Kinetics of Asymmetric Transfer Hydrogenation, Catalyst Deactivation, and Inhibition with Noyori Complexes As Revealed by Real-Time High-Resolution FlowNMR Spectroscopy

Catalytic hydrogen transfer from basic isopropyl alcohol to aryl ketones mediated by [(arene)(TsDPEN)RuCl] complexes has been investigated by operando ¹ H NMR spectroscopy using a recirculating flow setup. Selective excitation pulse sequences allowed fast and quantitative monitoring of the key [(mesitylene)(TsDPEN)RuH] intermediate during catalysis, which is shown to interact with both substrates by polarization transfer experiments. Comparison of reaction profiles with catalyst speciation traces in conjunction with reaction progress kinetic analysis using variable time normalization and kinetic modeling showed the existence of two independent catalyst deactivation/inhibition pathways: whereas excess base exerted a competitive inhibition effect on the unsaturated catalyst intermediate, the active hydride suffered from an inherent first-order decay that is not evident in early stages of the reaction where turnover is fast. Isotopic labeling revealed arene loss to be the entry point into deactivation pathways to Ru nanoparticles via hydride-bridged intermediates.