On the Synthesis and Isolation of Chlorocarbazoles Obtained by Chlorination of Carbazoles

Article abstract of DOI:10.1002/jhet.5570340327

Carbazole (1) undergoes electrophilic aromatic substitution with various chlorinating reagents. Although 3-chlorocarbazole (1b), 3,6-dichlorocarbazole (1d) and 1,3,6,8-tetrachlorocarbazole (1f) obtained by chlorination of carbazole were isolated and characterized sometime ago, 1-chlorocarbazole (1a), 1,6-dichlorocarbazole (1c) and 1,3,6-trichlorocarbazole (1e) had never been isolated from the reaction mixtures. The preparation and subsequent isolation and characterization of 1a, 1b, 1e, 1d, 1e and 1f are reported (mp, t_F, R_f, ¹H- and ¹³C-nmr, ms). Physical and spectroscopic properties of le are compared with those of 1b and 1d in order to show that the former is the major product obtained in several chlorinating processes. As chlorinating reagents, chlorine in glacial acetic acid, sulfuryl chloride, N-chlorosuccinimide, N-chlorosuccinimide-silica gel, N-chlorobenzotriazole, and N-chlorobenzotriazole-silica gel in dichloromethane and in chloroform have been used and their uses have been compared. The chlorination reaction of different carbazole derivatives such as 2-hydroxycarbazole (2), 2-acetoxycarbazole (3), 3-bromocarbazole (4) and 3-nitrocarbazole (5) was also studied and the corresponding chloro derivatives 2a, 2b, 2c, 2d, 3a, 3b, 3c, 3d, 3e, 3f, 4a, 4b, 4c, 4d, 5a and 5b are described for the first time. Semiempirical PM3 calculations have been performed in order to predict reactivity of carbazole (1), substituted carbazoles 2-5 and chlorocarbazoles (Scheme 1). Theoretical and experimental results are discussed briefly.