Microwave-assisted synthesis in water as solvent

A review that covers microwave-assisted organic synthesis (MAOS) using water as a solvent ranging from ambient temperature to near critical water at 300°C and 80 bar pressure is presented. The problems associated with the use of water as a solvent in microwave reactions are experienced under high dilution conditions or high temperature region where the loss tangent of water is reduced to such an extent that efficient absorbance of microwave energy is not possible. In most cases either the substrates or some of the reagents/catalysts dissolved in the aqueous reaction mixture are strongly polar and therefore microwave absorbing, raising the overall dielectric properties of the reaction medium to a level that allows sufficient heating by microwaves. Furthermore, in microwave reactors, the reaction vessels are made out of not entirely microwave transparent glass, and therefore, heating of the reaction mixture often occurs in part by conventional conduction and convection phenomena.