Angewandte
Chemie
DOI: 10.1002/anie.201108714
Homogeneous Catalysis
Platinum-Catalyzed Hydrosilylations of Internal Alkynes: Harnessing
Substituent Effects to Achieve High Regioselectivity**
Douglas A. Rooke and Eric M. Ferreira*
Dedicated to Professor Barry M. Trost on the occasion of his 70th birthday
The metal-catalyzed hydrosilylation of C–C multiple bonds is
a well-studied and effective transformation.[1] Studies con-
cerning catalyst activity, coordination environment, and
mechanism have provided substantial insight into the nature
of this reaction. Vinylsilanes, the product of the monohydro-
silylation of alkynes, have proven to be highly useful synthetic
compounds, and versatile reagents in both nucleophilic
additions and cross-couplings.[2] Interestingly, the vast major-
ity of research on alkyne hydrosilylation has focused on the
anti-Markovnikov addition of a silane across a terminal
alkyne. Hydrosilylations of internal alkynes, comparably,
have received significantly less attention. A primary reason
for this scarcity is that regioselectivity can be difficult to
control when applying hydrosilylations to substrates featuring
disubstituted alkynes.
As a component of our efforts toward accessing geo-
metrically complementary a silylenones, we recently de-
scribed the platinum-catalyzed hydrosilylations of ynones.[3]
Simply by using PtCl2 in PhCH3 at room temperature, the
reactions proceeded in good yields with high stereoselectivity
for the E silylenones [Eq. (1); TBS = tert-butyldimethylsilyl].
The regioselectivity was quite high in most of the cases
initially evaluated. The selectivity was diminished, however,
in the hydrosilylation of ynone 5, where we observed a 2.1:1 a/
b ratio of vinylsilane products [Eq. (2)].
(BzMe2Si)(m-H)Pt}2][4] he posited that an electronic effect
dictated the regioselectivity of hydrosilylations. Both terminal
and internal alkynes fit into this paradigm, but the only
unsymmetrical internal alkynes evaluated were 2-pentyne
and 2-hexyne. This electronic effect has been a recurrently
observed trend in platinum-catalyzed internal alkyne hydro-
silylations, but reports and applications have been somewhat
sporadic, primarily emphasizing singular specific cases.[5,6] An
exceptional example by Alami and co-workers evaluated
arylacetylenes using either PtO2 or H2PtCl6.[7] Marko and co-
workers also investigated a number of internal alkynes as part
of a study of hydrosilylations using NHC/Pt complexes.[8]
Catalysis based on other metals has also been investigated,
with organoyttrium complexes that provide steric-dictated
regioselectivity, representing the most general example of
non-platinum-catalyzed cis hydrosilylations.[9–11]
Intrigued by the chemo-, regio-, and stereoselectivity of
our observed hydrosilylation with ynones, we sought to
further expand upon these studies. Taking into account the
advent of significant developments in the synthetic utility of
vinylsilanes, most notably in cross-coupling, we believed an
evaluation of the aspects that define the reactivity profile of
hydrosilylations was in order. Herein, we describe a more
detailed analysis of the platinum-catalyzed hydrosilylation of
internal alkynes, thus illustrating the predominant and
potentially quantifiable electronic influence on the regiose-
lectivity of the process. We also highlight other factors,
namely steric and functional group effects that attenuate this
regioselection.
Varyingly regioselective hydrosilylations of internal
alkynes have been observed in multiple settings before.
From Tsipisꢀs seminal experimental analysis using [{(Cy3P)-
We initially attributed the ynone hydrosilylation regiose-
lectivity to the electronic effect based on the differential
inductive properties of a ketone and an aliphatic group.
Consistent with Tsipisꢀs rationale[4] and based on the Chalk–
Harrod mechanism for platinum-catalyzed hydrosilyla-
tions,[12] we reasoned that the alkyne-coordinated PtII com-
plex 9 was delivering hydride to the more electron-deficient
alkyne carbon atom (Scheme 1). Reductive elimination from
[*] D. A. Rooke, Prof. E. M. Ferreira
Department of Chemistry, Colorado State University
1872 Campus Delivery, Fort Collins, CO 80523 (USA)
E-mail: emferr@mail.colostate.edu
[**] Colorado State University is acknowledged for the support of our
program. Eli Lilly is acknowledged for a graduate research fellow-
ship to D. A. R.
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2012, 51, 3225 –3230
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3225