The visible-light-driven transfer hydrogenation of nicotinamide cofactors with a robust ruthenium complex photocatalyst†
The highly efficient regeneration of nicotinamide cofactors has been successfully achieved with a quantum yield (Φ) of 7.9 × 10−3 via photocatalytic transfer hydrogenation in the presence of the ruthenium complex Ru(tpy)(biq)Cl2 (where tpy = 2,2′:6′,2′′-terpyridine and biq = 2,2′-bisquinoline). The photocatalytic system is not only highly efficient but also tolerant to amino acid residues. The combination of this photocatalyst with glutamate dehydrogenase enabled the controllable and efficient synthesis of L-glutamate to be realized. A mechanism involving light-induced ligand exchange, decarboxylation and hydride transfer has been proposed. Kinetic isotope experiments revealed that the decarboxylation of [Ru(tpy)(biq)HCOO]+ to [Ru(tpy)(biq)H]+ was the rate-determining step with a small apparent activation energy of 3.2 ± 0.4 kcal mol−1. The hydricity of [Ru(tpy)(biq)H]+ was estimated, via reaction equilibrium, to be 40 ± 3 kcal mol−1.