Origin of π-Facial Stereoselectivity in Thiophene 1-Oxide Cycloadditions
Brian J Levandowski1, Dinushka Herath1, Nathan M Gallup1
1Department of Chemistry and Biochemistry, University of California , Los Angeles, California 90095, United States.
Abstract:
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.
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