MECHANISTIC INVESTIGATION OF [2+2] PHOTOANNULATIONS OF ENONES AND OLEFINS

The photoinitiated [2 + 2] cycloaddition reaction between acrolein and ethylene is investigated using ab initio molecular orbital calculatio ns. RHF and UHF geometry optimizations with die 6-31G basis set are r eported for ethylene, acrolein, triplet acrolein, gauche and trans tri plet biradical intermediates, and formylcyclobutane, and are used to a ddress the issues of reactivity and selectivity in enone-olefin photoa nnulations. In contrast to a model in which regioselectivity arises fr om the alignment of dipoles in the excited state, calculated dipole mo ments and electrostatic potentials show no basis for such an assignmen t. It is striking that the proposed dipole as well as FMO bawd selecti vity models in the literature used to explain observed product distrib utions are inadequate, leaving open the question of the mechanistic si gnificance of a triplet exciplex in determining regioselectivity.