Highly stereoselective Friedel–Crafts alkylations of unactivated benzenes by episulfonium ion cyclizations

Intermolecular additions of highly enantiomerically enriched episulfonium ions onto activated benzene rings are known to accomplish chiral Friedel–Crafts alkylations. Unactivated benzenes are either unreactive or can give low stereoselectivities. Intramolecular cyclizations should be faster than intermolecular additions; thus, cyclization reactions should be able to avoid undesired episulfonium ion racemization. The work reported here tests that hypothesis and demonstrates that cyclizations of enantiomerically enriched episulfonium ions onto unactivated benzene rings are highly stereoselective.