Nickel-catalyzed hydrosilylation/cyclization of difluoro-substituted 1,6-enynes

Article abstract of DOI:10.1021/ol102019w

Ni-catalyzed hydrosilylative cyclization of difluoro-substituted 1,6-enynes can be carried out. The presence of a geminal-difluoromethylene group at an alkene terminus in enynes is essential for the reaction to proceed.

Full text of DOI:10.1021/ol102019w

ORGANIC
LETTERS
2010
Vol. 12, No. 22
5132-5134
Nickel-Catalyzed Hydrosilylation/
Cyclization of Difluoro-Substituted
1,6-Enynes
Manabu Takachi and Naoto Chatani*
Department of Applied Chemistry, Faculty of Engineering, Osaka UniVersity,
Suita, Osaka 565-0971, Japan
chatani@chem.eng.osaka-u.ac.jp
Received August 26, 2010
ABSTRACT
Ni-catalyzed hydrosilylative cyclization of difluoro-substituted 1,6-enynes can be carried out. The presence of a geminal-difluoromethylene
group at an alkene terminus in enynes is essential for the reaction to proceed.
Catalytic hydrosilylation of unsaturated organic compounds
is a process of substantial importance for the synthesis of
organosilicon derivatives.1 Numerous transition metal com-
plexes are known to function as effective catalysts for
hydrosilylation. Hydrosilylation of enynes is also an impor-
tant method for the preparation of cyclic compounds bearing
a C-Si bond.2-4 In most cases, rhodium complexes were
used as catalysts for hydrosilylation of enynes reported thus
far.2 Enynes that are applicable to the Rh-catalyzed hydrosi-
lylation are limited to those having no substituents at the
alkene moiety except one specific example.2h In contrast to
the Rh-catalyzed hydrosilylation of enynes, which results in
the formation of a C-Si bond at the alkyne carbon,
palladium-3 and yttrium-catalyzed hydrosilylation4 of enynes
yields hydrosilylative cyclization products, in which a C-Si
bond forms at the alkene terminus carbon. To the best of
our knowledge, there is no example of hydrosilylation of
difluoro-substituted enynes, although the expected cyclic
products would have both C-Si and C-F bonds that are
useful in synthesissthe fluorine atom attached at an alkene
is expected to put a unique reactivity to the alkene.5 We
have already reported that the Ni-catalyzed reaction of
difluoro-substituted 1,6-enynes with organozinc reagents
resulted in alkylative or reductive cyclization in which a
trans-C-F bond is selectively substituted during cyclization.6
We wish to report a new type of Ni-catalyzed hydrosilylative
cyclization of difluoro-substituted 1,6-enynes in which the
presence of a difluoromethylene group at the alkene carbon
is essential for the reaction to proceed.
(1) Hiyama, T.; Kusumoto, T. In ComprehensiVe Organic Synthesis;
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The reaction of 1 with HSiMe2Ph in the presence of
Ni(cod)2/PBu3 as the catalyst in dioxane at 50 °C for 2 h
gave 2a as a single product in 75% isolated yield (Scheme
1). The product obtained was not the expected product, which
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10.1021/ol102019w  2010 American Chemical Society
Published on Web 10/14/2010

Scheme 1
Scheme 2
Scheme 3. Limitations
has an exosilylmethylene (vinylsilane) moiety, but instead
has an allylsilane moiety7 unlike reported examples of the
Rh-catalyzed hydrosilylation of simple enynes.2 In addition,
the product has a difluoromethyl moiety, which is often used
in the medicinal, agricultural, and material sciences.8 Among
the ligands examined, it was found that PBu3 is the ligand
of choice: PBu3 (75%, 2 h), PPh3 (56%, 2 h), PCy3 (41%,
24 h), dppe (no reaction, 24 h). While other hydrosilanes
Table 1. Ni-Catalyzed Hydrosilylation of Difluoro-Substituted
Enynesa
also gave the corresponding cyclic allylsilanes, the reaction
with HSiMe2Ph gave a better yield in a short reaction time.
The reaction was significantly affected by steric factors of
hydrosilanes (HSiMe2Ph > HSiMePh2, HSiMe2Et > HSiEt3).
The results of the Ni-catalyzed hydrosilylation of difluoro-
substituted 1,6-enynes are shown in Table 1. Various
difluoro-substituted 1,6-enynes having an internal alkyne
served as good substrates to give the corresponding cyclic
allylsilanes. Curiously, an enyne having a trimethylsilyl group
at the alkyne terminus, as in 7, gave the corresponding
products 8 in high yields, irrespective of the structure of the
hydrosilanes.9 An enyne with a nitrogen functional group,
as in 9, also gave the corresponding product 10 in a good
yield.
In contrast to enynes having internal alkynes, the
reaction of an enyne having a terminal alkyne moiety, as
in 11, gave a mixture of allylsilane 12 and vinylsilane 13
(Scheme 2).
Scheme 4. Rh-Catalyzed Hydrosilylation of 1
a Reaction conditions: enyne (0.3 mmol), HSiR3 (0.60 mmol), Ni(cod)2 (0.015
mmol), PBu3 (0.06 mmol), dioxane (1.5 mL) at 50 °C. b Isolated yield.
Org. Lett., Vol. 12, No. 22, 2010
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Scheme 5. Reaction Mechanism
The presence of a difluoromethylene group at an alkene
proposed that the reaction is initiated by silylmetalation.11
Irrespective of the mechanism, an initial product was
vinylsilane 20, which isomerized to thermodynamically stable
allylsilane 21 under the reaction conditions. When R′ ) H,
the isomerization of 20 to 22 also proceeded.
In summary, the first example of hydrosilylation of
difluoro-substituted 1,6-enynes is described. A nickel com-
plex showed high catalytic activity.
terminus in enynes was essential for the reaction to proceed
(Scheme 3). The reaction of a simple enyne 14, dichloro-
substituted enyne 15, and monosubstituted enyne 16 gave
either complex mixtures or no reaction at all. The length in
the tether was also an important factor. Thus, a complex
mixture was obtained in the case of 1,7-difluoro-substituted
enyne 17.
Use of Rh4(CO)12 as the catalyst gave a regioisomeric
mixture of hydrosilylation products 18, in which the alkene
moiety did not participate (Scheme 4).
The proposed reaction mechanism is shown in Scheme 5.
One possibility would have the pathway via a nickelacycle
19 as a key intermediate, which is formed by the oxidative
cyclization of an enyne and Ni(0).10 Another possibility could
be the silylmetalation pathway. Tamao and Ito reported the
Ni-catalyzed hydrosilylation of diynes in which it was
Acknowledgment. This work was supported, in part, by
grants from Ministry of Education, Culture, Sports, Science
and Technology, Japan. We wish to thank to Daikin
Industries Ltd. for financial support.
Supporting Information Available: Experimental pro-
cedures and spectroscopic data for new compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
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