Vinylic, allylic and homoallylic oxidations of alkenes via pi- and sigma-organopalladium complexes

The stoichiometric and catalytic pathways of oxidative esterification of al kenes via intermediate organopalladium complexes are discussed. The oxidati on of propylene, hex-1-ene and cyclohexene by Pd-II acido complexes contain ing achiral, racemic and chiral carboxylate ligands was first studied in a series of solvents other than acetic acid. Significant changes in the selec tivity of the Pd-II-promoted reaction with changes in the solvent nature an d ligand chirality were observed. A way to allylic esters based on low-vale nce Pd nanoclusters provide highly selective oxidation of acyclic alkenes i nto allylic esters, whereas cycloalkenes undergo mostly redox disproportion ation. The role of pi-alkene, sigma-alkenyl and pi-allyl complexes in the m echanism of the alkene oxidative esterification with Pd-II complexes and lo w-valence Pd clusters is discussed. (C) 2000 Elsevier Science S.A. All righ ts reserved.