Insight into Catalyst Speciation and Hydrogen Co-Evolution during Enantioselective Formic Acid-driven Transfer Hydrogenation with Bifunctional Ruthenium Complexes from Multi-technique operando Reaction Monitoring

Transfer hydrogenation of acetophenone from formic acid / triethylamine mixtures catalysed by the Ikariya-Noyori complex [(mesitylene)RuCl(R,R)-(TsDPEN)] has been investiated by simultaneous high-resolution FlowNMR and FlowUV-Vis spectroscopies coupled with on-line sampling head-space mass spectrometry and chiral high-performance liquid chromatography using an integrated, fully automated recirculating flow setup. In line with previous observations, our combined results show a gradual switch from formic acid dehydrogenation to hydrogen transfer mediated by the same Ru-hydride complex, and point a Ru-formate species as the major catalyst intermediate. Hydrogen bonding in the formic acid / triethylamine mixture emerges as a sensitive 1H NMR probe for the transfer hydrogenation activity of the system.