Effect of Novel Deep Eutectic Solvents on the Endo/Exo Ratio of Diels-Alder Reactions at Room Temperature.

The Diels-Alder reaction is a prototypical example of a thermally allowed [4 + 2] cycloaddition with good control over the regio- and stereochemical outcomes. Therefore, Diels-Alder reactions in which adjacent stereocenters are generated at the two ends of the newly formed single bonds imply two different possible stereochemical outcomes. In cases where the dienophile has a single electron-withdrawing substituent, the outcome can often be predicted by applying the known "endo rule". Furthermore, the use of chiral eutectic solvents in asymmetric synthesis has become a novel tool to maintain sustainability in organic synthesis. In the present work, a set of recyclable and sustainable bio-based deep eutectic solvents (DESs) was designed using hydrogen bond acceptors (HBAs) with a chiral center. These compounds were used in their racemic and enantiomerically enriched forms to prepare DESs with lactic acid (LA), glycerol (Gly), and ethylene glycol, which act as hydrogen bond donors (HBDs) in the corresponding eutectic mixture. These DESs were used as solvents to study the reaction between cyclopentadiene and ethyl acrylate or butyl acrylate in typical [4 + 2] cycloadditions. The best yields and endo-selectivity were achieved using LA as the HBD in the eutectic mixtures. The results and adduct ratios obtained show that these DESs were able to improve both reaction yields and selectivity when compared to those observed in organic solvents or ionic liquids. Moreover, the reaction products (adducts) were easily recovered with diethyl ether from the reaction mixture, where they appeared as an upper layer.