Kinetic and Mechanistic Studies of the Selective Hydrogenation of (E)-Chalcones in Biomass-Derived gamma-Valerolactone Catalyzed by Rh-PPh3 Complexes

gamma-Valerolactone as a renewable, polar aprotic solvent was evaluated in the selective hydrogenation of carbon-carbon double bond of chalcones from both catalytic and mechanistic aspects. By using triphenylphosphine-modified cationic rhodium catalyst systems reasonably high activities (TOFinitial similar to 1200 h(-1)) were achieved in the selective hydrogenation of (E)-chalcone (CHL), as a common model substrate for alpha,beta-unsaturated ketones, to dihydrochalcone (DHC). Various derivatives of CHL, which were substituted on the styryl phenyl groups also showed exclusive selectivity for the hydrogenation of the C=C double bond. The optimal PPh3/Rh ratio was between 2.5 to 3, but higher ratios were advantageously also tolerated, which is beneficial for the process chemistry. The solvent GVL proved to stable enough under the reaction conditions for waste-free recycling. Kinetic studies and NMR spectra from the analogous deuteration of chalcone by using the cationic Rh-PPh3 system clearly show the olefin insertion as the rate-determining step with definite irreversibility. By investigating the substituent effect on the phenyl ring of the styryl moiety of chalcone, a quasi-linear correlation was found between the electronic parameters of the para-substituents and the activity with electron donor groups being favorable for the reaction rate. In summary, an environmentally friendly and feasible method is presented for the production of valuable DHC intermediates in view of their pharmaceutical importance.