Photochemical Reactions of Regioisomeric 2,2-Dimethyl-5,5-diphenyl- and 5,5-Dimethyl-2,2-diphenyl-Substituted Diazo Ketones of a Tetrahydrofuran Series

The principal direction of conventional photolysis of the regioisomeric 2,2-dimethyl-5,5-diphenyland 5,5-dimethyl-2,2-diphenyl-substituted 4-diazodihydrofuran-3(2H)-ones la and 1b, respectively, is the Wolff rearrangement, while other photochemical processes, which are giving rise to the formation of C-H-insertion, 1,2-alkyl- or -aryl-shifts, as well as H-atom-abstraction products occur to a much lower degree (Schemes 2 and 3). The ratio of similar reaction products from both regioisomers 1a and 1b is essentially independent of their structure, and a substantial effect of the relative position of the Ph and diazo group to each other on the yield of C-H-insertion products does not occur. Based on stereochemical considerations, the Wolff rearrangement of diazodihydrofuran-3(2H)-ones apparently proceeds in a concerted manner, whereas the appearance in the reaction mixture of 1,2-shift and H-atomabstraction products points to the parallel generation during photolysis of singlet and triplet carbenes (Schemes 4 and 5).