B(C6F5)3-Catalyzed Hydrosilylation of Vinylcyclopropanes

Article abstract of DOI:10.1021/acs.orglett.0c02751

A hydrosilylation of vinylcyclopropanes (VCPs) catalyzed by the strong boron Lewis acid B(C6F5)3 is reported. For the majority of VCPs, little or no ring opening of the cyclopropyl unit is observed. Conversely, for VCPs with bulky R groups, such as ortho-substituted aryl rings or branched alkyl residues, ring opening is the exclusive reaction pathway. This finding is explained by the thwarted hydride delivery to a sterically shielded, β-silicon-stabilized cyclopropylcarbinyl cation intermediate.

Full text of DOI:10.1021/acs.orglett.0c02751

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Letter
B(C6F5)3‑Catalyzed Hydrosilylation of Vinylcyclopropanes
Peng-Wei Long, Tao He, and Martin Oestreich*
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ABSTRACT: A hydrosilylation of vinylcyclopropanes (VCPs) catalyzed by the
strong boron Lewis acid B(C6F5)3 is reported. For the majority of VCPs, little
or no ring opening of the cyclopropyl unit is observed. Conversely, for VCPs
with bulky R groups, such as ortho-substituted aryl rings or branched alkyl
residues, ring opening is the exclusive reaction pathway. This finding is
explained by the thwarted hydride delivery to a sterically shielded, β-silicon-
stabilized cyclopropylcarbinyl cation intermediate.
he rhodium-catalyzed hydrosilylation of vinylcyclopro-
panes (VCPs)1 typically results in the opening of the
by a different hydride source and therefore turned toward the
known B(C6F5)3-catalyzed hydrosilylation of alkenes7 (Scheme
1, bottom).8 We report here an ionic9 hydrosilylation of VCPs
that predominantly affords the conventional products, that is,
with little or no ring opening.10
T
cyclopropyl group2 with the exception of an isolated example3
(Scheme 1, top). A recent report on an iron-catalyzed
Scheme 1. Rhodium-Catalyzed Hydrosilylation of VCPs and
Planned B(C6F5)3-Catalyzed Hydrosilylation
A brief optimization of a model reaction showed that the
B(C6F5)3-catalyzed hydrosilylation indeed proceeds with
hardly any opening of the cyclopropyl ring (Table 1). With
1.0 mol % of B(C6F5)3 at room temperature, VCP 1a reacted
with dihydrosilane 2a to mainly give product 3aa in various
solvents. Arene solvents as well as polar aprotic solvents
worked well (entries 1−4), whereas the yield was low in a
hydrocarbon solvent (entry 5). The amount of ring-opened
4aa was usually <10%, and the highest yield of 3aa was
a
Table 1. Selected Examples of the Optimization
yield of
yield of
yield of
b
b
b
entry
solvent
benzene
toluene
chlorobenzene
CH2Cl2
n-hexane
3aa (%)
(E)-4aa (%)
(Z)-4aa (%)
1
2
3
4
5
85
93
73
89
23
7
7
6
11
<5
<5
<5
<5
<5
<5
a
b
All reactions were performed on a 0.15 mmol scale. Yields were
1
hydrosilylation also includes a VCP with an internal double
bond where the cyclopropyl unit has remained intact (not
shown).4 We recently found that VCPs undergo an unusual (5
+ 1) cycloaddition when subjected to trityl-cation-initiated and
subsequent silylium-ion-propagated hydrosilylation (Scheme 1,
middle).5 The reaction commences with the formation of the
corresponding β-silicon-stabilized6 cyclopropylcarbinyl cation
(gray box). Because the hydrosilane is a poor hydride donor,
this intermediate engages in various bond reorganizations. We
wondered whether that same carbenium ion could be captured
determined by H NMR spectroscopy using CH2Br2 as an internal
standard.
Received: August 17, 2020
https://dx.doi.org/10.1021/acs.orglett.0c02751
© XXXX American Chemical Society
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obtained in toluene. A catalyst loading of 0.50 mol % resulted
in a decreased yield (76% instead of 93%).
Scheme 3. Scope II: Variation of the VCP in the
Hydrosilylation with Et2SiH2
a
A broad range of hydrosilanes could be employed in this
reaction (Scheme 2). Other dihydrosilanes reacted in good
a
Scheme 2. Scope I: Variation of the Hydrosilane
a
Unless otherwise noted, reactions were performed using 5.0 mol %
of B(C6F5)3. The conversion in parentheses and the distribution of
ring-opened byproducts in square brackets were determined by NMR
analysis. Yields are of analytically pure material obtained after flash
b
c
chromatography on silica gel. 1.0 mol % of B(C6F5)3. 2.0 mol % of
d
e
B(C6F5)3. 0.50 mol % of B(C6F5)3. 75% (99%) along with <5% of
ring-opened products obtained on a 1.0 mmol scale.
a
Unless otherwise noted, reactions were performed using 0.50 mol %
of B(C6F5)3. The conversion in parentheses and the distribution of
ring-opened byproducts in square brackets were determined by NMR
analysis. Yields are of analytically pure material obtained after flash
yields: 81% for Ph2SiH2 (2a), 78% for MePhSiH2 (2b), and
72% for Et2SiH2 (2c). In turn, the success with mono-
hydrosilanes was dependent on their steric demand. No
reaction was seen with Ph3SiH (2d), but mixed alkyl/aryl-
substituted derivatives did convert; isolated yields were 48 and
66% with MePh2SiH (2e) and Me2PhSiH (2f), respectively.
According to this trend, fully alkyl-substituted Et3SiH (2g) and
EtMe2SiH (2h) brought about even higher yields. The
trihydrosilane PhSiH3 (2i) furnished the product in 39% yield.
We continued the study with Et2SiH2 (2c) to assess the
substrate scope (Scheme 3). Aside from a few exceptions,
electronically modified aryl-substituted VCPs 1b−l reacted in
moderate to good yields. Electron-rich 1e with an OMe group
in the para position did not participate, likely because of Lewis
pair formation with the catalyst. Yields were generally lower for
VCPs with electron-deficient aryl groups, and the CF3-
substituted congener 1f did not fully convert, even with an
increased catalyst loading of 5.0 mol %. We attribute this to the
poorer stabilization of the carbenium-ion intermediate. Also,
the formation of the ring-opened product became competitive
in these cases. Several other R groups were tolerated: benzyl, as
in 1m, cyclopropyl, as in 1n, primary alkyl, as in 1o, and
alkynyl, as in 1p, underwent the hydrosilylation in reasonable
yields. However, the hydrosilylation of benzyl- and n-hexyl-
substituted VCPs 1m and 1o was accompanied by substantial
b
c
chromatography on silica gel. 5.0 mol % of B(C6F5)3. 3.0 mol % of
B(C6F5)3.
ring opening (32% of 3mc along with 47% 4mc and 53% of
3oc along with 22% 4oc, respectively). VCPs with internal
double bonds did not react.
The above substrate selection is biased in the sense that it
does not include any sterically hindered VCPs. The reason for
this is that these do exclusively follow the ring-opening
pathway (Scheme 4). VCPs having an ortho-substituted aryl or
an α-naphthyl group, as in 1q−s, as well as a branched alkyl
group, as in 1t, do not produce any of the conventional
hydrosilylation product 3. The ring-opening products 4 were
formed in moderate yields throughout. Moreover, the same
observation was made for VCP 1u with a quaternary carbon
atom (R′ ≠ H). We think that steric shielding of the β-silicon-
stabilized cyclopropylcarbinyl cation intermediate thwarts the
hydride delivery from the borohydride.
To support the assumed intermediacy of a β-silicon-
stabilized cyclopropylcarbinyl cation (cf. Scheme 1, gray
box), we conducted deuterium-labeling experiments with two
representative VCPs (Scheme 5, top). With Et3SiD (2g-d1),
B
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gauged, and the functional-group tolerance is acceptable, given
that catalysis with B(C6F5)3/hydrosilane combinations is often
not compatible with oxygen Lewis-basic sites.8
Scheme 4. Scope III: VCPs Undergoing Ring Opening in
the Hydrosilylation with Et2SiH2
a
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c02751.
General procedures, experimental details, character-
ization, and spectral data for all new compounds
(PDF)
AUTHOR INFORMATION
Corresponding Author
■
Martin Oestreich − Institut fur Chemie, Technische Universitat
̈
̈
Berlin, 10623 Berlin, Germany; orcid.org/0000-0002-
1487-9218; Email: martin.oestreich@tu-berlin.de
Authors
Peng-Wei Long − Institut fur Chemie, Technische Universitat
̈
̈
Berlin, 10623 Berlin, Germany; orcid.org/0000-0003-
4740-1541
̈
̈
Tao He − Institut fur Chemie, Technische Universitat Berlin,
10623 Berlin, Germany; orcid.org/0000-0001-5327-8186
a
Yields are of analytically pure material obtained after flash
chromatography on silica gel. Unless otherwise noted, alkene isomers
have been quantitatively separated.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c02751
Scheme 5. Deuterium-Labeling Experiments and Simple
Mechanistic Picture
a
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The work was funded by the Deutsche Forschungsgemein-
schaft (DFG, German Research Foundation) under Germany’s
Excellence Strategy (EXC 2008/1-390540038). P.-W.L. thanks
the China Scholarship Council for a predoctoral fellowship
(2019−2023), and M.O. thanks the Einstein Foundation
Berlin for an endowed professorship.
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