A Predictor of Reactivity in Allowed Photodimerizations and Photocycloadditions

Abstract

A paradigm for prediction of reactivity in allowed, [2 + 2] and [4 + 4] singlet-state photodimerizations and photocycloadditions is developed and compared to results obtained from the literature. Favorable features for high reactivity are high singlet energy, low triplet energy, and high frontier orbital density at reacting positions. The quantitative algorithm γ(rc) = (ETA + ETB - ESA)/c2 is derived, where γ(rc) is the resonance integral for end-on interaction of carbon 2p orbitals at distance rc, ETA and ETB are triplet energies of reactants A and B, ESA is the singlet energy of the excited reactant A, and c2 is the sum of HOMO and LUMO orbital coefficient products over reacting positions. The algorithm correlates a substantial body of known photoreactivities with considerable success. © 1980, American Chemical Society. All rights reserved.