A photocatalytic acid- and base-free Meerwein-Ponndorf-Verley-type reduction using a [Ru(bpy)3]2+/viologen couple

A photocatalytic system to effect the Meerwein-Ponndorf-Verley reduction of carbonylic compounds to alcohols has been developed. The system comprises Ru(bpy)3]2+ as a photosensitizer, triethanolamine as a sacrificial electron donor, viologen as an electron acceptor, and the carbonyl compound and iPrOH as Meerwein-Ponndorf-Verley reagents. The photocatalytic reaction can be performed in neat iPrOH or in 1-butyl-3-methylimidazolium ionic liquid. Mass spectrometric detection of the viologen hydride derivative VH+ confirms that this species is the reducing agent responsible for the carbonyl compound reduction. The reaction intermediates involved in the photocatalytic system have also been characterized by laser flash photolysis.