Photochemical Transformations. 40. Syn and Anti Migration in Photo-Wagner-Meerwein Rearrangements

Article abstract of DOI:10.1021/jo00223a026

The photochemistry of the trans and cis isomers of 7,8-dichloro-2,3:5,6-dibenzobicyclo<2.2.2>octa-2,5-diene (1 and 3) has been explored.The singlet excited states of these compounds are photoactive.In acetonitrile, mixtures of exo- and endo-4,anti-8-dichloro-2,3:6,7-dibenzobicyclo<3.2.1>octa-2,6-diene (2-Cl), the syn-8-chloro epimers exo- and endo-4-Cl, N-(anti-8-chloro-2,3:6,7-dibenzobicyclo<3.2.1>octa-2,6-dien-exo-4-yl)acetamide (2-NHAc) and its syn-8-chloro epimer exo-4-NHAc were produced.In acetic acid, the dichloro compounds and a mixture of the anti- andsyn-8-chloro-2,3:6,7-dibenzobicyclo<3.2.1>octa-2,6-dien-exo-4-yl acetates (2-OAc and 4-OAc) were produced.In cyclohexane, irradiation of 2 gave the dichlorides epimeric at C-4 and C-8.All of the photoreactions proceeded with a preponderance of migration with retention at the migration terminus (syn migrations), in contrast to the ground-state reactions which proceed with clean inversion at the migration terminus (anti migration).Failure to see stereospecificity was shown not to be due to C-8 bridge migration.Quantum yields for the various reactions are reported.The results are discussed in terms of several reaction channels following excitation and electron transfer.Radical reductive monodechlorinations of 1 and 3 lead to stereoconvergent radical rearrangements.The syntheses of 2-NHAc and 4-NHAc by Ritter reactions from 2-OAc and 4-OAc respectively are described.