INTERMOLECULAR ORBITAL THEORY .3. THERMAL AND PHOTOCHEMICAL DIMERIZATION OF UNSATURATED KETONES

The theory of the interaction between conjugated molecules with overlapping p orbitals3 has been extended to include the effect of electron repulsion. Self-consistent field π orbitals on the separate molecules are used as starting point. The total interaction energy is expressed in terms of the overlap Srr between interacting orbitals and of the net electronic charge ∆qr on the atoms. The results are used to draw incipient reaction paths for the Diels-Alder dimerization of acrolein and to calculate stabilization energies in the photodimerization of 3,5-diene-7-one and 4,6-diene-3-one type keto steroids. The high selectivity in the thermal reaction can be ascribed to the combined effect of favorable overlap and polar interactions. In the photodimerization reactions the product is determined, among the few sterically allowed configurations of approach, by favorable overlap interactions in the first case but no simple physical interpretation of the selectivity can be found in the second case. © 1969, American Chemical Society. All rights reserved.