Organic photochemical Rearrangements of triplets and zwitterions; Mechanistic and exploratory organic photochemistry

There has been some controversy about the mechanisms of reactions of enones and dienones. The question has been whether a zwitterion or diradical is involved as the intermediate. In the case of ground state species it has not been recognized that zwitterions may have diradical character. In triplet reactions there is the question of whether the rearrangements take place at the initial T1 stage or subsequently as SO ground state zwitterions or diradicals. We now have some new rearrangements bearing on these questions. In some cases it is the triplet which rearranges, while in others it is the SO zwitterion. The zwitterion vs. diradical nature of the SO species has been assessed by both experimental as well as theoretical means. Similarly, both experiment and computational theory have been used to determine at what stage the triplet rearrangements occur and to determine the characteristics of the rearranging species. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007).