Solvent effects in catalysis: rational improvements of catalysts via manipulation of solvent interactions

In homogeneous catalysis, the main emphasis on improving catalyst performance (rate, yield and selectivity) is directed towards manipulation of the ligands. The steric and electronic effects of ligands are extremely well understood and the rational design of new ligands that leads to improved catalysts is feasible. Similarly, in heterogeneous catalysis structural changes to the catalyst are used to improve their properties. In contrast, the role that solvents play in catalytic processes is often given cursory attention. The environmental impact of solvents is often considered, as it should be, and the use of organic-immiscible solvents, e.g. water, ionic liquids, etc., is frequently considered with respect to catalyst recovery, product isolation, and recycling. Nevertheless, the direct role of solvents in reactions is often overlooked, although solvents interact directly with the catalyst, the substrates and products, and all these interactions can increase or decrease reaction rate and/or selectivity. Herein we consider the role of solvents in catalysis and illustrate the critical role of solvents viewed from a mechanistic approach. In particular, we focus largely, but not exclusively, on hydrogenation reactions and cross-coupling reactions as the main types of solvents used for these two classes of reactions tend to be very different and illustrate different functions of solvents in catalysis.