PHOTO-CHEMICAL TRANSFORMATIONS .24. COMPARISON OF IONIC INTERMEDIATES PRODUCED PHOTOCHEMICALLY WITH CORRESPONDING GROUND-STATE INTERMEDIATES - INITIAL STUDIES IN SOME CHLOROBENZOOCTADIENYL SYSTEMS

Rearrangements and solvolyses in acetic acid and/or in acetonitrile have been studied with exo and endo 4-functionalized derivatives of 3-chloro-6,7-benzobicyclo[3.2.1]octa-2,6-diene (3 and 4, respectively) and their Wagner-Meerwein rearrangement isomers, the anti 7-functionalized 2-chloro-5,6-benzobicyclo[2.2.2]octa-2,5-dienes (5). Both ground-state and photochemical reactions with a variety of nucleofugal groups X have been investigated. Ground-state reactions appear to be “normal”, 3 and 4 isomers being produced under kinetic control and a preponderance of 5 isomers produced upon equilibration. No mixing of this system with the syn-benzylic chlorobenzobicyclooctadienyl systems 6-8 is seen. The results are compatible with the idea that the lowest lying cationic intermediate in this system is the allylic cation 15 and that the benzo-bridged (phenonium ion) cation 14 is of low enough energy to be readily accessible in the equilibration studies. Direct irradiation of either of the allylic isomers 3 and 4 (X = Cl, OCOCHCl2, and OMs) in acetonitrile led readily to the 5 isomers, without mixing of 6-8 systems, accompanied by photosolvolysis to 3-NHCOCH3 (water added). Solvent effects show that polar solvents favor such reactions, while radicals are produced from 3-C1 in nonpolar solvents. With X = OH or OAc, only di-ύ-methane rearrangement was observed, and the latter type of rearrangement was observed as well with a variety of 3-X and 4-X compounds, with ketone-triplet sensitization. The unsensitized results seem inconsistent with reaction paths involving radical intermediates or concerted rearrangements, but can be rationalized by the assumption that reaction from the excited state leads to an intimate ion pair involving the bridged cation 14, which may relax to 5-X or 3-X by ion-pair collapse or to the more stable allylic ion 15, prior to or attendant upon formation of a solvent-separated ion pair. © 1979, American Chemical Society. All rights reserved.