Catalytic Hydrosilylation of Alkenes by a Ruthenium Silylene Complex. Evidence for a New Hydrosilylation Mechanism

Article abstract of DOI:10.1021/ja037620v

Novel ruthenium and osmium silylene species containing Si?H bonds have been synthesized and characterized by NMR spectroscopy. The ruthenium complex [Cp*(ⁱPr₃P)(H)₂Ru=Si(H)Ph·Et₂O][B(C₆F₅)<

Full text of DOI:10.1021/ja037620v

Published on Web 10/18/2003
Catalytic Hydrosilylation of Alkenes by a Ruthenium Silylene Complex.
Evidence for a New Hydrosilylation Mechanism
Paul B. Glaser and T. Don Tilley*
Department of Chemistry and Center for New Directions in Organic Synthesis (CNDOS), UniVersity of California,
Berkeley, Berkeley, California 94720-1460
Received July 29, 2003; E-mail: tdtilley@socrates.berkeley.edu
Table 1. Results from a Series of Catalytic Runs
Hydrosilylation, the addition of silicon and hydrogen across a
multiple bond, is an important reaction type in organic synthesis
and polymer chemistry, and for the production of organosilicon
compounds.1 The wide variety of catalysts available for these
reactions permits synthetic control over the stereo-, regio-, and
enantioselectivities of product formation. The largest group of
catalysts for the hydrosilylation of alkenes features electron-rich
transition metals such as platinum and rhodium, which typically
operate by Chalk-Harrod type mechanisms that result in cis
addition of the Si-H bond to an alkene.2 Catalysts based on d0
metal centers mediate Si-C bond formation via a σ-bond metathesis
process and give fewer side products due to alkene isomerization.3
Finally, strong Lewis acids have been employed as hydrosilylation
catalysts that affect trans additions of silanes to alkenes.4
a
silane
alkene
product
yield %
PhSiH3
PhSiH3
PhSiH3
PhSiH3
PhSiH3
PhSiH3
1-hexene
ethene
ethene-d4
cyclohexene
styrene
PhSi(H)2Hex (7)
77
54
N/A
72
94
PhSi(H)2CH2CH3 (8)
PhSi(H)2CD2CD2H (9)
PhSi(H)2Cy (10)
PhSi(H)2CH2CH2Ph (11)
1-methylcyclohexene (() trans-1-PhSiH2-
70
2-Me-cyclo-C6H10 (12)
HexSiH3 1-hexene
Hex2SiH2 (14)
57
a Isolated yields. Catalyst loadings were 0.5-5 mol % of 5, reaction
times were 3-18 h. NMR yields were g98% in each case. See Supporting
Information for experimental details.
3Et2O afforded the base-stabilized silylene complex [Cp*(iPr3P)(H)2-
RudSi(H)Ph‚Et2O][B(C6F5)4] (5). The Et2O in 5 is loosely bound
to the silicon center in solution, as demonstrated by its rapid
displacement in CD2Cl2 solution by 1 equiv of either THF or Ph2-
CO. The reaction of 5 with 1 equiv of 1-hexene afforded [Cp*-
(iPr3P)(H)2RudSiPhHex] [B(C6F5)4] (6), as determined by 1H, 13C,
and 29Si NMR spectroscopy. Interestingly, 6 does not bind Et2O in
CD2Cl2 solution (by NMR spectroscopy). Treatment of a solution
of 5 with a 50-fold excess of both 1-hexene and PhSiH3 cleanly
generated PhSi(H)2Hex as the only silicon-containing product. This
catalytic reaction was found to be general for a variety of alkenes,
but was restricted to monosubstituted silanes (Table 1). For example,
no catalysis was observed with substrates such as Et2SiH2 and Ph2-
SiH2. No further reaction of the products to tertiary or quaternary
silanes was observed, even in the presence of excess alkene at long
reaction times. Vinyl silanes were not detected by NMR spectros-
copy or GC/MS. The clean hydrosilylation of difficult substrates
such as 1-methylcyclohexene,11 the observed selectivity for mono-
substituted silanes, the absence of unsaturated products, and the
exclusive anti-Markovnikov regiochemistry of addition are unusual.
In considering possible mechanisms for this reaction, it is worth
noting that the silylene ligand seems to be necessary for the
observed catalysis. Catalytic behavior was observed neither for Cp*-
(iPr3P)(H)2RuSi(H)Ph(OTf) nor for the silyl chloride 4. The Lewis
acidity of the silylene center suggested a mechanism in which 5
behaves similarly to B(C6F5)3 as proposed by Gevorgyan.4a
However, the exclusive cis-hydrosilylation of 1-methylcyclohexene
by 5 and the lack of reactivity of a range of secondary and tertiary
silanes appears to exclude this type of mechanism. A second
mechanistic possibility involves a [2π+2π] cycloaddition of the
metal silicon bond with an alkene. Such processes have been
Transition metal silylene complexes, in which a three-coordinate
silicon center is bound to a metal center, are reactive species that
appear to play roles in a number of catalytic and stoichiometric
processes.5 However, no currently accepted hydrosilylation mech-
anism includes a silylene intermediate. Nevertheless, it seems that
silylene complexes may be potentially useful intermediates in new
hydrosilylation reactions. This is suggested, for example, by the
strong Lewis acidic character of such complexes. In addition,
silylene complexes may participate in cycloaddition reactions, as
indicated by the observation of metal-catalyzed transfers of silylene
fragments to unsaturated carbon-carbon bonds.5d-g Here, we report
an unusual type of hydrosilylation that appears to proceed by the
direct addition of an Si-H bond in a silylene complex to an alkene.
Initial attempts to observe silylene-mediated hydrosilylation
chemistry involved derivatives of the electron-rich Cp*(iPr3P)Os
fragment (Cp* ) η5-C5Me5). As previously communicated, Cp*-
(iPr3P)OsBr readily adds PhSiH3 to afford Cp*(iPr3P)Os(Br)(H)(SiH2-
Ph).6 This reaction type was found to be general for a series of
primary silanes ArSiH3, where Ar ) Mes (2,4,6-Me3), trip (2,4,6-
iPr3C6H2), dipp (2,6-iPr2C6H3), and C6F5.7 Reactions of the resulting
Cp*(iPr3P)Os(Br)(H)(SiH2Ar) with Li[B(C6F5)4] afford unusual,
hydrogen-substituted silylene complexes of the type [Cp*(iPr3P)(H)2-
OsdSi(H)Ar][B(C6F5)4].8 These species are distinguished by a
silicon-bound proton resonance shifted to remarkably low field (e.g.,
+11.5 ppm for Ar ) trip, 2), and by a characteristically downfield
29Si NMR resonance (315 ppm for 2). Reaction of 2 with 1 equiv
of 1-hexene in CD2Cl2 rapidly generated a new silylene complex
formulated as [Cp*(iPr3P)(H)2OsdSi(Hex)trip][B(C6F5)4] (3). How-
ever, attempts to incorporate this Si-C bond-forming reaction into
an osmium-mediated catalytic cycle (e.g., PhSiH3 + 1-hexene +
5% 2, 100 °C in C6H5F, 18 h) were not successful.
suggested, but they have never been directly observed.5c
A
As compared to analogous osmium complexes, ruthenium
complexes are generally more labile and therefore more useful in
catalysis.9 Ruthenium silylene complexes were therefore targeted
as part of a more reactive system for the hydrosilylation of alkenes.
Reaction of Cp*(iPr3P)Ru(H)(Cl)(SiH2Ph)10 (4) with Li[B(C6F5)4]‚
mechanism including such a cycloaddition as the key Si-C bond-
forming step seems unlikely in this case, as the resulting ruthenium-
(VI) metallasilacyclobutane intermediate would have to adopt a
highly crowded conformation to give the observed anti-Markovni-
kov regiochemistry. Chalk-Harrod type mechanisms2 can also be
9
13640
J. AM. CHEM. SOC. 2003, 125, 13640-13641
10.1021/ja037620v CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1. Proposed Catalytic Cycle
Professors Charles Casey, Richard Andersen, and Robert Bergman
for helpful discussions.
Note Added after ASAP. In the Supporting Information file
published 10/18/03, the SiH coupling constants for compounds 1
and 2 were incorrect. The current Supporting Information file
published 10/21/03 is correct.
Supporting Information Available: Experimental details for the
synthesis of new compounds, procedures for catalytic runs, and schemes
illustrating alternate mechanisms (PDF). This material is available free
of charge via the Internet at http://pubs.acs.org.
References
postulated if the silylene complex merely represents a resting state
for the catalyst. In such a mechanism, hydride migration to silicon
would produce the coordinatively unsaturated intermediate [Cp*-
(iPr3P)(H)RuSiH2Ph]+, which could then bind an alkene. Although
it is difficult to entirely discount this mechanism, it would not
explain the observed selectivity toward primary silanes, especially
because the catalysis is insensitive to the steric properties of the
alkene. Furthermore, reaction of PhSiH3 with an excess of C2D4 in
the presence of 5 mol % 5 afforded PhSi(H)2CD2CD2H as the only
observed isotopomer. When deuterated substrates are employed,
catalytic hydrosilylation reactions have previously been found to
afford mixtures of isotopomers via reversible olefin insertion
steps.2c,11b,12
In light of these considerations, we propose that the key Si-C
bond-forming step in the hydrosilylation reaction catalyzed by 5
proceeds by the concerted addition of the Si-H bond of the silylene
across the CdC bond of the substrate in a manner analogous to
the B-C bond-forming step in the hydroboration of alkenes13 (eq
1). Three-coordinate cationic silicon centers are formally isoelec-
tronic with monomeric boranes and can therefore be expected to
participate in similar reactions. Direct addition of alkenes to the
Si-H bond of a metal-silylene complex also finds precedent in
hydrocarbation,14 in which a cationic diiron-supported C-H ligand
adds directly15 to an alkene without the intermediacy of metal-
alkene complexes.
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A proposed mechanism for this catalysis (Scheme 1) features
activation of two Si-H bonds of the silane substrate, direct addition
of an (sp2)Si-H bond to the alkene, and finally 1,2-H migration
and reductive elimination steps.16 This mechanism accounts for all
of the salient features of catalytic hydrosilylation by 5 including
the selectivity for primary silanes (required for formation of a
hydrogen-substituted silylene ligand), catalytic competence toward
cyclic and trisubstituted alkenes, the observed anti-Markovnikov
regiochemistry, and the cis-stereochemistry of addition. Further
details of the reaction pathway are currently being investigated.
(14) (a) Casey, C. P.; Fagan, P. J. J. Am. Chem. Soc. 1982, 104, 4950. (b)
Casey, C. P.; Meszaros, M. W.; Fagan, P. J.; Bly, R. K.; Colborn, R. E.
J. Am. Chem. Soc. 1986, 108, 4053.
(15) Hydrocarbation appears in some cases to operate through a stepwise
mechanism including carbocation intermediates.14b Equation 1 represents
the limiting case of [2σ+2π] cycloaddition in which Si-C bond formation
is synchronous with Si-H bond breaking. The other limiting mechanism
involves formation of a Si-C bond in a four-coordinate, silyl-substituted
carbocation followed by intramolecular hydride migration to carbon. The
addition of alkenes to R3Si+ species to afford silyl-substituted carbocations
may be coupled to intermolecular hydride transfer.4c For a stepwise
addition, exclusive cis-stereochemistry of addition would not necessarily
be expected.
(16) The fragment resulting from reductive elimination of the product in Scheme
1 is drawn as a 14-electron species for simplicity. This intermediate would
more likely exist as a labile adduct with coordinated solvent or substrate,
or with a metalated ligand.
Acknowledgment. This work was supported by the National
Science Foundation. CNDOS is supported by Bristol-Meyers Squibb
as a Sponsoring Member and Novartis Pharama as a Supporting
Member. The authors wish to acknowledge Dr. Terry Krafft, and
JA037620V
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