Concerning the Mechanism and Selectivity of Palladium(II)-Catalyzed Aerobic Oxidation Reactions

Oxidation is one of the most fundamental and important processes in nature. It would be advantageous to chemically replicate the high substrate specificity and selectivity observed in oxidative enzymes. Several such synthetic processes have been developed that involve the transfer of a heteroatom to a substrate in an asymmetric fashion. Enantioselective oxidative dehydrogenations, which do not involve transfer of a heteroatom, are much less common. Reactions of this type have recently been developed for the oxidative kinetic resolution of secondary alcohols using palladium(II) catalysis, dioxygen, and the chiral ligand (–)-sparteine. This general approach (palladium(II), dioxygen, ligand) was applied to the development of oxidative heteroatom/olefin cyclizations to form dihydrobenzofurans, cyclic ethers, lactones and lactams. The nonenantioselective reaction employs pyridine as a ligand. These conditions could be extended to the enantioselective cyclization of allyl-appended phenols through the use of (–)-sparteine as a ligand. The mechanism of the oxidative heteroatom/olefin cyclizations was explored via stereospecifically deuterium-labeled substrates. These studies indicate that the stereochemistry of oxypalladation for primary alcohol substrates is syn, whether a mono- or bidentate ligand is used. In contrast, cyclizations of deuterium-labeled carboxylic acid substrates undergo anti oxypalladation. The origins of stereoselectivity in the oxidative kinetic resolution of secondary alcohols using the C1 symmetric ligand (–)-sparteine were investigated through structural and reactivity studies of a variety of ((–)-sparteine)palladium(II) complexes. A model for the observed selectivity was developed, and is supported by theoretical calculations. Experiments with the C2 symmetric diastereomers of (–)-sparteine highlight the special properties of (–)-sparteine that make it a uniquely effective ligand in the kinetic resolution.