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 Bronsted 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 P-31 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 Bronsted acids.