Merging Platinum-Catalyzed Alkene Hydrosilylation with SiH4 Surrogates: Salt-Free Preparation of Trihydrosilanes

Article abstract of DOI:10.1021/acs.organomet.6b00505

The monosilane (SiH₄) surrogate di(cyclohexa-2,5-dien-1-yl)silane is shown to be compatible with platinum-catalyzed hydrosilylation of α-olefins. The cyclohexa-2,5-dien-1-yl substituents in the monohydrosilylation adducts serve as protecting groups, and treatment with catalytic amounts of B(C₆F₅)₃ liberates the Si-H bonds along with benzene. By this, trihydrosilanes become accessible in two steps without the formation of salt waste.

Full text of DOI:10.1021/acs.organomet.6b00505

Communication
pubs.acs.org/Organometallics
Merging Platinum-Catalyzed Alkene Hydrosilylation with SiH4
Surrogates: Salt-Free Preparation of Trihydrosilanes
Polina Smirnov and Martin Oestreich*
Institut fur Chemie, Technische Universitat Berlin, Strasse des 17. Juni 115, 10623 Berlin, Germany
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* Supporting Information
ABSTRACT: The monosilane (SiH4) surrogate di(cyclohexa-2,5-
dien-1-yl)silane is shown to be compatible with platinum-catalyzed
hydrosilylation of α-olefins. The cyclohexa-2,5-dien-1-yl substitu-
ents in the monohydrosilylation adducts serve as protecting
groups, and treatment with catalytic amounts of B(C6F5)3 liberates
the Si−H bonds along with benzene. By this, trihydrosilanes
become accessible in two steps without the formation of salt waste.
he development of different methods for alkene hydro-
The preparation of trihydrosilanes 5 is generally challenging,
T
silylation continues to attract attention.1 However, the
smallest member of the hydrosilane family, monosilane (SiH4),
is essentially not used, as its handling comes along with
inherent hazards of flammability and toxicity. To provide a safer
alternative to this dangerous gas, a new type of stable
hydrosilane surrogate was recently introduced by us, enabling
the installation of SiH4−n moieties (n = 1−4).2,3 For example,
solid cyclohexa-2,5-dien-1-yl-substituted silane 1 reacts with
various terminal and internal alkenes 2 in the presence of
tris(pentafluorophenyl)borane, B(C6F5)3,4 in an ionic transfer
hydrosilylation (2 → 3−5, Scheme 1, top).3c This one-pot
and the literature-known methods rely on the reduction of
6
either AlkylSiCl3 or AlkylSi(OEt)3.7 As part of our earlier
work,3c we had already demonstrated that solid surrogate 1 and
its liquid congener 6 are compatible with transition-metal-
catalyzed alkene hydrosilylation (one example each). Platinum-
catalyzed hydrosilylation of oct-1-ene had worked equally well
with 1 (69%) and 6 (58%), but the intermediate derived from 6
(quantitative) was far superior to that of 1 (30%) in B(C6F5)3-
catalyzed release of the Si−H bonds. This two-step sequence
consisting of transition-metal-catalyzed hydrosilylation and
Lewis-acid-promoted deprotection offers a straightforward
entry into the salt-free preparation of trihydrosilanes (2 → 7
→ 5, Scheme 1, bottom). In this Communication, we
summarize the elaboration of this methodology to access this
useful class of compounds.8
Scheme 1. SiH4 Surrogates in Alkene Hydrosilylation
We continued testing various α-olefins (Table 1). The
initially obtained yield of 58% in the hydrosilylation of oct-1-
ene3c was further improved to 74% in excess alkene9 (2a → 7a,
entry 1). Likewise, an α-olefin with a cyclohexyl group in the
homoallylic position reacted in 56% yield within 24 h (2b →
7b, entry 2). However, more sterically hindered systems
required longer reaction times for full consumption of surrogate
6 (2c/2d → 7c/7d, entries 3 and 4). Interestingly, when 2c was
used in excess, 2-fold hydrosilylation involving both Si−H
bonds in 6 was observed, furnishing 8c in 51% isolated yield
(not shown; see the Supporting Information for character-
ization data). Reducing the loading of 2c to 1.2 equiv led to the
chemoselective formation of adduct 7c. Allylbenzene was also
suitable for this reaction (2e → 7e, entry 5), although the yield
was diminished due to the formation of β-methylstyrene as
byproduct. That yield was improved when allylpentafluor-
obenzene was used instead (2f → 7f, entry 6). Moreover,
C(sp3)−Br and C(sp3)−Si bonds as well as a silyl ether were
tolerated in the hydrosilylation step (2g−i → 7g−i, entries 7−
process involves two consecutive catalytic cycles where the
same borane catalyst promotes liberation of the Si−H bonds
and then mediates the hydrosilylation of the C−C double bond
through Si−H bond activation.5 This strategy was effective for
the chemoselective synthesis of mono- (3; n = 3) and
dihydrosilanes (4; n = 2)3c but not trihydrosilanes (5; n = 1).
Received: June 20, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.6b00505
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
a
Wilkinson’s catalyst (Ph3P)3RhCl. In contrast, the same
protocol applied to Ph3SiH afforded the hydrosilylation adduct
in good yield. Also, 1,1- and 1,2-disubstituted alkenes did not
undergo hydrosilylation.
Table 1. Preparation of Trihydrosilanes from α-Olefins
The newly developed two-step synthesis provides an
alternative route for the preparation of synthetically useful
trihydrosilanes.8 The use of the liquid monosilane surrogate
3c
6
avoids handling of gaseous SiH4 but also AlkylSiCl3 and
AlkylSi(OEt)3.7 The cyclohexa-2,5-dien-1-yl substituents serve
as an easy-to-remove protecting group at the silicon atom, only
releasing benzene as waste. Moreover, these groups could also
be engaged in transfer hydrosilylation2 to access heteroleptic di-
and triorganosilanes (or dihydro- or monohydrosilanes).3a,b
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00505.
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Synthetic procedures and figures giving NMR spectra of
the compounds synthesized in this paper (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: martin.oestreich@tu-berlin.de.
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a
Unless otherwise noted, the platinum-catalyzed hydrosilylations were
Notes
performed on a 0.4 mmol scale (based on 6) in alkene 2 (0.4 mL) at
The authors declare no competing financial interest.
b
room temperature under an inert atmosphere in a glovebox. Isolated
c
yield after flash chromatography on silica gel. Determined by 1H
ACKNOWLEDGMENTS
d
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NMR spectroscopy using mesitylene as an internal standard. 1.2
equiv of 2c used. Alkene migration observed. Decomposition.
e
f
P.S. gratefully acknowledges the Minerva Foundation for a
postdoctoral fellowship (2016−2017). M.O. is indebted to the
Einstein Foundation (Berlin) for an endowed professorship.
9). All hydrosilylation adducts 7a−h except for 7i underwent
the B(C6F5)3-catalyzed deprotection in CD2Cl2 quantitatively
(7a−h → 5a−h, entries 1−8). The silyl ether in 7i was not
stable toward B(C6F5)3.
To demonstrate the scalability of this methodology, we
performed the two-step synthesis of n-octylsilane on a 4 mmol
scale based on surrogate 6 (cf. 2a → 7a → 5a, entry 1). After
20 h, intermediate 7a was obtained in 82% yield after flash
chromatography on silica gel. Purified 7a was then subjected to
deprotection, and trihydrosilane 5a was isolated by distillation
after 5 h. Attempts to turn this two-step transformation into a
one-pot procedure were unsuccessful.
REFERENCES
(1) Marciniec, B.; Maciejewski, H.; Pietraszuk, C.; Pawluc,
Advances in Silicon Science; Marciniec, B., Ed.; Springer: Berlin, 2009;
Vol. 1, pp 3−51.
(2) For a review of transfer hydrosilylation, see: Oestreich, M. Angew.
Chem., Int. Ed. 2016, 55, 494−499.
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́
P. In
(3) (a) Simonneau, A.; Oestreich, M. Angew. Chem., Int. Ed. 2013, 52,
11905−11907. (b) Keess, S.; Simonneau, A.; Oestreich, M. Organo-
metallics 2015, 34, 790−799. (c) Simonneau, A.; Oestreich, M. Nat.
Chem. 2015, 7, 816−822.
(4) For a recent review of B(C6F5)3-catalyzed Si−H and H−H bond
activation, see: Oestreich, M.; Hermeke, J.; Mohr, J. Chem. Soc. Rev.
2015, 44, 2202−2220.
(5) Sakata, K.; Fujimoto, H. Organometallics 2015, 34, 236−241.
(6) (a) Finholt, A. E.; Bond, A. C., Jr.; Wilzbach, K. E.; Schlesinger,
H. I. J. Am. Chem. Soc. 1947, 69, 2692−2996. (b) Sakurai, H.; Shoji,
M.; Yajima, M.; Hosomi, A. Synthesis 1984, 598−600.
(7) (a) Hosomi, A.; Iijima, S.; Sakurai, H. Chem. Lett. 1981, 243−
246. (b) Mucha, N. T.; Waterman, R. Organometallics 2015, 34, 3865−
3872.
Several other linear α-olefins, 2j−m, functionalized at the
aliphatic terminus as well as α,β-unsaturated acceptors 2n−p
were subjected to platinum-catalyzed hydrosilylation employing
surrogate 6 (Chart 1). Unfortunately, none of them were
Chart 1. α-Olefins and α,β-Unsaturated Acceptors Not
Compatible with the Hydrosilylation Step
(8) For the use of trihydrosilanes in surface modification, see for
example: (a) Fadeev, A. Y.; McCarthy, T. J. J. Am. Chem. Soc. 1999,
121, 12184−12185. (b) Owens, T. M.; Nicholson, K. T.; Banaszak
Holl, M. M.; Suzer, S. J. Am. Chem. Soc. 2002, 124, 6800−6801.
̈
(c) Pelzer, K.; Havecker, M.; Boualleg, M.; Candy, J.-P.; Basset, J.-M.
̈
Angew. Chem., Int. Ed. 2011, 50, 5170−5173.
compatible with the reaction setup, and only small amounts of
the desired adducts 7j−p (not shown) were detected despite
complete conversion of 6. The hydrosilylation of styrene with 6
furnished a complex mixture along with traces of the expected
adduct (2q → 7q; not shown). A similar outcome was found
when the platinum catalyst (cod)PtCl2 was replaced by
(9) The large excess of the alkene is not necessary. For example, 70%
yield was achieved in the model reaction when using 2.0 equiv of the
alkene substrate; surrogate 6 was fully consumed within 20 h.
B
DOI: 10.1021/acs.organomet.6b00505
Organometallics XXXX, XXX, XXX−XXX