PHOTOINDUCED REACTIONS .110. MECHANISM OF INDOLE-SINGLET OXYGEN REACTIONS - INTERCEPTION OF ZWITTERIONIC INTERMEDIATES AND ENE REACTION

The trapping reaction of zwitterionic peroxides formed in singlet oxygen reaction of N-methylindoles is described. The zwitterionic peroxide derived from 1,3-dimethylindole (1) was intercepted by methanol, ethanol, isopropyl alcohol, and β-methoxyethanol. The trend of the efficiency of the trapping reactions by these alcohols was parallel to that for the interception of zwitterions from tetracyanoethylene and enol ethers by alcohols. It has been shown that the introduction of an electron-withdrawing substituent into the indole ring favors the trapping reaction over the oxidative cleavage of the 2,3 double bond. Thus, the photooxygenation of 1,2,3-trimethyl-5-nitroindole (7c), 9-methyl-6-nitro-1,2,3,4-tetrahydrocarbazole (10a), and 9-acetyl-1,2,3,4-tetrahydrocarbazole (10b) in methanol gave the corresponding trapping products 9,11a, and lib, respectively, whereas 1,2,3-trimethylindole (7a), 5-methoxy-1,2,3-trimethylindole (7b), and 9-methyl-1,2,3,4-tetrahydrocarbazole (10c) yielded only the ring cleavage products. Photooxygenation of 7a in alcohols gave the ring cleavage product 8a as the major product, whereas in aprotic solvents 7a produced the ene-type product 13a. In contrast, photooxygenation of 1,2-dimethyltryptophol (7e) gave only the trapping product 16 in both protic and aprotic solvents. The result suggests that a common intermediate for both “ene” and 1,2 cycloaddition is captured intramolecularly by the nucleophilic group of the side chain. The experimental results have been explained by the mechanism involving gauche and cis zwitterions as the intermediates. © 1979, American Chemical Society. All rights reserved.