Asymmetric Transfer Hydrogenations of 2,3-Disubstituted Quinoxalines with Ammonia Borane.

An asymmetric transfer hydrogenation of 2,3-disubstituted quinoxalines using a chiral frustrated Lewis pair of Piers' borane and (R)-tert-butylsulfinamide as the catalyst with ammonia borane as the hydrogen source has been successfully realized. For 2-alkyl-3-arylquinoxaline substrates, cis-tetrahydroquinoxalines were obtained as the predominant products in high yields with 77-86% ee. In contrast, trans isomers were often furnished as major products for the reactions of 2,3-dialkylquinoxalines with up to >99% ee.