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 H-1 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: Delta G(0) = -13 kJ mol(-1) at 273 K, Delta H-0 = -58 kJ mol(-1), Delta S-0 = -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 E-a = 83 kJ mol(-1). On the basis of these data, a mechanism is proposed for the overall hydrogen transfer reaction.