Synthesis and isolation of iodocarbazoles.: Direct iodination of carbazoles by N-iodosuccinimide and N-iodosuccinimide-silica gel system

Carbazole (1) undergoes electrophilic aromatic substitution with various iodinating reagents. Although, 3-iodocarbazole (1b) and 3,6-diiodocarbazole (1d) obtained by iodination of carbazole were isolated and characterized sometime ago, 1-iodocarbazole (1a), 1,6-diiodocarbazole (1c) and 1,3,6-triiadocarbazole (1e) had never been isolated from the reaction mixture. The preparation and subsequent isolation and characterization of 1a, 1b, 1c, 1d and 1e are reported (mp, t(r), R-f, H-1-nmr, C-13-nmr and ms). As iodinating reagents, NaIO4/I-2 and NaIO4/KI mixtures in (i) ethanol doped with catalytical amount of sulfuric acid and in (ii) acetic acid, and N-iodosuccinimide and N-iodosuccinimide-silica gel in dichloromethane and in chloroform have been used and their uses have been compared. The iodination reaction of different carbazole derivatives such as 2-acetoxycarbazole (2), 3-bromocarbazole (3) and 3-nitrocarbazole (4) was also studied and the corresponding iododerivatives, 2a, 2b, 2c, 3a, 3b, 4a and 4b, are described for the first time. Semiempirical PM3 calculations have been performed in order to predict reactivity of carbazole (1), substituted carbazoles (2-4) and iodocarbazoles (1a-1e, 2a-2c, 3a-3b, 4a and 4b) (Scheme 1). Theoretical and experimental results are discussed briefly.