Electronic overlap population as a measure of reactivity in electrocyclic reactions. Part 2.—Photocyclization and photodimerization reactions

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Abstract
Electronic overlap populations in the excited and in the ground states, and the differences in these resulting from one electron excitations are suggested as general reactivity indices for electrocyclic processes, in particular photocyclizations and photodimerizations. The applications of this method for predicting the possibility of reaction according to a given path or for comparing different reaction paths are described. The following cases are treated: (a) ground and excited state cyclizations of butadiene, of hexatriene and of tetraphenyl-o-quinodimethane; (b)cis-stilbene-like photocyclizations of 1,2-di-(β-naphthyl)ethylene, and of 1-(α-naphthyl)-2-(β-naphthyl)ethylene; (c) photocyclizations of various sulphur and oxygen heterocyclic stilbene derivatives; (d) photocyclizations of sulphur heterocyclic derivatives of 1,4-diphenylbutadiene; (e) photocyclization of diphenylamine; and (ƒ) photodimerizations of anthracene, 2-methoxynaphthalene, sorbic acid and trans-stilbene. The intermolecular electronic overlap population method is found to describe accurately the reactivity of systems in which the outcome is due to interactions between distant groups, as for trans-stilbene photodimerization. The electronic overlap population method is found to have several advantages compared to the HMO-free valence method for analyzing the reactivity in systems (a)-(ƒ). It gives a correct description of reactivity in several cases where the HMO-free valence method fails. Within the present context, results obtained using EH wavefunctions agree with results obtained from ab initio STO-3G calculations.

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