Effect of pH on the Biphasic Catalytic Hydrogenation of Benzylidene Acetone Using CpRu(PTA)2H

The hydrogenation of benzylidene acetone has been accomplished under biphasic conditions (water/diethyl ether) utilizing CpRu(PTA) 2 H as the catalyst under mild conditions. The results show high selectivity toward hydrogenation of the olefin in benzylidene acetone with 4-phenylbutan-2-one as the major product. The effects of pH, hydrogen pressure, and salts on the reduction are described. The presence of anions such as BF 4 - and PF 6 - significantly improve catalytic activity, while Cl - shuts down catalysis due to the formation of the catalytically inactive CpRu(PTA) 2 Cl. The aforementioned factors do not influence the selectivity of the reaction. Spectroscopic studies of the nature of CpRu(PTA) 2 H under the reaction conditions enabled the identification of mono- and dihydride intermediates, [CpRu(PTA)(PTAH)H] + and [CpRu(PTA) 2 (H) 2 ] +, respectively. The distribution of these intermediates as a function of pH and the effect on reaction rates are presented. Reaction rate is found to increase with an increase in concentration of [CpRu(PTA)(PTAH)H] +, which is the presumed active catalytic species. Based on the data presented a 1,4-conjugate addition mechanism is proposed. Finally, the solid-state structures of the mono- and diprotonated complexes of CpRu(PTA) 2 Cl, [CpRu(PTA)(PTAH)Cl](PF 6 ), and CpRu(PTAH) 2 Cl](PF 6 ) 2 are reported.