Brønsted acid activation strategy in transition-metal catalyzed asymmetric hydrogenation of N-unprotected imines, enamines, and N-heteroaromatic compounds.

Asymmetric hydrogenation plays an important role in organic synthesis, but that of the challenging substrates such as N-unprotected imines, enamines, and N-heteroaromatic compounds (1H-indoles, 1H-pyrroles, pyridines, quinolines, and quinoxalines) has only received increased attention in the past three years. Considering the interaction modes of a Brønsted acid with a Lewis base, Brønsted acids may be used as the ideal activators of C=N bonds. This Minireview summarizes the recent advances in transition-metal-catalyzed, Brønsted acid activated asymmetric hydrogenation of these challenging substrates, thus offering a promising substrate activation strategy for transformations involving C=N bonds.