Catalytic asymmetric hydrogenation of heteroaromatic compounds, indoles

Catalytic asymmetric hydrogenation of olefins, ketones, and imines has widely been used to prepare various optically active compounds. Meanwhile, few asymmetric hydrogenations of aromatic groups had been reported until recently. We found hydrogenation of indoles to proceed with high enantioselectivity in the presence of a rhodium catalyst modified with a trans-chelating chiral ligand PhTRAP. The rhodium catalyst transformed N-acetylindoles bearing a substituent at 2-position into the corresponding indolines with up to 95% ee. 3-Substituted indoles were hydrogenated with high enantioselectivity (up to 98% ee) when their nitrogen was protected with a sulfonyl group. A PhTRAP-ruthenium catalyst exhibited high enantioselectivity (up to 95% ee) for the hydrogenation of W-Boc-indoles. The ruthenium complex catalyzed the hydrogenation of 2, 3-disubstituted indoles as well as 2- or 3-monosubstituted indoles. Furthermore, the recent progress of asymmetric hydrogenation of aromatic compounds was surveyed in this paper.