EFFECT OF SUBSTRATE STRUCTURE ON THE LIQUID-PHASE REGIOSELECTIVE 1,4-HYDROGENATION OF E-BENZYLIDENE KETONES ON RH/SEPIOLITE CATALYST

The liquid-phase catalytic hydrogenation of benzylidene ketones, in the p-XC6H4-CH = CHCOR form (E isomers; X = H, Me, MeO, Cl; R = Me, Et, n-Pr, i-Pr, t-Bu, n-Pe, Ph) was carried out using a sepiolite-supported rhodium catalyst (1 wt.% Rh) in methanol solvent under low hydrogen pressure (0.55 MPa) and at room temperature (298 K). The reactions were selective with respect to the formation of the conjugate reduction product (p-XC6H4CH2CH2COR) although a very slight amount of benzyl ketone (hydrogenolysis of the C-Cl bond) is found in p-chlorobenzylidene ketones. The influence of both X and R substituents was analyzed by the Pavelich-Taft equation and both conjugative and steric effects of X and R groups were found to simultaneously influence the reaction process. A cis-concerted mechanism, including a transition state with some partially developed charge, is assumed for the hydrogenation of E-benzylidene ketones over Rh/sepiolite catalysts. Here, the concerted character of the hydrogenation is considered as a reaction taking place in a single step on a rhodium surface site with three degrees of coordinative unsaturation.