Palladium (II)‐catalyzed homogeneous alcohol oxidations: Disclosing the crucial contribution of palladium nanoparticles in catalysis

Abstract

Versatile redox catalysts play the significant roles in alcohol oxidations, in which the mechanisms for homogeneous and heterogeneous alcohol oxidations are generally different. This work introduced a Lewis acid (LA) promoted homogeneous alcohol oxidation with Pd (OAc)2 catalyst by using oxygen balloon as the oxidant source. It was found that adding Lewis acid such as Sc (OTf)3 significantly accelerated Pd (II)‐catalyzed alcohol oxidations; notably, the time courses of oxidations monitored by GC and 1H NMR disclosed that there existed two processes including the initial sluggish oxidation followed by a rapid oxidation. The promotional effect of Lewis acid was attributed to the formation of heterobimetallic Pd (II)/LA species, which improved the oxidizing power of the palladium (II) species, thus accelerating alcohol oxidation in the induction period. Correlating the sizes of in situ generated palladium nanoparticles with the time course of alcohol oxidation further disclosed that the loosely, spherically large nanoparticles, which were composed of many tiny nanoparticles having the size less than 10 nm, were responsible for the rapid oxidation, whereas those highly dispersed, tiny nanoparticles having the size less than 10 nm were not responsible for the rapid oxidation.