A tentative rationalization of microwave effects in organic synthesis according to the reaction medium, and mechanistic considerations

• Published in: Tetrahedron Vol. 57; no. 45; pp. 9199 - 9223
• Main Authors: Perreux, Laurence, Loupy, André
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 05.11.2001
• ISSN: 0040-4020; 1464-5416
• DOI: 10.1016/S0040-4020(01)00905-X

A rationalization of microwave effects in organic synthesis based on the nature of the reaction medium and mechanistic considerations is proposed. With accurate and strict control of experiments, it may be possible to distinguish thermal and specific (non-purely thermal) effects. Specific microwave effects may be understood by considering evolution of the polarity of systems during the course of the reaction and the position of the transition state along the reaction coordinates. The most favourable encountered cases concern unimolecular or bimolecular reactions between neutral molecules (as dipoles are developed in the TS) and anionic reactions of tight ion pairs, i.e. involving charge localized anions (leading to ionic dissociation in TS). Specific effects are most likely to be observed under solvent-free conditions (green chemistry procedures) or in non-polar solvent and when the reaction mechanisms involve the formation of polar and late transition states (difficult reactions).