The reaction of dimethyldioxirane with 1,3-cyclohexadiene and 1,3-cyclooctadiene: monoepoxidation kinetics and computational modeling

Abstract

Abstract The reaction of dimethyldioxirane (1) with excess 1,3-cyclohexadiene (2a) and 1,3-cyclooctadiene (2b) in dried acetone yielded the corresponding monoepoxides in excellent yield. Second-order rate constants for monoepoxidation were determined using UV methodology. The k2 value for the monoepoxidation of 1,3-cyclohexadiene was found to be 1.14±0.06 m -1 s-1, whereas that for the monoepoxidation of 1,3-cycloctadiene was 0.31±0.03 m -1 s-1. Basic density functional calculations at the B3LYP/6-31G level were employed to model the monoepoxidations. As expected, the calculations were consistent with a concerted, electrophilic process with a spiro-transition state. As found for the epoxidation of simple alkenes, the calculated transition-state geometry showed a slight asynchronous tilt of the dioxirane plane relative to that of the remaining alkene portion of the diene and a slight tilt back from the face of the diene. Relative reactivities (relative k2 values) were determined using the difference in the calculated electronic activation energies and were consistent with the experimental relative k2 values without the need to correct for the medium (solvent). Reactivity differences for epoxidation can be quickly predicted by this approach as long as there are reasonable structural similarities between the substrates.