EPOXIDATION BY DIMETHYLDIOXIRANE - EFFECTS OF INTRAMOLECULAR AND INTERMOLECULAR INTERACTIONS

The Density Functional Theory B3LYP/6-31G method is used to provide a detailed understanding of the origins of infra-and intermolecular (so lvent) effects on the epoxidation of C-C double bonds by dimethyldioxi rane (DMDO) in a model system, 2-methyl-2-butene. We found that the pr esence of hydrogen bond donor substituents, such as hydroxyl and amino groups, at the allylic position on the olefin leads to substantially decreased activation barriers for epoxidation. This effect is observed exclusively when a hydrogen bond interaction is present between the h ydroxyl or amino substituent and the attacking DMDO molecule, and is n ot caused by inductive electronic effects of the substituents. An even more significant lowering of the activation barrier is seen when DMDO forms a hydrogen bond with methanol (representing a hydrogen bond don or solvent) in the transition state. Solvent polarity, studied using t he SCIPCM model, influences the epoxidation barrier to a much smaller degree than do hydrogen bonding interactions.