Aldol condensation: green perspectives

Aldol reaction is one of the most established reactions employed for the construction of new C–C bond with application in chemical synthesis and biochemical domains. Conventionally, aldol reaction involved the use of basic catalyst in hydroalcoholic medium or use of strong bases in toxic and flammable organic solvents. Apart from long reaction times, such conditions result in mixtures of ketols and α, β unsaturated ketones and side products from competitive side reactions along with aldol products. In recent years, considerable efforts and interest have been put for the development of catalytic methods in green context for this transformation. Though numerous variants of homogenous and heterogeneous catalysts have been examined for better results, there are environment concerns associated with catalytic aldol reaction. Increasing ecological awareness and environment-friendly reactions has prompted chemists to involve improved strategies such as micellar medium, microwave irradiation and ultrasonics as alternative routes for aldol condensation. This review summarizes and updates the research on these routes with a green perspective. Reactions performed under these methodologies with or without use of heterogeneous catalysis have been highlighted with reference to the principles of green chemistry. The protocols have been primarily complied from pedagogical journals with depiction of green component wherever possible.