Promoting Effect of Lewis Acid on the Olefin Hydroesterification Catalyzed by Triphenylphosphine–Palladium Complex

A number of Lewis acids obtained by a reaction of metal oxides with Brønsted acids such as para-toluenesulfonic acid (p-TsOH) and methanesulfonic acid (CH3SO3H) have been employed as promoter for styrene hydroesterification catalyzed by triphenylphosphine–palladium (PPh3–Pd) complex. The acidities of the Lewis acids were analyzed by potentiometric titration and the PPh3 consumption during the hydroesterification was characterized by 31P NMR. The results indicated that the use of Lewis acid with apparent pH similar to p-TsOH can bring about a marked enhancement in the reaction rate and a significant depression in the PPh3 consumption. Such kind of Lewis acids may be more suitable as promoter for the olefin hydroesterification than the conventional Brønsted acids.