Cobalt-Catalyzed Cyclization/Hydrosilylation Reaction of 1,6-Diynes Enabled by an Oxidative Cyclization–Hydrosilylation Mechanism

Transition-metal-catalyzed cyclization/hydrosilylation
of 1,6-diynes
is a useful method for the preparation of five-membered ring-fused
silyl dienes that are useful reagents in organic synthesis. Only a
handful of noble metal catalysts facilitating this transformation
are known, and nonprecious metal catalysts effecting the reaction
have remained elusive. Herein, we report that low-coordinate Co(0)-N-heterocyclic
carbene complexes can catalyze the cyclization/hydrosilylation of
1,6-diynes with tertiary and secondary hydrosilanes, furnishing five-membered
ring-fused (Z)-1-silyldienes in good yields and excellent
stereoselectivity. Mechanistic study disclosed that the catalytic
cycle likely has oxidative cyclization of 1,6-diynes with Co(0) species
as the key step. This mechanism accounts for the high stereoselectivity
and absence of uncyclized hydrosilylation byproducts in the cobalt-catalyzed
cyclization/hydrosilylation reaction, which is different from the
hydrosilylation-cyclization mechanism of the noble metal-catalyzed
reactions.