Organic Synthesis without Conventional Solvents

Organic synthesis without an organic solvent is still the exception. In this microreview, we discuss and compare different synthetic methods that proceed in the absence of conventional organic solvents. This includes solvent-free methods in solids (mechanochemistry and solid-state photochemistry), and in neat liquids (thermal and photocatalytic), as well as the application of unconventional solvents, such as ionic liquids and deep-eutectic solvents. The different approaches are briefly introduced and examples illustrate their specific advantages, followed by a critical discussion of their limitations. Some methods have the intrinsic benefits of solvent-free operation, such as avoiding solubility problems and side-reactions with the solvent. However, every method has specific advantages and the demand of the synthetic application is crucial for selecting the best technique.