Vinylic, allylic and homoallylic oxidations of alkenes via π- and σ-organopalladium complexes

The stoichiometric and catalytic pathways of oxidative esterification of alkenes via intermediate organopalladium complexes are discussed. The oxidation of propylene, hex-1-ene and cyclohexene by Pd-II acido complexes containing achiral, racemic and chiral carboxylate ligands was first studied in a series of solvents other than acetic acid. Significant changes in the selectivity of the Pd-II-promoted reaction with changes in the solvent nature and ligand chirality were observed. A way to allylic esters based on low-valence Pd nanoclusters provide highly selective oxidation of acyclic alkenes into allylic esters, whereas cycloalkenes undergo mostly redox disproportionation. The role of pi-alkene, sigma-alkenyl and pi-allyl complexes in the mechanism of the alkene oxidative esterification with Pd-II complexes and low-valence Pd clusters is discussed. (C) 2000 Elsevier Science S.A. All rights reserved.