydrogenation of (MesPDI)CoCH3 have
been unsuccessful. The more sterically hindered example,
(iPrPDI)CoH, has been established to be observable in
solution but known to convert to the corresponding
dinitrogen complex, (iPrPDI)CoN2, upon exposure to 1 atm
of N2.48,49 This specific cobalt hydride compound was used as
a surrogate for the mesityl variant for understanding olefin
insertion chemistry. Addition of 1 equiv of 1-butene to a
benzene-d6 solution of (iPrPDI)CoH afforded (iPrPDI)Co(n-
butyl), the identity of which was confirmed by independent
Stepwise cross-linking experiments were conducted using
bis(imino)pyridine cobalt-catalyzed dehydrogenative silylation.
Addition of 1-octene (3 equiv per Si−H group) to Si-H A in
the presence of 2000 ppm of (MesPDI)CoCH3 followed by
analysis of the resulting material by 1H and 13C NMR
spectroscopy established formation of octenyl side chains in a
2:1 E/Z ratio. No remaining silyl hydrides were detected by
1H NMR or IR spectroscopies or by degradation with
ethanolic KOH. Fresh Si-H A was then added to the
octenylated polymer in the presence of 2000 ppm of
synthesis from addition of n-butyllithium to (iPrPDI)CoCl.48
A
deuterium labeling experiment was also performed. Addition
of (Me3SiO)2MeSiD50 to the solution of (iPrPDI)CoH in
benzene-d6 resulted in appearance of the (Me3SiO)2MeSi-H
resonance, indicating rapid H/D exchange between the Si−D
and Co−H (eq 2). Importantly, this experiment establishes a
different activation mode for bis(imino)pyridine cobalt alkyls
versus hydrides. For the former, treatment with silane
generates the corresponding cobalt silyl, while for the latter,
degenerate exchange occurs. While the origin of this
difference is currently under investigation, these results have
profound implications for cobalt-catalyzed dehydrogenative
silylation as attempts to intercept the putative cobalt hydride
following β-H elimination with excess silane to regenerate the
cobalt−silyl and H2 will not occur due to these reactivity
preferences.
(
MesPDI)CoCH3. A significantly lower degree (24
4%) of
disilylation was observed in the stepwise process, likely due to
more rapid gel formation and difficult diffusion in the absence
of solvent.
Exploring the Mechanism of Bis(imino)pyridine
Cobalt-Catalyzed Alkene Dehydrogenative Silylation.
The observation of bis(imino)pyridine cobalt-catalyzed
dehydrogenative silylation to selectively form allylsilanes
prompted studies into the mechanism of the transformation.
Questions under consideration included: (i) why do bis-
(imino)pyridine cobalt complexes promote dehydrogenative
silylation while hydroboration selectivity was observed with
the same catalyst? (ii) How is the mechanism different from
Brookhart’s cyclopentadienyl cobalt(III) catalyst, which is
selective for alkene hydrosilylation? (iii) Why are the cobalt
catalysts selective for allylsilane relative to vinylsilane? To
answer these questions, the mechanism of catalytic turnover
was investigated.
Stoichiometric Experiments. Our investigations were
initiated by examining the activation mode of (MesPDI)CoCH3
under catalytic conditions. Addition of 1 equiv of 1-octene to
a benzene-d6 solution of (MesPDI)CoCH3 produced no change
over the course of hours at 23 °C. In contrast, addition of 1
equiv of 1-(Me3SiO)2MeSiH to (MesPDI)CoCH3 resulted in
immediate liberation of a stoichiometric quantity of methane
as determined by a Toepler pump experiment. Analysis of the
1
cobalt product by H and 13C NMR spectroscopy established
a mixture of unidentified products, suggesting that the
putative bis(imino)pyridine cobalt silyl is too reactive for
observation under these conditions. Performing the reaction at
lower temperatures also did not result in observation of the
cobalt silyl. Addition of 1 equiv of (Me3SiO)2MeSiD47 to a
benzene-d6 solution of (MesPDI)CoCH3 resulted in formation
of both CH4 and CH3D. Hydrolysis of the resulting mixture
of cobalt complexes and analysis of the free bis(imino)-
pyridine ligand by 2H NMR spectroscopy established
Catalytic Reations with (Me3SiO)2MeSiH. To gain
insight into the catalyst resting state and possibly the
turnover-limiting step of the cycle, the catalytic reaction
using (MesPDI)CoCH3 was monitored in benzene-d6 solution
1
by H NMR spectroscopy. At approximately 10% conversion,
a mixture of two bis(imino)pyridine cobalt alkyl complexes,
1
(
MesPDI)Co(n-Oct) and (MesPDI)CoCH3, was observed by H
G
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Scheme 7. Deuterium Labeling Studies in Bis(imino)pyridine Cobalt-Catalyzed Dehydrogenative Silylation of 4-Octene
Scheme 8. Determinaton of Deuterium Kinetic Isotope Effects in Bis(imino)pyridine Cobalt-Catalyzed Dehydrogenative
Silylation of 1-Octene
NMR spectroscopy. While (MesPDI)Co(n-Oct) is the catalyst
resting state, the slow activation of (MesPDI)CoCH3 by the
tertiary silane complicates more detailed kinetic measure-
ments. We do note that, at longer reaction times, quantitative
formation of methane was observed (vide supra) and
corroborated by a Toepler pump experiment.
(Me3SiO)2SiMeH and (Me3SiO)2SiMeD were conducted in
the presence of 0.4 mol % of (MesPDI)CoCH3 and
Mes‑d24
(
PDI)CoCH3, respectively. In the deuterated cobalt
compound, the methyl groups on the mesityl substituent of
the imine ligands were also labeled with deuterium to avoid
complications with cyclometalation. After 5 min, the reactions
were quenched and the products analyzed by gas chromatog-
raphy. The natural abundance reaction produced 13% yield of
allylsilane, whereas the reaction with (Me3SiO)2SiMeD
furnished the allylsilane in 5% yield. Comparison of these
values established a deuterium KIE of 2.6 at 23 °C (Scheme
8).
Catalytic dehydrogenative silylation experiments were also
conducted with deuterated silane. The neat dehydrogenative
silylation of 1-octene with (Me3SiO)2MeSiD in the presence
of 1 mol % of (MesPDI)CoCH3 and analysis of the organic
products by 2H NMR spectroscopy after 1 h at 23 °C
established exclusive incorporation of the isotopic label in the
terminal position of the alkane byproduct (Scheme 6).
Deuterium incorporation was lower than expected, likely a
result of competing cyclometalation chemistry within the
cobalt complex. Performing a similar isotopic labeling
experiment with a 2:1 mixture of neat 1-octene and
(EtO)3SiD in the presence of 1 mol % of (MesPDI)CoCH3
also yielded octane-1-d1 along with the expected allylsilane.
Deuterium labeling experiments were also conducted with
trans-4-octene and (TMSO)2MeSiD in the presence of 1 mol
% of (MesPDI)CoCH3 (Scheme 7). Analysis of the organic
The deuterium kinetic isotope effect was also determined
by internal competition. In this experiment, a mixture of 1-
octene with 10 total equiv of silane was prepared. The
composition of the silane was an equimolar mixture of
(Me3SiO)2SiMeH and (Me3SiO)2SiMeD. Addition of 1 mol
% each of (MesPDI)CoCH3 and (Mes‑d PDI)CoCH3 to this
24
mixture followed by quenching after 10 min resulted in
complete conversion of the alkene. Analysis of the octane by
quantitative 13C NMR spectroscopy established a 1.0:0.16
ratio of octane to 1-d1-octane (Scheme 8). Notably, this
measurement also produced a deuterium KIE of 2.6,
consistent with the independent rate measurements.
Proposed Mechanism. A mechanism consistent with the
experimental observations for bis(imino)pyridine cobalt-
catalyzed dehydrogenative silylation is presented in Scheme
9. The sequence begins with activation of (MesPDI)CoCH3
with the tertiary silane to form methane and the putative
cobalt silyl. This activation mode contrasts with alkene
hydroboration where cobalt hydride is formed upon treatment
of the corresponding alkyl complex with HBPin.41 Following
2
products by H and 13C NMR spectroscopies established 70%
incorporation of the deuterium label into octane with the
remaining 30% distributed statistically throughout the allyl-
silane. The observation of deuterium incorporation into the
allylsilane product is likely a result of reversible insertion of
the starting alkene into a cobalt hydride (vide infra).
Determination of Deuterium Kinetic Isotope Effect
for Dehydrogenative Silylation. To determine the
deuterium kinetic isotopic effect for cobalt-catalyzed dehydro-
genative silylation, two parallel catalytic reactions with
H
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Scheme 9. Proposed Mechanism for Bis(imino)pyridine Cobalt-Catalyzed Dehydrogenative Silylation of α-Olefins
generation of the cobalt silyl, the first equivalent of the
terminal alkene undergoes 2,1-insertion, forming a secondary
bis(imino)pyridine cobalt alkyl complex, accounting for the
anti-Markovnikov selectivity in the dehydrogenative silylation.
Subsequent fast β-hydrogen elimination furnishes the
observed allylsilane. The selective formation of allyl- rather
than vinylsilane is derived from preferential β-hydrogen
elimination away from the large tertiary silane substituent
and forms the cobalt hydride, (MesPDI)CoH. The stoichio-
metric experiments and those previously reported by Gibson48
suggest that the insertion of a second equivalent of olefin is
fast to furnish the bis(imino)pyridine cobalt alkyl intermedi-
ate. When internal alkenes such as 4-octene are used as
substrates, either the cobalt hydride or the silyl alkyl complex
can promote chain-running processes through reversible β-H
elimination and olefin insertion events, evident from the
observation of 2- and 3-octenes during the dehydrogenative
silylation process. The observation of 2- and 3-octenes during
the dehydrogenative silylation process suggests that isomer-
ization from the hydride can occur in the absence of C−Si
bond formation. The reaction of bis(imino)pyridine cobalt
alkyl with silane is the turnover-limiting step, liberating the
corresponding alkane and regenerating the cobalt silyl,
consistent with the observation of bis(imino)pyridine cobalt
alkyl as the resting state. This proposal is also in agreement
with the primary deuterium kinetic isotope effects determined
from both independent rate measurement and intermolecular
competition.51
A similar mechanistic proposal has been offered by Grant
and Brookhart for the hydrosilylation of 1-hexene by
cyclopentadienyl cobalt(III) complexes with Et3SiH.34 Nota-
bly, isotopic labeling studies with Et3SiD placed deuterium in
the 6-position of the hexyl silane, consistent with rapid chain
running prior to C−Si bond formation. Activation of the
cobalt alkyl by Et3SiH was proposed to form a cobalt silyl
which undergoes 2,1-insertion of 1-hexene to furnish the silyl
alkyl. Chain running without olefin dissociation forms a
terminal cobalt hexyl complex that upon reaction with
additional Et3SiH releases the observed product and
regenerates the active silyl complex. The difference between
the cyclopentadienyl complex of Brookhart and bis(imino)-
pyridine derivative reported here is that the former does not
dissociate olefin following β-hydrogen elimination while the
latter does. The origin of this discrepancy is likely due to the
presence of an electrophilic and relatively substitutionally inert
Co(III) complex in the cyclopentadienyl case versus a more
reducing and labile Co(II) center in the bis(imino)pyridine
derivative.39,42
The selective formation of allyl- rather than vinylsilane is
derived from preferential β-hydrogen elimination away from
the large tertiary silane substituent and forms the cobalt
hydride. Support for sterically induced selectivity for allylsilane
formation is derived from variation of the alkene substrates
I
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1
dichroic, exhibiting a pink color with a green hue. H NMR (500
MHz, C6D6): δ = 0.26 (s, 6H, C(CH3)), 1.07 (s, 1H, CoOH), 2.10
(s, 12H, o-CH3), 2.16 (s, 6H, p-CH3), 6.85 (s, 4H, m-aryl), 7.49 (d,
2H, 3-pyridine), 8.78 (t, 1H, 4-pyridine). 13C {1H} NMR (125 MHz,
C6D6): δ = 19.13 (o-CH3), 19.42 (C(CH3)), 21.20 (p-CH3) 114.74
(4-pyridine), 121.96 (3-pyridine), 129.22 (m-aryl), 130.71 (o-aryl),
134.78 (p-aryl), 149.14 (i-aryl), 153.55 (2-pyridine), 160.78 (CN).
IR (C6H6): νOH = 3582 cm−1. Anal. Calcd for C27H32CoN3O: C,
68.49; H, 6.81; N, 8.87. Found: C, 68.40; H, 7.04; N, 8.77.
General Procedure for the Dehydrogenative Silylation of
1-Octene with Different Silanes Using (MesPDI)CoCH3. A
scintillation vial was charged with 0.100 g (0.891 mmol) of 1-
octene and 0.449 mmol (0.5 equiv) of the desired silane (0.100 g of
(Me3SiO)2MeSiH, 0.075 g of (EtO)3SiH, or 0.052 g of Et3SiH), and
0.001 g (0.002 mmol, 0.5 mol %) of (MesPDI)CoCH3 was then
added to the mixture. The reaction was stirred at room temperature
for the desired amount of time and then quenched by exposure to
air. The product mixture was analyzed by gas chromatography and
NMR spectroscopy (see Table S3). The alkenylsilane was purified by
passing the mixture through a silica gel column with hexane followed
by removal of the volatiles in vacuo. The reaction of 1-octene and
(Me3SiO)2MeSiH was further optimized using 1.000 g (8.912 mmol)
of 1-octene, 1.000 g (4.494 mmol, 0.504 equiv) of (Me3SiO)2MeSiH,
and 0.003 g (0.006 mmol, 0.14 mol %) of (MesPDI)CoCH3, and
1.413 g (94%) of the allylsilane product was obtained after filtration
through a silica plug.
reported in Table 3. For relatively small α-olefins, such as
propene and 1-octene, the steric profile of the tertiary silane is
dominant and directs β-hydrogenation elimination away from
the large substituent, resulting in exclusive formation of
allylsilane. Introduction of a [CMe3] (Table 3, entry 5) or
[SiMe3] (Table 3, entry 6) along the hydrocarbon chain
increases the steric profile along the alkyl and results in
formation, albeit in minor amounts, of vinylsilane co-product.
A model for these observations is presented in Scheme 10.
Similar control of allyl- versus vinylsilane selectivity using
Scheme 10. Model for Altering the Selectivity for Selective
Allylsilane Formation in Bis(imino)pyridine Cobalt-
Catalyzed Dehydrogenative Silylation
Silylation of 1-Bis(trimethylsiloxy)methylsilyl-2-octene with
(Me3SiO)2MeSiH Using (MesPDI)CoCH3. This experiment was
performed in a manner similar to the silylation of 1-octene using
0.100 g (0.301 mmol) of 1-bis(trimethylsiloxy)methylsilyl-2-octene,
0.034 g (0.152 mmol, 0.51 equiv) of (Me3SiO)2MeSiH, and 0.001 g
(0.002 mmol, 1 mol %) of (MesPDI)CoCH3. The reaction was stirred
at room temperature for 24 h and quenched by exposure to air.
Analysis of the mixture by GC-FID, GC-MS, and NMR spectroscopy
showed an approximately 1:0.85 mixture of 1-bis(trimethylsiloxy)-
methylsilyloctane and 1,8-bis(bis(trimethylsiloxy)methylsilyl)-2-oc-
tene (0.15 equiv are regioisomers). The disilylated product was
determined to be a 3:1 E/Z mixture by NMR spectroscopy. Similar
results were obtained when the vinylsilane, (E)-1-bis(trimethyl-
siloxy)methylsilyl-1-octene, was used.
Alternative Procedure for the Double Silylation of 1-
Octene with (Me3SiO)2MeSiH Using (MesPDI)CoCH3. This
experiment was performed in a manner similar to the silylation of
1-octene using 1.000 g (8.912 mmol) of 1-octene, 1.500 g (6.742
mmol, 0.756 equiv) of (Me3SiO)2MeSiH, and 0.010 g (0.021 mmol,
0.3 mol %) of (MesPDI)CoCH3. The reaction was stirred at room
temperature for 24 h and quenched by exposure to air. Analysis of
the crude mixture by GC-FID showed an approximately 2:1:1
mixture of octane, 1-bis(trimethylsiloxy)methylsilyloctane and 1,8-
bis(bis(trimethylsiloxy)methylsilyl)-2-octene (major isomer, 85%).
The mixture was passed through silica gel using hexane, and the
volatiles were removed in vacuo. The mono- and disilylated products
were separated by vacuum distillation. 1-Bis(trimethylsiloxy)methyl-
silyloctane was isolated as the fraction that boiled at 45 °C (5
mmHg), and the disilylated product was collected as the residue
(1.126 g, 92%).
ligand control has been reported by Watson and co-workers
in palladium-catalyzed silyl Heck chemistry.26
CONCLUDING REMARKS
■
A cobalt-catalyzed method for the dehydrogenative silylation
of alkenes has been discovered. With α-olefins and tertiary
silanes, the catalytic process is highly selective for formation of
allylsilane with the silyl group located exclusively at the
terminus of the hydrocarbon chain. Notably, this selectivity
has been extended to internal alkenes such as cis- and trans-4-
octene, providing a method for the remote silylation of
unactivated C−H bonds with retention of unsaturation. This
method has also been extended to the cross-linking of silicone
fluids where commercially available α-olefins serve as synthetic
equivalents of α,ω-dienes. An additional application of this
method has been made to the synthesis of unsaturated
silahydrocarbons.52 Deuterium labeling experiments in con-
junction with substrate scope and in situ monitoring
experiments support a mechanism involving initial formation
of a cobalt silyl complex followed by 2,1-olefin insertion and
selective β-hydrogen elimination to form a cobalt hydride and
the allylsilane. Insertion of a second equivalent of alkene
followed by turnover-limiting reaction with silane completes
the catalytic cycle. The selectivity for cobalt silyl rather than
cobalt hydride formation as the mode of catalyst activation is
likely the origin of dehydrogenative silylation reactivity.
ASSOCIATED CONTENT
■
EXPERIMENTAL SECTION53
S
* Supporting Information
■
Synthesis of (MesPDI)CoOH. A 20 mL scintillation vial was
charged with 0.100 g (0.203 mmol) of (MesPDI)CoCl, 0.012 g (0.30
mmol, 1.5 equiv) of NaOH, and approximately 10 mL of THF. The
reaction mixture was stirred for 2 days, and a color change from dark
pink to red was observed. The volatiles were removed in vacuo, and
the residue was dissolved in approximately 20 mL of toluene. The
resulting solution was filtered through Celite, and the solvent was
removed from the filtrate in vacuo. Recrystallization of the crude
product from 3:1 pentane/toluene yielded 0.087 g (90%) of dark
pink crystals identified as (MesPDI)CoOH. The compound is
Additional experimental details, including general consider-
ations, complete characterization data for allylsilanes and
polymeric silanes. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
pchirik@princeton.edu
J
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Article
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Notes
The authors declare the following competing financial
interest(s): A patent application has been filed but is yet to
be published: Atienza, C. C. H.; Chirik, P. J.; Lewis, K. M.;
Boyer, J. L.; Delis, J. G. P.; Roy, A.; US Patent Application
filed May 6, 2013 (unpublished).
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̈
ACKNOWLEDGMENTS
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■
We thank Momentive Performance Materials Inc. for financial
support. We also thank Charles Dehany for assistance with
solid-state 29Si NMR spectroscopy.
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