Ionic Liquids as Novel Media and Catalysts for Diels-Alder Chemistry

Background: The Diels-Alder reaction has played a fundamental role in advancing synthetic chemistry over the years and continues to greatly impact modern synthetic methodology. Ever increasing interest in the application of ionic liquids (ILs) as catalyst and solvents in organic synthesis has provided the impetus to explore their application in Diels-Alder chemistry with the goal to improve efficiency as well as chemo-, regio-and enantio-selectivity. Objective: The review focuses on recent progress in the area of IL-mediated Diels-Alder reactions through illustrative examples that demonstrate their efficacy as solvent and catalysts. Conclusion: It is clear from the review of the topic that a vast amount of work has been done in this area, employing diverse types of ILs which were employed either alone or in conjunction with Lewis-acids, metal ions, chiral additives, organocatalysts, and/or other additives. The results in comparison to conventional solvents have demonstrated improved yields, faster rates, as well as improved regio-and enantioselectivity. These attributes along with the advantages associated with recycling/reuse clearly point to ILs as superior media for Diels-Alder chemistry. Much of the emphasis has been on synthesis but some fundamental mechanistic work aimed at understanding rate enhancement and endo-selectivity have also been reported.