One-pot synthesis of benzaldehyde derivatives in PdCl2-catalyzed reactions with H2O2 in alcoholic solutions

Abstract

The activity of the palladium salt catalysts was investigated on the benzaldehyde reactions with H2O2. The PdCl2 salt was the most efficient catalyst for the formation of the goal products. In CH3OH, benzaldehyde was selectively oxidized to benzoic acid and posteriorly esterified to methyl benzoate. In this process, the reaction of the PdCl2 catalyst with aqueous H2O2 provided H+ ions, which also promoted the esterification and acetalization reactions. The oxidant load played a key role in the selectivity controlling; while a large excess (9:1) gave benzoic acid (90%), with a lower ratio (3:1), acid and ester were formed with selectivity of 58 and 35%, respectively. With H2O2 at an equimolar ratio, the Pd(0) reduced the benzaldehyde to benzyl alcohol (30%). In absence of the oxidant, PdCl2 (2.5 mol %) condensated benzaldehyde and alcohol to acetal (90%). Therefore, PdCl2 directly catalyzed the oxidation and reduction reactions, and participated in acid-catalyzed reactions (i.e., condensation and esterification), generating H+ ions, demonstrating to be a highly versatile catalyst. An adequate adjusting of reaction conditions allowed to control the reaction selectivity toward the desired products (i.e., benzoic acid or ester benzyl, acetal, or benzyl alcohol).