Asymmetric Transfer Hydrogenation of N-Unprotected Indoles with Ammonia Borane.

A metal-free asymmetric transfer hydrogenation of unprotected indoles was successfully realized using a catalyst derived from HB(C6F5)2 and (S)-tert-butylsulfinamide with ammonia borane as a hydrogen source. A variety of indolines were achieved in 40-78% yields with up to 90% ee.