Web Release Date: February 24,
Dienophile Twisting and Substituent Effects Influence Reaction Rates of Intramolecular Diels-Alder Cycloadditions: A DFT Study
Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095-1569, and Department of Chemistry, University of California, Berkeley, California 94720
Received January 9, 2005
Abstract:
Intramolecular cycloadditions of 5-vinyl-1,3-cyclohexadienes were studied with B3LYP/6-31G(d) density functional calculations. The one-atom tether dictates that the Z substituent becomes exo and the E substituent becomes endo in the TS. The geometry of the cycloaddition TS is typical of a pericyclic transformation except unusual twisting of the dienophile places the endo substituent in a relatively steric-free position and the exo substituent in a highly crowded position. The experimental rate differences between isomeric pairs of vinylcyclohexadienes can be explained by comparing reactant destabilization when a bulky group occupies the Z position of the starting alkene and transition state stabilization when a bulky group is endo in the cycloaddition TS.
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