Photocatalytic hydrogenation of alkenes using ammonia-borane

The hydrogenation of olefins is a fundamental transformation in both industry and academia. Traditional methods to achieve this process involve the use of hydrogen gas in combination with a transition metal catalysis. Although efficient, these protocols are associated with challenges, such as safety concerns, cost, and waste management, and they are often unselective. An idealized hydrogen-free and metal-free hydrogenation of olefins remains elusive. Herein, a metal-free photochemical hydrogenation of unactivated alkenes is reported employing ammonia-borane as a hydrogen surrogate. The method is reliant on amine-ligated boryl radical generation from a diaryl ketone photocatalyst, which then undergoes a complex series of radical transformation. Through a detailed mechanistic study, it is proposed that the steps include halogen-atom transfer with the solvent, in situ generation of alkyl borane intermediates, H-atom transfer to generate a borane-ligated aminyl radical, and subsequent β-fragmentation, all of which are turned over by a thiol co-catalyst. Overall, this reaction enables a mild, hydrogen-free, and metal-free hydrogenation of olefins tolerating functional groups typically incompatible with standard methods.