Olefin Epoxidation of α-β Unsaturated Esters. Comparison of Reactivity of Some Simple Esters and Enantiomerically Pure Diesters of TADDOL and BINOL: A Computational Study

Epoxidation reaction has been the subject of numerous investigations and a number of useful methodologies have been studied. The electrophilic reagents used in this epoxidation, react preferentially with electron-rich olefins. For this reason, electron-deficient olefins such us α,β-unsaturated esters react very slowly with low yields. Our group has previously reported the synthesis of enantiomerically pure α-β unsaturated diesters of TADDOL and BINOL. Optically active epoxides are highly versatile intermediates that can be converted into a wide variety of enantiomerically enriched molecules. Because of this, we decided to study the stereoselective di-epoxidation of diacrylate derivatives with C₂-symmetry. First, we studied simple substrates with the intention to extend the results to more complex molecules. Thus, several epoxidation methods were tested on α-β-unsaturated esters with the aim to improve the previously reported data. Among all the experimental conditions tested, mCPBA reagent was the best, giving the glycidic esters derivatives with very good yields and short reaction time compared with reported methods. Due to the fact that the reaction conditions were unsuccessful with the C₂-diacrylate systems, we decided to study the reactivity of these substrates applying DFT calculations. The reactivity of mCPBA, in olefins epoxidation reactions, can be rationalized by frontier orbital interactions. For this reason, we evaluated the coefficients and shape of HOMO OMs of a series of simple esters and TADDOL and BINOL derivatives. We achieved interesting results concerning the reactivity of these compounds.