Reactions of cyclohexenyl halides with tributylstannane. Stereoelectronic effects on S_H2 reactions at halogen

Abstract

Allylic halogenation of t-butylcyclohexene (2a) with t-butyl hypochlorite or N-bromosuccinimide affords mixtures containing mainly trans-5-t-butyl-3-chlorocyclohexene (2b) and the corresponding trans bromo compound (2c) respectively. The copper-catalysed reaction of (2a) with t-butyl perbenzoate shows similar but less pronounced stereoselectivity. Halogen-atom abstraction by tributyltin radicals, generated from tributylstannane, proceeds more rapidly from the trans halides (2b) and (2c) than from their cis epimers (3b) and (3c). These results suggest that homolytic fission or formation of allylic C-halogen bonds is sensitive to stereoelectronic effects.