Dense phase carbon dioxide as a medium for selective oxidation catalysis: Understanding the opportunity for enhanced chemistry

This dissertation explores the potential of dense phase carbon dioxide as a reaction medium for selective catalytic transformations. The aim of this research is to benchmark reactivities and selectivities in dense phase CO ₂ to those in organic solvents, understand ligand effects, and delineate the potential of this environmentally benign solvent. Two important classes of reactions were investigated; selective oxidation and carbon-carbon bond forming reactions. A number of homogeneous catalytic oxidation reactions were carried out in dense phase CO₂, with an emphasis on reactions of olefins with organic peroxides to form epoxides. The vanadium-catalyzed epoxidation of allylic and homoallylic alcohols in liquid and supercritical CO₂ was studied in detail to measure the effect of this medium on the catalytic reactivity and selectivity. Reaction rates, activation parameters, and selectivities were used to compare this reaction in dense phase CO₂ with reactions in organic solvents. Preliminary strategies for heterogenizing this chemistry were also developed including polymer supported catalysis and two phase CO ₂/H₂O reaction conditions. In addition to vanadium, titanium-catalyzed epoxidations, including asymmetric epoxidation, and molybdenum-catalyzed oxidations were also studied. Another important class of reactions, palladium-catalyzed carbon-carbon bond formation, was also studied for benefits from solvent replacement as well as reactivity and selectivity. This work demonstrates that palladium-catalyzed coupling reactions can proceed with high conversions and selectivities in supercritical CO₂, particularly in the presence of the tris[3,5-bis(trifluoromethyl)phenyl] phosphine ligand. Compressed dimethyl ether was also explored as a polar dense phase alternative solvent (to replace peroxidizable ethers such as THF) for palladium-catalyzed coupling reactions and preliminary results suggest that the reactivity of Pd(0)-catalyzed coupling reactions in dimethyl is higher than that in carbon dioxide, toluene or tetrahydrofuran.