Organocatalytic Asymmetric Transfer Hydrogenation of (Hetero)Arenes

Organocatalytic asymmetric transfer hydrogenations of heteroarenes constitute a powerful, generalizable, and environmentally friendly strategy to access biologically important chiral saturated heterocycles. The combination of asymmetric Brønsted acid catalysis and hydrogen transfer from Hantzsch-type dihydropyridines is the key for a successful reaction outcome. In particular, chiral BINOL-phosphoric acids and their derivatives show the highest activities and selectivities. Furthermore, step-economic cascade reactions are valuable advanced tools to generate bioactive molecules through an organocatalytic reduction in a targeted fashion. Recently developed cooperative and relay catalysis protocols combine organo- and metal-catalyzed reactions and make use of substoichiometric amounts of the hydrogen donor or solely employ hydrogen gas, the most cost–effective reductant.