FORMAL [3 + 2] CYCLOADDITION OF BENZYLIC CATIONS WITH ALKENES

The reaction of benzylic cations with styrenes affords dihydro(1H)indenes in good yield via a formal [3 + 2] atom cycloaddition. The cations were generated from quinone methides and benzylic alcohols. (E)-Styrenes participate in the reaction with remarkable stereoselectivity affording dihydro(1H)indenes with three stereogenic centers with >40:1 diastereoselectivity. A possible transition state for the reaction is discussed. Less activated alkenes such as dihydropyran and methylcyclohexene afforded cycloadducts in 66% and 51% yields, respectively.