The role of spin-orbit coupling and symmetry in photochemical rearrangements of alpha,beta-unsaturated cyclic ketones

The spin-orbit coupling parts in the effective one-electron Hamiltonian operator, with the inclusion of symmetry, have been used to formulate spin-inversion mechanisms based on the Shaik and Epiotis theory in triplet photoreactions for the radiationless decay of triplet complexes to singlet ground-state products. This has been applied to investigate the stereochemical behavior of alpha,beta-unsaturated cyclic ketones. It is shown that triplet photoreaction complexes can be classified according to the number and the direction of the atomic orbital rotations required to maximize intramolecular spin-orbit coupling and simultaneously maintain the intramolecular bonding, from which it may produce different stereoisomers. Six spin-inversion reaction mechanisms are suggested to produce the various photoproducts, including aryl-migrated bicyclohexanone (endo and exo), aryl-migrated cyclohexenone, cyclobutanone, lumiketone, cyclopentenone, and type-A rearrangement. It is found that, because of its larger spin-orbit coupling expressions as well as better initial orbital overlap (owing to the less conformational change), the 1,2-aryl-migrated endo-bicyclic ketone is in general the most favorable product formed by 4,4-diarylcyclohexenones in solution, crystal, and polymer matrix photochemistry. The substituents (including their nature and the sites to be occupied), solvent polarity, heavy atom effects and environments must all play a decisive role in predicting the final photoproducts of alpha,beta-unsaturated cyclic ketones. The results obtained are in good agreement with the available experimental results and permit a number of predictions to be made.