A palladium complex catalyst for the regioselective hydrocarboxylation of 4-methylstyrene

Abstract

With a homogeneous catalyst system of PdCl2-CuCl2-PPh3 dissolved in a polar solvent, highly regioselective hydrocarboxylation of 4-methylstyrene to the branched alpha-(4-methylphenyl) propionic acid has been achieved. Effects of additives (mineral acids and O-2) and reaction conditions (amounts of catalyst components, reaction temperature, pressure and solvents) have been studied. Compared with the previous results of hydrocarboalkoxylation of 4-methyl-styrene to the ester by the same catalyst, the hydrocarboxylation required severer reaction conditions and the presence of a polar solvent that could solubilize water. Efficient hydrocarboxylation was achieved only after fine adjustments of the reaction conditions and addition of a halogen compound. And the addition of HCl improved the reaction rate significantly. This improvement was due to the effect of chlorine, not the effect of the proton. The addition of oxygen reduced reaction rates by oxidizing carbon monoxide and PPh3.