Visible light photocatalytic reduction of aldehydes by Rh(III)-H: a detailed mechanistic study

The chemoselective photoreduction of aldehydes in the presence of ketones was achieved using triethanolamine (TEOA) as sacrificial electron donor, proflavine (PF) as photocatalyst and [Cp*Rh(III)(bpy) Cl]Cl (Rh-cat) as mediator. The reducing agent, which reacts with the carbonyl group was found to be [Cp*Rh(III)(bpy)H]Cl (Rh(III)-H). Contrary to formate-based reduction, its slow photochemical in situ generation enables to kinetically distinguish aldehydes from ketones. The inherent reactivity difference of the carbonyl compounds is transferred by the method into synthetically useful reaction selectivities. The substrate scope is broad with excellent yields. A detailed study of the reaction mechanism reveals that the photoreduction of the PF triplet and the subsequent reduction of the Rh-cat leading to Rh(III)-H represents the major reaction pathway, which is highly oxygen sensitive. The oxidative quenching of the PF singlet state by Rh-cat is a competing mechanism, which prevails in non-degassed systems.