Graebe-Ullmann synthesis

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). 

Molina, A., Vaquero, J.J., Garcianavio, J.L. and Alvarezbuilla, J., One-pot Graebe-Ullmann synthesis of gamma-carbolines under microwave irradiation, Tetrahedron Lett., 1993, 34, 2673. 

As a recent modification, the Graebe-Ullmann synthesis of. y-carbolines under microwave irradiation has been conducted (Scheme 37). Yields of the reaction were comparable to those obtained by conventional heating at 150-210°C (30-81% yield), but the reaction times were significantly reduced <93TL2673>. Similarly, using flash vacuum pyrolysis techniques, nitroar-ylbenzotriazoles have been converted to pyridoindoles <88BSB245>. 

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. 

Other references related to the Graebe-Ullmann synthesis are cited in the literature. ... 

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. 

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

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

The extension of the method to the synthesis of y-carboline from 1-y-pyridylbenztriazole (199) and of a 3,4-benz-y-carboline from a l-(4-quinolyl)benztriazole proceeded smoothly. In an alternative approach excellent yields of y-carboline were obtained by heating l-phenylpyrido[3,4-d] -triazole (200) at 320-350°. The synthesis of halogeno-substituted j8- and y-carbolines via the Graebe-Ullmann reaction has been reported. ... 

The application of the Graebe-UUmann earbazole synthesis to the carboline series has led to syntheses of a-, /3-, and y-carboline. This reaction has already been reviewed in Vol. Ill of this series.419 The only point worth reemphasizing at this stage is the fact that Graebe-Ullmann cyclization of 1 -/J-pyridylbenztriazole (189) gives mainly, if not only, j8-carboline—cyclization taking place at C-4 of the pyridine ring—in marked contrast to the results obtained in the Fischer cyclizations. 

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... 

The Graebe-Ullmann methodology has proved especially valuable in the synthesis of ellipticine and analogues. Using flash vacuum pyrolysis, Miller and StoweU... 

As featured in reviews, the Graebe-Ullmann reaction has played an important role in the synthesis of... 

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