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 moiet y from addition to the central carbon of allene, while two others resu lt from addition to a terminal carbon of the allene. The two biradical s containing the allyl fragment are predicted to be approximately 20 k cal/mol lower in energy than the other two. Using a model for the reac tion mechanism in which the relaxed triplet excited state of allene re acts with ground state cyclopentenone, the reaction barriers to formin g these allylic substituted systems are lower by ca. 8-10 kcal/mol in comparison to the other two. The conformation of the biradicals upon s pin inversion is in all probability one important factor in determinin g whether the biradicals close to products or revert to starting mater ials. The sp(3) hybridization at the bond-forming carbon in the vinyl (nonallylic) systems leads to nearly free rotation about that bond, an d three shallow minima and the corresponding transition states for int ernal rotation were located. One minimum has the two carbon centers wh ich carry the excess spin density about 3.8 Angstrom, apart while in t he other two minima the distances are similar to 3.1 Angstrom. Taking these distances into account, it is estimated that very few of the int ermediates formed from the addition a to the carbonyl close to product s. In contrast, a high percentage of the systems formed from attack be ta to the carbonyl are predicted to yield products. In the allylic sub stituted systems, carbon centers with some radical character are alway s relatively close to the site of radical character in the cyclopenten one 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 biradi cals.