Reaction and Characterization of Hypervalent Organo-λ^<3->bromanes

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(I) Reaction of difluoro-λ^3-bromane with alkynylstannanes (2.2 equiv) in the presence of BF_3 Et_2O in dichloromethane results in oxidative homocoupling to give symmetrical 1,3-diynes. The reaction involves a Michael addition of alkynylstannanes to the initial formed alkynyl-λ^3-bromanes. Cross-coupling of alkynyl-λ^3-bromanes with alkynylstannanes affords unsymmetrical 1,3-diynes.(II) Cyclohexenyl- and cyclopentenyl-λ^3-bromanes were prepared from vinyl borates by the reaction with difluoro-k^3-bromane via borane-bromane (III) exchange. Solvolysis of cyclopentenyl-λ^3-bromane in aqueous alcohols afforded a good yield of cyclopentanone via generation of cyclopentenyl cation. We have measured the rates of solvolysis, which show very high leaving group ability of aryl-λ^3-bromanyl groups.Intermolecular transylidation between halonium ylides under thermal and catalytic (rhodium (II) acetate) conditions, which makes it possible to synthesize a hitherto unknown kind of aliphatic chloronium ylides as well as a variety of bromonium and iodonium ylides, was developed. Compared to the iodonium ylide, the chloronium and bromonium ylides serve as a better progenitor for generation of carbenes (or carbenoids) and efficiently undergo cyclopropanation of olefins such as cyclooctadiene under uncatalyzed thermal conditions. A greater leaving group ability of the λ^3-chroranyl and λ^3-bromanyl groups compared to that of the λ^3-iodanyl group seems to be responsible for the observed differences in reactivity between these halonium ylides.