Selective Hydrogenation of Cinnamaldehyde Catalyzed by Ruthenium ( II ) Complexes Based on Azoimine Ligands

Abstract

In this work, four Ru (II) complexes based on azoimine ligands were tested with respect to their catalytic behavior in the liquid phase hydrogenation of cinnamaldehyde. The introduction of a thiophenyl substitution in the azoimine ligand was found to enhance significantly the catalytic activity and selectivity of the Ru complex. The catalytic behavior of the most active complex in this work was studied with respect to the effect of the co-catalyst KOH, the Ru complex and the cinnamaldehyde (CALD) concentrations as well as with respect to the effect of the reaction temperature. KOH is needed to transform the Ru complex into a reactive form and the higher the KOH concentration in the system, the higher is the efficiency of the catalytic system. The order of this activation process with respect to KOH and with respect to the Ru complex was found to be one in both cases. With a CALD-to-Ru ratio of 84:1 at T=86 o C, pH2=4 bar and [KOH]= 5.82×10 -3 M, a complete conversion could be accomplished in only 90 minutes with selectivity to cinnamyl alcohol approaching 95%. Increasing the initial concentration of cinnamaldehyde has, under the reaction conditions investigated in this work, a negative effect on the rate of hydrogenation with an order decreasing with increased concentration from -1 to -3. Based on our results, an outer-sphere mechanism is ruled out to operate in this system.