S_H2' reaction in organocobaloximes: synthesis and 1,3-rearrangement of exo-methylene alicyclic allyl sulfones

Abstract

Photostimulated reactions of cyclopentenylmethyl-, cyclohexenylmethyl- and α-pinenyl cobaloximes with arenesulfonyl halides result in the formation of corresponding exo-methylene cycloalkylsulfones in 82-98% isolated yields. The product formation is rationalized by a radical chain mechanism involving an S_H2' displacement of cobaloxime(II) from organocobaloximes by sulfonyl radical. Reaction under thermal condition, however, provides a mixture of exo-methylene cycloalkylsulfone and its endoisomer. Independent experiments confirmed that the endo-isomer arises from a facile 1,3-allylic rearrangement of the exo-isomer. From mechanistic and theoretical studies the rearrangement is proposed to involve a [1,3]-sigmatropic migration of sulfonyl group.