Supercritical carbon dioxide as a substitute solvent for chemical synthesis and catalysis

Supercritical carbon dioxide represents an inexpensive, environmentally benign alternative to conventional solvents for chemical synthesis. In this chapter, we delineate the range of reactions for which supercritical CO2 represents a potentially viable replacement solvent based on solubility considerations and describe the reactors and associated equipment used to explore catalytic and other synthetic reactions in this medium. Three examples of homogeneous catalytic reactions in supercritical CO2 are presented: the copolymerization of CO2 with epoxides, ruthenium-mediated phase transfer oxidation of olefins in a supercritical CO2/aqueous system, and the catalyic asymmetric hydrogenation of enamides. The first two classes of reactions proceed in supercritical CO2 but no improvement in reactivity over conventional solvents was observed. Hydrogenation reactions, however, exhibit enantioselectivities superior to conventional solvents for several substrates.