Light-driven reduction of aromatic olefins in aqueous media catalysed by aminopyridine cobalt complexes.

A catalytic system based on earth-abundant elements that efficiently hydrogenates aryl olefins using visible light as the driving-force and H 2 O as the sole hydrogen atom source is reported. The catalytic system involves a robust and well-defined aminopyridine cobalt complex and a heteroleptic Cu photoredox catalyst. The system shows the reduction of styrene in aqueous media with a remarkable selectivity (>20 000) versus water reduction (WR). Reactivity and mechanistic studies support the formation of a [Co-H] intermediate, which reacts with the olefin via a hydrogen atom transfer (HAT). Synthetically useful deuterium-labelled compounds can be straightforwardly obtained by replacing H 2 O with D 2 O. Moreover, the dual photocatalytic system and the photocatalytic conditions can be rationally designed to tune the selectivity for aryl olefin vs. aryl ketone reduction; not only by changing the structural and electronic properties of the cobalt catalysts, but also by modifying the reduction properties of the photoredox catalyst.