Carbazoles Tauber carbazole synthesis

The infrequently used carbazole synthesis discovered by Ernst Tauber in 1890 involves the acid-promoted, high-temperature conversion of 2,2 -diaminobiphenyls to carbazoles. Tauber s original observation was the reaction of meta-diaminobenzidine with 18% hydrochloric acid at 180-190 °C to afford 2,7-diaminocarbazole (Scheme 1, equation 1) [ 1-3], In a second paper, in which... 

Scheme 3 Olah and Cho Modifications of the Tauber Carbazole Synthesis...

Among the two cyclization procedures that utilize 2,2 -diaminobiphenyls, the Tauber synthesis has been most widely used and proceeds in higher yields than the diazotization reaction. The application of this method to the synthesis of carbazoles is limited by the availability of 2,2 -diaminobiphenyls. 

As an alternative to the Tauber method, 2,2 -diaminobiphenyl (509) was transformed to carbazole (1) using diazotization conditions. This reaction proceeds intra-molecularly by displacement of nitrogen. With this procedure, only moderate yields of carbazoles were obtained. The reaction is mechanistically similar to the Graebe-Ullmann synthesis (494) (Scheme 5.6). 

The first person to explore the utility of the Tauber synthesis in any depth was Leditschke, who prepared several carbazoles using concentrated phosphoric acid (Scheme 2,... 

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