A Tutorial on the Mechanisms of Group 9 Transition-Metal-Catalyzed Asymmetric Olefin Hydrogenation

Asymmetric alkene hydrogenation catalyzed by group 9 transition-metal complexes has become one of the most widely used methods for producing enantio-enriched compounds, particularly single-enantiomer pharmaceuticals. This Tutorial is focused on the mechanistic aspects of asymmetric alkene hydrogenation mediated by group 9 transition-metal catalysts with an emphasis on elucidating reaction pathways and the origins of enantio-induction. Among these catalysts, bis-(phosphine)-rhodium-(I) complexes are the most extensively studied, due to their broad applicability and historical importance in the development of asymmetric hydrogenation. Early mechanistic studies provided strong support for an unsaturated pathway consistent with the Curtin-Hammett principle, in which the minor rhodium alkene diastereomer reacts more rapidly with H₂ to yield the preferred enantiomer of the alkane. In contrast, later investigations with more electron-rich phosphines favored a metal dihydride pathway. Iridium catalysts bearing chiral bidentate phosphines have expanded the scope of asymmetric hydrogenation to minimally functionalized alkenes, and extensive experimental and computational studies support an Ir-(III)/Ir-(V) redox cycle. More recently, attention has shifted toward more earth-abundant cobalt catalysts, which can form both neutral and cationic complexes. Emerging evidence indicates that neutral cobalt catalysts operate through either non-redox Co-(II) metallacycle pathways or more rhodium-like Co-(0/II) redox cycling mechanisms.