SOLVENT EFFECT AS THE RESULT OF FRONTIER MOLECULAR ORBITAL INTERACTION. VI. THE DIELS-ALDER REACTIONS OF AN N-ACYL-o-QUINONE MONOIMIDE BEHAVING AS DIENE OR DIENOPHILE

Article abstract of DOI:10.1016/S0040-4020(01)86536-4

The solvent effect of the Diels-Alder (D.A.) reactions of N-(2',4'-dichloro-6'-oxo-2',4'-cyclohexadien-1'-ylidene)-4-nitrobenzamide (1) was kinetically investigated. 1, with 2,3-dimethylbutadiene (DMB), behaves mainly as heterodienophile whereas, with ethyl vinyl ether (EVE), reacts as heterodiene.The mechanism of solvation is the same in both D.A. reactions and involves 1 since log k_DMB is linearly related to log k_EVE.The monoparametric inverse relationship between rate and basicity D_? parameters shows that solvent behaves as a nucleophile.This relationship is largely improved by a bi-parametric equation with solvent basicity and polarity contributions.The negative polarity contribution of the solvent to the rate is discussed in terms of change of polarity for these D.A. reactions going from reactants to transition states.