Effect of substrate structure on the liquid-phase regioselective 1,4-hydrogenation of E-benzylidene ketones on Rh/sepiolite catalyst

Abstract

The liquid-phase catalytic hydrogenation of benzylidene ketones, in the p-XC 6H 4-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-XC 6H 4CH 2CH 2COR) 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.