Cobalt-Catalyzed Trimethylsilylmethylmagnesium-Promoted Radical Alkenylation of Alkyl Halides: A Complement to the Heck Reaction

A cobalt complex, [CoCl2(dpph)] (DPPH = [1,6-bis(diphenylphosphino)hexane]), catalyzes an
intermolecular styrylation reaction of alkyl halides in the presence of Me3SiCH2MgCl in ether to yield
β-alkylstyrenes. A variety of alkyl halides including alkyl chlorides can participate in the styrylation. A radical
mechanism is strongly suggested for the styrylation reaction. The sequential isomerization/styrylation
reactions of cyclopropylmethyl bromide and 6-bromo-1-hexene provide evidence of the radical mechanism.
Crystallographic and spectroscopic investigations on cobalt complexes reveal that the reaction would begin
with single electron transfer from an electron-rich (diphosphine)bis(trimethylsilylmethyl)cobalt(II) complex
followed by reductive elimination to yield 1,2-bis(trimethylsilyl)ethane and a (diphosphine)cobalt(I) complex.
The combination of [CoCl2(dppb)] (DPPB = [1,4-bis(diphenylphosphino)butane]) catalyst and Me3SiCH2MgCl induces intramolecular Heck-type cyclization reactions of 6-halo-1-hexenes via a radical process.
On the other hand, the intramolecular cyclization of the prenyl ether of 2-iodophenol would proceed in a
fashion similar to the conventional palladium-catalyzed transformation. The nonradical oxidative addition
of carbon(sp2)−halogen bonds to cobalt is separately verified by a cobalt-catalyzed cross-coupling reaction
of alkenyl halides with Me3SiCH2MgCl with retention of configuration of the starting vinyl halides. The cobalt-catalyzed intermolecular radical styrylation reaction of alkyl halides is applied to stereoselective variants.
Styrylations of 1-alkoxy-2-bromocyclopentane derivatives provide trans-1-alkoxy-2-styrylcyclopentane
skeletons, one of which is optically pure.