Microwave-Assisted Organic Syntheses in Deep Eutectic Solvents: A Win-Win Association for Sustainable Chemistry

The 12 principles of green chemistry guide the scientific community toward the development of chemical processes that are more respectful of the environment and safer for human health. In organic synthesis, this mainly involves the use of sustainable alternatives to conventional organic solvents, energy-efficient processes, and waste minimization. In this context, this review focuses on the use of deep eutectic solvents (DES) in microwave-assisted organic synthesis. Indeed, DES, due to their nonvolatility, nonflammability, and low toxicity compared to conventional organic solvents, are considered desirable "green solvents" for the development of environmentally friendly processes. Moreover, their physicochemical properties make them ideal media for microwave heating. Thus, all organic syntheses using DES as solvent and microwave heating documented in the literature are reported, including heterocycle synthesis, nitrogen quaternization reactions, 5-hydroxymethylfurfural production, Knoevenagel reactions, and miscellaneous transformations. The recyclability of DES-based systems and their scalability, where applicable, are reported. Mechanistical considerations when DES are involved are also described. Compared with conventional heating methods, microwave heating of DES media generally results in good yields and a significant reduction in reaction times. This DES-MW combination appears promising for more sustainable organic syntheses.