Theoretical Studies of Regioselectivity in the Photochemical Cycloaddition of Allene to Cyclopentenone

In the photocycloaddition of allene to cyclopentenone, four different triplet 1,4-biradicals can form, two of which contain an allylic moiety from addition to the central carbon of allene, while two others result from addition to a terminal carbon of the allene. The two biradicals containing the allyl fragment are predicted to be approximately 20 kcal/mol lower in energy than the other two. Using a model for the reaction mechanism in which the relaxed triplet excited state of allene reacts with ground state cyclopentenone, the reaction barriers to forming these allylic substituted systems are lower by ca. 8−10 kcal/mol in comparison to the other two. The conformation of the biradicals upon spin inversion is in all probability one important factor in determining whether the biradicals close to products or revert to starting materials. The sp 3 hybridization at the bond-forming carbon in the vinyl (nonallylic) systems leads to nearly free rotation about that bond, and three shallow minima and the corresponding transition states for internal rotation were located. One minimum has the two carbon centers which carry the excess spin density about 3.8 Å, apart while in the other two minima the distances are ∼3.1 Å. Taking these distances into account, it is estimated that very few of the intermediates formed from the addition α to the carbonyl close to products. In contrast, a high percentage of the systems formed from attack β to the carbonyl are predicted to yield products. In the allylic substituted systems, carbon centers with some radical character are always relatively close to the site of radical character in the cyclopentenone ring. The relative ratio of cycloadducts can be estimated from the distribution of the biradicals. Important factors considered in this study include energetics, conformations, spin−orbit coupling constants, and the singlet−triplet energy splittings of the intermediate biradicals.