Origin of π-Facial Stereoselectivity in Thiophene 1-Oxide Cycloadditions

摘要

We report a DFT computational study (M06-2X) of π-facial selectivity in the Diels–Alder reactions of thiophene 1-oxide. The preference for the syn cycloaddition arises because the ground state geometry of thiophene 1-oxide is predistorted into an envelope conformation that resembles the syn transition state geometry. The syn distortion occurs to minimize the effect of hyperconjugative antiaromaticity in the thiophene 1-oxide, arising from overlap of the σ*_(SO) with the π-system. The syn selectivity follows through to the product structure that is stabilized by a π–σ*_(SO) interaction, related to the 7-norbornenyl ion stability.