Phosphine-Ruthenium(II)-Aquo Redox Chemistry: The Aerobic Catalytic Oxidation of Cyclohexene

Abstract

Recently, our laboratory synthesized ruthenium(IV)-oxo complexes containing tertiary phosphine ligands, which act as stoichiometric oxidation reagents toward a variety of organic and inorganic substrates (alcohols, olefins, aldehydes, phosphines, sulfides, and sulfoxides). Currently, we are investigating the aerobic oxidation of cyclohexene, phosphineruthenium( II)-aquo complexes. The activation of molecular oxygen occurs at room pressure and temperature, without the need for a coreductant. The active oxidant in the catalytic cycle appears to be the phosphineruthenium( IV)-oxo species, for the product distribution from the catalytic oxidation of cyclohexene is identical to the product distribution from the stoichiometric oxidation of cyclohexene by a phosphine-ruthenium(IV)-oxo complex. The rate of product formation over a 24 hour period is constant, with the catalytic reaction sampled after 1, 2, 4, 8, and 24 hours. The initial catalyst was isolated intact at the end of the reaction. The oxidation of cyclohexene produces 2-cyclohexene-l-one, 2-cyclohexene-l-ol, and cyclohexene oxide in a product ratio of 16:8:1. After twenty-four hours, the oxidation of a 2.2 M solution of cyclohexene using a 5.0 x 10-4 M solution of catalyst yields a turnover number of 1560, where the production of 2-cyclohexen-l-one from cyclohexene requires two turnovers of catalyst per molecule. From experimental observations, a catalytic cycle can be suggested for the aerobic oxidation of cyclohexene. Initially, a five-coordinate phosphine-ruthenium(II) complex is generated, by the the loss of an aquo ligand. The combination of a molecule of dioxygen with two of these five-coordinate complexes forms a dinuclear, oxygen-bridged intermediate. Homolytic cleavage of the 0-0 bond yields two phosphineruthenium(IV)-oxo molecules, the proposed active oxidant. The phosphineruthenium(IV)-oxo complex oxidizes the target organic substrate, forming a phosphine-ruthenium(II)-oxidized substrate complex. Dissociation of the oxidized organic substrate by this complex yields the five-coordinate phosphine-ruthenium(II) complex, which continues in the catalytic cycle.