SOLVENT EFFECT AS THE RESULT OF FRONTIER MOLECULAR-ORBITAL INTERACTION .6. THE DIELS-ALDER REACTIONS OF AN N-ACYL-ORTHO-QUINONE MONOIMIDE BEHAVING AS DIENE OR DIENOPHILE

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-pi 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.