Organic Letters
Letter
1
monitored by H NMR. (n-Octadecyl)2SiH2 (6a) was fully
respective hydrosilanes in good yields (eq 8, Scheme 4).
Accordingly, we propose that the present reactions proceed first
by base-catalyzed disproportionation of alkoxysilanes to give
hydrosilanes; upon catalysis by 1a, hydrosilylated alkenes gave
the alkylhydrosilanes.
consumed in 1.5 h, and (n-octadecyl)3SiH was produced in a
nearly quantitative yield. On the other hand, if 3 equiv of
(MeO)4Si were added to the reaction mixture, the conversion
of 6a was less than 40% in 6 h. This result suggests that the
(MeO)4Si byproduct inhibits the hydrosilylation of alkenes 6a−
6f with (alkyl)2SiH2, leading to selective formation of
dialkylsilanes in the reaction of alkenes with (MeO)3SiH.
The above reactions could be applied for the synthesis of
alkyl hydrosilanes containing different alkyl groups. For
example (2-(cyclohex-3-en-1-yl)ethyl)(methyl)(octyl)silane
(7a) was obtained in a 68% yield by stepwise addition of 1-
octene and 4-vinylcyclohex-1-ene (eq 5).
Because Me2SiH2, MeSiH3, and SiH4 were proposed as
intermediates in the hydrosilylation reactions reported here, it
is important to compare the rate of their generation and that of
hydrosilylation. For large scale applications, the rate of
hydrosilylation should be faster than the rate of disproportio-
nation in order to avoid the buildup of flammable
intermediates, especially the pyrophoric SiH4. When an actual
reaction of 1-decene with Me2(MeO)SiH was monitored by 1H
NMR, a substoichiometric amount of Me2SiH2 was detected in
the beginning, but it disappeared after 15 min. This result
suggests that the generation of Me2SiH2 is faster than
hydrosilylation, but hydrosilylation is sufficiently fast so that
the intermediate is quickly consumed. In the reaction of 1-
decene with Me(EtO)2SiH, only Me(Decyl)SiH2, but not
MeSiH3, was detected as an intermediate in the beginning of
the reaction. This result suggests that hydrosilylation is faster
than the generation of MeSiH3. And in an actual reaction of 1-
Several experiments were carried out to provide preliminary
mechanistic insights into these reactions. The addition of a
catalytic amount of NaOtBu to Me2(MeO)SiH led to a quick
release of Me2SiH2, detected by 1H NMR. Likewise, the
addition of a catalytic amount of NaOtBu to (MeO)3SiH led to
the formation of SiH4, again detected by H NMR. Thus, the
alkoxysilanes were proposed to react first with a base to give
hydrosilanes in a sequence shown in eqs 6−7, Scheme 4.
1
decene with (MeO)3SiH, no SiH4 was detected by H NMR.
The above-mentioned results suggest that the reactions of
alkenes with Me(EtO)2SiH or (MeO)3SiH described here are
potentially suitable for large scale applications because there
was no significant buildup of flammable MeSiH3 or pyrophoric
SiH4.10 Even the reactions of alkenes with Me2(MeO)SiH
might be suitable for large scale application given proper
handling, because only a small amount of Me2SiH2 was formed
as an intermediate in the beginning of the reaction. Me2SiH2
has good solubility in organic solvents and is not pyrophoric.
The superior activity of 1a compared to other nickel
complexes (Table 1) is tentatively attributed to the former’s
inability to catalyze isomerization of alkenes under basic
conditions, as well as its low efficiency for the hydrosilylation
with alkoxysilanes. It was found that, under the reaction
conditions of Table 1, various amounts of internal alkenes were
formed using 1b−1d and NiCl2(PPh3)2 as catalysts. Only a
minor amount of internal decenes were formed during the
reaction (<5% by GCMS) when 1a was used as the catalyst.
Moreover, less than 3% of conventional hydrosilylation product
(Decyl)(Me)2Si(OMe) was formed in the reaction of 1-decene
with Me2(MeO)SiH using 1a as precatalyst, while up to 19% of
this compound was formed using other nickel catalysts in Table
1. Likewise, for the reactions with 2b and 2c (Scheme 3) using
1a as precatalyst, only 3−8% of conventional hydrosilylation
products were formed.
1
Scheme 4. Proposed Mechanism
Nucleophilic attack of an alkoxide anion on Me2(MeO)SiH
gave the five-coordinate species (A), which reacted with
another molecule of Me2(MeO)SiH to produce Me2SiH2 and
Me2(MeO)2Si, in a disproportionation manner. The base was
also regenerated in this step. Similar base-catalyzed dispro-
portionation reactions of alkoxysilanes were previously
reported.9 The disproportionation of alkoxysilanes was
followed by 1H NMR. Me2(MeO)SiH was converted to
Me2SiH2 and Me2(MeO)2Si under 0.5 mol % of NaOtBu in
THF-d8 in 50 min. An analogous reaction of Me(EtO)2SiH
required 20 min to complete. The dispropotionation of
trialkoxysilane (MeO)3SiH was even faster: in the presence of
0.1 mol % of NaOtBu the conversion was completed after 5
min. Thus, the rate of disproportionation follows the order:
(MeO)3SiH > Me(EtO)2SiH > Me2(MeO)SiH. It was noted
that the nickel complex did not accelerate the rate of
In summary, we have developed a novel catalytic method
allowing the use of alkoxy hydrosilanes as surrogates of gaseous
silanes in hydrosilylation reactions of alkenes. While serving as a
convenient and safer alternative to methods directly using
Me2SiH2, MeSiH3, and SiH4, the present approach actually
facilitates the “formal” use of these underexplored reagents in
chemical synthesis.
ASSOCIATED CONTENT
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* Supporting Information
The Supporting Information is available free of charge on the
Complex 1a was found to be an efficient catalyst for the
hydrosilylation of 1-octene by Ph2SiH2 and Et2SiH2 in the
presence of 1 equiv of NaOtBu (relative to 1a), giving the
C
Org. Lett. XXXX, XXX, XXX−XXX