Carbazole Graebe-Ullmann synthesis

The thermal decomposition of triazole 1 to form carbazole 2 is regarded as the Graebe-Ullmann carbazole synthesis. 

In 1968, Burgess and colleagues discovered the photochemical Graebe-Ullmann carbazole synthesis. Thus, irradiation of a solution of 1-phenylbenzotriazole in benzene (0.1M) afforded a nearly quantitative yield of carbazole (Scheme 4, equation 1) [26]. Schmid and colleagues... 

As we saw briefly in Chapter 53, the photochemical version of the Graebe-Ullmann carbazole-carboUne synthesis can be an excellent route to these heterocycles. Similar to this benzotriazole reaction is the photolysis of aryl azides to give carbazoles and related fused indoles. 

GOMBERG - BACHMANN - GRAEBE - ULLMANN Arylatwn Atykaryl bond formation via diazonium salts Carbazole synthesis by intramolecular aryl-aryl bond formation. 

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). 

Transformation of 1-arylbenzotriazole (520) to carbazole (1) under thermal conditions is known as the Graebe-Ullmann synthesis (502). This transformation is very sensitive to the nature of the substituents on the benzotriazole. However, unsubstituted 1-arylbenzotriazole (520) affords carbazole in nearly quantitative yield. The required starting material, 1-arylbenzotriazole (520), is prepared by the diazotization of N-(2-aminophenyl)aniline (519) (503,504). Limited reports on the mechanism of the Graebe-Ullmann reaction have appeared. Presumably, a diradical intermediate is involved in the thermolysis of the triazole (505) (Scheme 5.9). 

Graebe-Ullmann synthesis. Formation of carbazoles by the action of nitrous acid on 2-amino-diphenylamines, followed by decomposition of the resulting benzotriazoles. 

The Graebe-Ullmann synthesis of carbazoles by gas-phase pyrolysis80-83 or photolysis69 of l-aryl-l,2,3-benzotriazoles involves the cyclization of imidoylcarbenes (or the mesomeric 1,3-diradicals) onto aromatic rings (Eq. 24). This cyclization is apparently very rapid, for the use of substituted... 

This reaction is related to the Bucherer Carbazole Synthesis, Fischer Indole Synthesis, Graebe-Ullmann Synthesis and Piloty-Robinson Pyrrole Synthesis. 

The Graebe-Ullmann reaction of Af-arylbenzotriazoles is an extraordinarily versatile and powerful synthesis of carbazoles and carbolines. Despite the commonly encountered low yields, the ease of preparation of the requisite Al-arylbenzotriazoles, either by the diazotization of 2-ami-nodiarylamines or the coupling of benzotriazole with the appropriate aromatic or heteroaromatic partner, elevates the Graebe-Ullmann reaction to one of distinction for the synthesis of carbazoles and carbolines. 

The Graebe-Ullmann synthesis, converts 1-arylbenzotriazoles into carbazoles or their heterocyclic analogs. It has been accelerated under microwave irradiation, where the l-(4-pyridyl)-benzotriazole was converted into a y-carboline (Molina et al, 1993). This synthesis was completed within 2.5 h by conventional method while it takes only 15 min under microwave exposure and the yield was also increased. 

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