Advances in Homogeneous, Heterogeneous, and Biphasic Metal-Catalyzed Reactions in Dense-Phase Carbon Dioxide

Abstract

The use of compressed carbon dioxide as a reaction medium, either as a liquid or a supercritical fluid (sc CO2), offers the opportunity not only to replace conventional hazardous organic solvents but also to optimize and potentially control the effect of solvent on chemical synthesis. Although synthetic chemists, particularly those employing catalysis, may be relative latecomers to the area of supercritical fluids, the area of catalysis in carbon dioxide has grown significantly since around 1975 to the point that a number of excellent reviews have appeared (Baiker et al., 1999; Buelow et al., 1998; Jessop and Leitner, 1999; Jessop et al., 1995c, 1999; Morgenstern et al., 1996). Developing and understanding catalytic processes in dense-phase carbon dioxide could lead to “greener” processing at three levels: (1) solvent replacement, (2) improved chemistry (e.g., higher reactivity, selectivity, less energy), and (3) new chemistry (e.g., use of CO2 as a C-1 source). In this chapter, we will highlight a number of examples from the literature in homogeneous and heterogeneous transition-metal catalysis, as well as the emerging area of biphasic catalysis in H2O/sc CO2 mixtures. The intent is to provide an illustrative rather than a comprehensive overview to four classes of catalytic transformations: acid catalysis, reduction via hydrogenation, selective oxidation catalysis, and catalytic carbon–carbon bond-forming reactions. The reader is referred to other chapters in this book and other reviews (King and Bott, 1993) for discussion of uncatalyzed reactions, phase-transfer catalysis, polymerization, and radical reactions in sc CO2. From a synthetic chemist’s viewpoint, sc CO2 has a number of potential advantages that one would like to capitalize upon. • Solvent Replacement Carbon dioxide is a nontoxic, nonflammable, inexpensive alternative to hazardous organic solvents. Simple solvent replacement will not be a sufficient driver for all chemical reactions; however, as described below, the use of carbon dioxide could lead to better chemistry for certain reactions. • Gas Miscibility Gases such as H2, O2, and CO are sparingly soluble in liquid solvents but they are highly miscible with sc CO2.