Theoretical approach of steric effects in the Diels-Alder reaction

The molecular volume of a set of reference molecules composed of 21-substituted methyl, ethyl, ethenyl and ethynyl radicals have been obtained from a theoretical study. The computed volume of each substituent is shown to be constant in the various molecules considered for the study and independent of the hybridization of the linked carbon atom. An additivity scheme allows to determine these substituent volumes, which may be used as a steric factor and be introduced in a structure/reactivity model. The introduction of this steric factor significantly improves the model relating the activation barriers to the well known Field and Resonance parameters for the 1-substituted dienes in the Diels-Alder reaction. As expected, this factor has only a small effect on 2-substitued dienes in the Diels-Alder reaction and failed to improve the correlation in the case of endo/exo additions of substituted ethylene.