Recent progress in Wacker oxidations: Moving toward molecular oxygen as the sole oxidant

Wacker oxidations, the conversion of terminal olefins to methyl ketones with Pd(II) catalysts, have seen widespread use in synthetic applications. Standard synthetic Wacker conditions use catalytic PdCl2 with stoichiometric CuCl under an aerobic atmosphere in a mixed-solvent system of N,N-dimethylformamide and H2O. Though much attention has been directed toward elucidating the rate-determining step and the mechanism of nucleopalladation, the assumption that Cu does not participate in this portion of the catalytic cycle has recently been called into question based on an isolated Pd/Cu bimetallic species and the influence of Cu on product selectivity. Fortunately, recent advancements have been made toward the elimination of Cu additives, thereby alleviating these issues. Success in this area has come from the application of information gained in studying other direct-O-2-coupled Pd(II) oxidation systems, including ligand modulation, something which could not be achieved in the presence of Cu. The developments in peroxide-mediated and direct-O-2-coupled Wacker oxidations are highlighted herein.