Synthesis of small ring-containing conjugated dienes via the coupling reaction of cyclopropyl- and cyclobutylmagnesium carbenoids with α-sulfonylallyllithiums

A variety of allylidenecyclopropanes and allylidenecyclobutanes were synthesized via the coupling reaction of cyclopropyl- and cyclobutylmagnesium carbenoids with α-sulfonylallyllithiums. Small ring cycloalkylmagnesium carbenoids were generated from aryl 1-chlorocycloalkyl sulfoxides and a Grignard reagent, and the resultant magnesium carbenoids were reacted with α-sulfonylallyllithiums, which were prepared via the deprotonation of p-tolyl vinyl sulfones or allyl p-tolyl sulfones. Allylidenecycloalkanes were obtained in moderate to good yields, and the Diels–Alder reaction of allylidenecycloalkanes with tetracyanoethylene afforded spirocyclic compounds. The coupling reaction of acyclic alkylmagnesium carbenoids with α-sulfonylallyllithiums provided conjugated dienes as mixtures of geometric isomers. The results from DFT calculations showed that small ring cycloalkylmagnesium carbenoids have a long C–Cl bond and expanded bond angles on the backside of the C–Cl bond relative to those of the corresponding chlorocycloalkanes.