Water in Organocatalytic Reactions

Water is a green solvent: it is cheap, safe and environmentally friendly compared with organic solvents. Pfizer Corp. has defined a solvent guide that encourages the use of preferred solvents, for example, water or ethyl acetate, over usable solvents, for example, dimethyl sulfoxide (DMSO), and undesirable solvents, for example, dimethylformamide (DMF) [1]. However, chemical reactions carried out in or on water are not necessarily green reactions. Indeed, if we consider a reaction involving two species (A and B) and a product (C), the requirement for an ideal situation is when the product C has zero solubility and is quantitatively recovered by filtration, leaving no starting compounds A and B. In this case pure water can be recovered. This is, of course, an ideal case and in reality the filtrate will contain A, B, and C. In this case, the greenness and economic benefit of the process will depend on the kind of process needed to obtain pure water without traces of organic molecules and, if employed, any kind of metal-based catalyst or organocatalyst.