title: Cobalt-catalyzed trimethylsilylmethylmagnesium-promoted radical alkenylation of alkyl halides: a complement to the Heck reaction.
abstract: 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 beta-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.
authors.affiliation: Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Kyoto-daigaku Katsura, Kyoto 615-8510, Japan.
authors.forename: Walter
authors.initials: W
authors.lastname: Affo
authors.forename: Hirohisa
authors.initials: H
authors.lastname: Ohmiya
authors.forename: Takuma
authors.initials: T
authors.lastname: Fujioka
authors.forename: Yousuke
authors.initials: Y
authors.lastname: Ikeda
authors.forename: Tomoaki
authors.initials: T
authors.lastname: Nakamura
authors.forename: Hideki
authors.initials: H
authors.lastname: Yorimitsu
authors.forename: Koichiro
authors.initials: K
authors.lastname: Oshima
authors.forename: Yuki
authors.initials: Y
authors.lastname: Imamura
authors.forename: Tsutomu
authors.initials: T
authors.lastname: Mizuta
authors.forename: Katsuhiko
authors.initials: K
authors.lastname: Miyoshi