Deep eutectic solvents for next-generation cyclodextrin science

In cyclodextrin science, water is employed as a solvent almost exclusively, and this imposes non-negligible limitations to the scope of applications. Accordingly, deep eutectic solvents, constructed from hydrogen-bonding donors and acceptors, have been attracting much interest as important substitutes for water. This review comprehensively covers chemical and physicochemical features of cyclodextrins in these eco-friendly solvents. In one category, cyclodextrins or their derivatives are dissolved as solutes in conventional deep eutectic solvents. All of alpha-, beta-, and gamma-cyclodextrins efficiently form inclusion complexes with various guest molecules, exactly as observed in water. Notably, chemically modified cyclodextrins (e.g. 2-hydroxypropyl-cyclodextrins) form even more stable inclusion complexes than native cyclodextrins. Alternatively, deep eutectic solvents are prepared by combining cyclodextrins with other hydrogen-bonding components. The cyclodextrin units in these mixtures also form inclusion complexes with guest molecules. It has been proposed that enhanced flexibility of cylindrical structures of cyclodextrins allows effective induced-fit to stabilize inclusion complexes. The applications of these systems range widely, from catalysis for organic synthesis to extraction, analysis, pharmaceutics, and many other fields. High solubilities of cyclodextrins and various chemicals in these solvents guarantee high productivity in target transformations. These unique systems promise further innovation.