Kinetics and Mechanism of Ruthenacycle-Catalyzed Asymmetric Hydrogen Transfer

The mechanism of asymmetric hydrogen transfer catalyzed by a ruthenacycle has been studied. Ruthenium-hydride intermediates have been characterized by 1H NMR and IR. These hydride complexes appeared as a diastereomeric mixture (de = 64%) responsible for the enantiomeric excess of the hydrogen transfer reaction (ee = 60%). Kinetic measurements have been performed as a function of substrate concentration and temperature. Michaelis−Menten kinetics were observed indicating substrate−catalyst complex formation (thermodynamic data for the complexation reaction: ΔG0 = −13 kJ mol−1 at 273 K, ΔH0 = −58 kJ mol−1, ΔS0 = −170 J K−1 mol−1). The complexation process is accomplished prior to an intracomplex chemical reaction. As the substrate is still bound to the catalyst, pericyclic hydrogen transfer is deduced with an activation energy Ea = 83 kJ mol−1. On the basis of these data, a mechanism is proposed for the overall hydrogen transfer reaction.