Microwave Dielectric Heating for Solvent-free Organic Transformations

The use of solvents in organic synthesis is today one of the main causes of pollution and risk for humans and the environment. Although many green solvents have been recently developed, the best solvent is still no solvent. The use of microwaves (MWs) for heating reaction mixtures provides an efficient tool to perform organic reactions in the absence of solvents. The combination of solvent-free protocols and microwave-assisted organic synthesis (MAOS) is therefore one the most challenging tools to abide by the 12 green chemistry principles. This overview reports on the recent results, benefits and challenges of the synergy between MW heating and neat reactions. Important milestones of organic chemistry such as oxidation, alkylation, cross-coupling, Knoevenagel, Claisen and Michael addition reactions, together with innovative sustainable organic chemistry reactions such as borrowing hydrogen protocols, are discussed with critical attention to their benefits compared to conventional heating and the use of ‘hidden’ solvents employed during work-up and purification steps.