Olefin isomerization and hydrosilylation catalysis by lewis acidic organofluorophosphonium salts

Article abstract of DOI:10.1021/ja410379x

Organofluorophosphonium salts of the formula [(C₆F ₅)_3-xPh_xPF][B(C₆F₅) ₄] (x = 0, 1) exhibit Lewis acidity derived from a low-lying σ* orbital at P opposite F. This acidity is evidenced by the reactions of these salts with olefins, which catalyze the rapid isomerization of 1-hexene to 2-hexene, the cationic polymerization of isobutylene, and the Friedel-Crafts-type dimerization of 1,1-diphenylethylene. In the presence of hydrosilanes, olefins and alkynes undergo efficient hydrosilylation catalysis to the alkylsilanes. Experimental and computational considerations of the mechanism are consistent with the sequential activation and 1,2-addition of hydrosilane across the unsaturated C-C bonds.

Full text of DOI:10.1021/ja410379x

Communication
pubs.acs.org/JACS
Olefin Isomerization and Hydrosilylation Catalysis by Lewis Acidic
Organofluorophosphonium Salts
Manuel Perez, Lindsay J. Hounjet, Christopher B. Caputo, Roman Dobrovetsky, and Douglas W. Stephan*
́
Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada
S
* Supporting Information
under dilute conditions. 1H and 13C{1H} NMR spectra indicate
the formation of both cis- and trans-2-hexene as the major
products (Scheme 1) along with small quantities of polyolefin
ABSTRACT: Organofluorophosphonium salts of the
formula [(C6F5)3−xPhxPF][B(C6F5)4] (x = 0, 1) exhibit
Lewis acidity derived from a low-lying σ* orbital at P
opposite F. This acidity is evidenced by the reactions of
these salts with olefins, which catalyze the rapid isomer-
ization of 1-hexene to 2-hexene, the cationic polymer-
ization of isobutylene, and the Friedel−Crafts-type
dimerization of 1,1-diphenylethylene. In the presence of
hydrosilanes, olefins and alkynes undergo efficient hydro-
silylation catalysis to the alkylsilanes. Experimental and
computational considerations of the mechanism are
consistent with the sequential activation and 1,2-addition
of hydrosilane across the unsaturated C−C bonds.
Scheme 1. Phosphonium-Catalyzed Isomerization of 1-
Hexene to 2-Hexene
(see Supporting Information). Clearly, this observation
suggests a direct interaction of the 1 with the olefin. In this
regard, it is interesting to note that electrophilic boranes, which
are less Lewis acidic than 1, have been shown to exhibit van der
Waals interactions with olefins.12,13 Thus, isomerization of the
terminal olefin is thought to proceed via its interaction with the
Lewis acidic phosphonium center, generating an incipient
carbocation that can undergo a 1,3-proton migration (Scheme
1). In a separate experiment, cis-2-hexene was added to a
C6D5Br solution containing 1. After 2 h the partial isomer-
ization to trans-2-hexene was observed spectroscopically,
indicating that 1 also catalyzes the cis/trans isomerization of
an internal olefin.
In a similar fashion, a stirred CH2Cl2 solution of 1 at −78 °C
was subjected to 1 atm of isobutylene for 30 min. From the
resulting viscous mixture, poly(isobutylene) was isolated. GPC
data showed MW of 4.0 × 105 Da, with a polydispersity of 1.7.
1H and 13C{1H} NMR analysis were consistent with the linear
polymer being the major component.10 Interaction of
isobutylene with 1 is thought to produce a relatively stable
tertiary carbocation that facilitates rapid cationic polymer-
ization. In contrast, the analogous reaction of 1,1-diphenyl-
ethylene with 1.5 mol % 1 in CH2Cl2 resulted in a Friedel−
Crafts-type cyclodimerization product. Such reactivity is known
to occur in the presence of Lewis acids including B(C6F5)3.14,15
Having observed interactions of fluorophosphonium salts
with olefins, trials to effect olefin hydosilylation catalysis were
undertaken. In an initial run, Et3SiH and 1-hexene were added
to a CD2Cl2 solution containing 1.5 mol % of catalyst 1. 1H and
13C{1H} NMR spectroscopic analyses of the mixture showed
espite the extensive use of phosphorus compounds as
DLewis bases in organometallic chemistry and catalysis,
there are a few examples of such species exhibiting Lewis
acidity. Gudat,1 Burford,2 Yoshifuji,3 Bertrand,4 and others have
explored phosphenium cations as Lewis acids. Such P(III)
centers are predicted to exhibit fluorophilicities comparable to
known neutral Lewis acids.5 P(V) centers such as those in ylide
reagents are Lewis acidic, prompting their classic reactions with
ketones,6 whereas phosphonium cations mediate additions to
polar unsaturates7 and Diels−Alder reactions.8 More recently,
the Lewis acidity of phosphonium cations has been exploited by
̈
Gabbai and co-workers to enhance the fluoride ion selectivity of
adjacent B/P electrophiles.9 Targeting further applications of
Lewis acidic phosphonium cations, we incorporated electron-
withdrawing aryl substituents, recognizing that these groups
would offer additional stabilization of the σ* LUMO at P.10,11
This feature, together with the cationic charge, was expected to
generate a highly electrophilic center. Indeed, the species
[(C6F5)3−xPhxPF][B(C6F5)4] (1, x = 0; 2, x = 1) have been
shown to exhibit Lewis acidity, binding the oxygen-donors
DMF and Et3PO.11 The latter result indicated that the salt 1 is
more Lewis acidic than B(C6F5)3.11 This characteristic renders
these species sufficiently fluorophilic to effect C−F bond
activations of fluoroalkanes via hydrodefluorination catalysis.11
Given their Lewis acidity, we herein report the reactivity of
organofluorophosphonium salts with olefins and alkynes,
demonstrating their unique ability to effect olefin isomerization
and hydrosilylation of olefins and alkynes.
The reaction of 1-hexene with phosphonium salt 1 was
investigated. For example, using 1 mol % 1 in CD2Cl2, nearly
complete conversion of 1-hexene to 2-hexene was observed
within <1 h at ambient temperature. This reaction is slowed
Received: October 9, 2013
© XXXX American Chemical Society
A
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Communication
that the reaction was complete after 30 min, and the product
triethyl(hexyl)silane (3a) was isolated in 88% yield (Table 1).
units, consistent with a Si···H···P interaction. This feature is
consistent with previously reported data11 and is reminiscent of
observations made by Piers and co-workers who described
hydrosilylation catalysis by B(C6F5)3, which serves to activate
the silane.16−21 Thus, the phosphonium cation catalysts appear
to activate both olefin and silane in independent experiments,
bringing into question the pathway for hydrosilylation. It is
noteworthy that during hydrosilylation catalysis, no evidence
for olefin isomerization or polymerization was observed.
a
Table 1. Hydrosilylation Reactions
Calculations at the wB97XD/def2-TZV level of theory22,23
were conducted by modeling interactions of the phosphonium
ion with silane and olefin. Computations have previously shown
that the LUMO of the phosphonium cation contains the σ*
orbital oriented opposite the P−F bond. It is this orbital that
was found to be the acceptor, prompting interaction of
hydrosilane with 1. The calculations also reveal that this
interaction results in a ΔH of −15.2 kcal mol−1 This
stabilization was found to be greater than the corresponding
interaction of 1 with olefin (ΔH = −8.1 kcal mol−1; see
Supporting Information). These data are, thus, consistent with
a mechanism in which phosphonium activates hydrosilane. This
view is further supported by the observation that attempts to
effect hydrosilylation with the sterically encumbered silane
iPr3SiH shows no reaction over a 24 h period. Experimental
data for the hydrosilylation of methylcyclohexene (Scheme 2)
Scheme 2. Proposed Mechanism of Hydrosilylation
a
Hydrosilane/olefin mixtures were added to 1 or 2 in CH2Cl2,
CD2Cl2, or C6D5Br and left at ambient temperature, except for 3j (left
at 60 °C). Isolated yields are reported, except for 3b (determined by
1H NMR).
Using the less-electrophilic phosphonium salt 2 in an analogous
reaction required that the catalyst loading be increased to 9 mol
% to achieve complete conversion after 5 h. Similar
hydrosilylations of 1-hexene were also carried out using silanes
Me2ClSiH or Ph3SiH in the presence of 1.5 mol % 1, resulting
in excellent product yields. The above protocol was compatible
with styrenes, resulting in their complete conversions and near-
quantitative yields in all cases (Table 1). It is noteworthy that
the p-chloro-substituted product (3g) is accessible, allowing the
possibility of further functionalization of the arene. In the case
of p-methoxy-α-methylstyrene, 1 catalyzed the addition of
Et3SiH to the olefin but also resulted in the replacement of the
O-methyl group by Et3Si. Phosphonium cation catalyst 1 was
tolerant of the silyl ether functionality, affording 3i in excellent
yield after 12 h. Internal olefins also underwent hydrosilylation,
as heating trans-2-hexene to 60 °C with Et3SiH in the presence
of 1 gave selective conversion to 3j, whereas cyclohexene
reacted with this silane to produce 3k in good yield at room
temperature. Finally, the alkynes dec-1-yne and diphenylacety-
lene were hydrosilylated with Et3SiH, producing selectively the
cis-olefinic products 4a and 4b, respectively, in high yields.
Efforts to garner some mechanistic insights were undertaken.
To this end, the 1H NMR spectrum of Et3SiH with 5 mol % 1
in CD2Cl2 at ambient temperature shows a dramatically
using Et3SiH afforded the product 3l using either B-
24−26
(C6F5)3
or 1 as the catalyst. In the case of borane-
catalyzed hydrosilylation, previous studies by the groups of
Gervorgyan26 and Oestreich27 have unambiguously established
an anti 1,2-addition of Si−H to olefin. A similar mechanism has
been demonstrated by Oestrich for ketone and imine
hydrosilylation.24,25 Thus, in the present case, a similar route
is proposed in which addition of olefin to phosphonium-
activated silane generates a carbocation24,25 to which the
hydride is delivered in an anti-fashion. The stability of the
fluorophosphonium cations throughout the catalysis support
the view that free silylium cation is not generated as silylium
ions would be expected to degrade the fluorophosphonium
cation to yield the notoriously strong Si−F bond.10,11
3
broadened SiH signal with no observable JHH to the CH2
B
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Journal of the American Chemical Society
Communication
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phosphonium cations with suitable substituents is sufficiently
low in energy to result in strong Lewis acidity. This feature is
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hydrosilylation of olefins and acetylenes. This approach affords
a new avenue to Lewis acid catalysis and further expands the
applications of metal-free catalysts into reactivity traditionally
achieved by transition metal catalysts. We are continuing to
probe the reactivity of these and related electrophilic
phosphonium cations.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Preparations of compounds, reaction procedures, NMR spectra,
and computational details. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
D.W. Stephan, dstephan@chem.utoronto.ca
■
(37) Oestreich, M.; Rendler, S. Angew. Chem., Int. Ed. 2005, 44, 1661.
Notes
(38) Konigs, C. D. F.; Klare, H. F. T.; Oestreich, M. Angew. Chem.,
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
The authors gratefully acknowledge the support of NSERC of
Canada. D.W.S. is grateful for the award of a Canada Research
Chair. C.B.C. gratefully acknowledges the receipt of an NSERC
of Canada CGS-D and a Walter C. Sumner Fellowship.
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dx.doi.org/10.1021/ja410379x | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX