The Chemoselective Catalytic Oxidation of Alcohols, Diols, and Polyols to Ketones and Hydroxyketones

Book excerpt: The chemoselective oxidation of vicinal diols to α-hydroxyketones is a challenge in organic syntheses because not only does the diol need to be oxidized selectively to a monocarbonyl compound, but diols are also prone to overoxidation and oxidative cleavage. Employing a cationic palladium complex, [(neocuproine)Pd(OAc)]2(OTf)2, we were able to demonstrate the selective oxidation of glycerol to dihydroxyacetone mediated by either benzoquinone or O2 as the terminal oxidant, an accomplishment that has little precedent in homogeneous catalysis. Mechanistic studies were undertaken to uncover the nature of this remarkable chemoselectivity. Kinetic and deuterium-labeling studies implicate reversible β-hydride elimination from isomeric Pd alkoxides and turnover-limiting displacement of the dihydroxyacetone product by benzoquinone. We successfully extended this methodology to other terminal 1,2-diols and symmetric vicinal 1,2-diols and have carried out aerobic oxidation of these substrates catalyzed by 1. Examination of the reactivity of 1 with conformationally-restricted 1,2-cyclohexanediols suggests that the diol must chelate to the Pd center for effective oxidation to the hydroxyketone product. In a separate project, we have also investigated the electrocatalytic reduction of dioxygen by several dinuclear copper complexes, an important reaction for fuel cell applications.