Graebe-Ullmann reaction

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. ... 

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). 

Through the photolytic extrusion of dimethyl phenylphosphonate from oxazaphos-phoranes, the corresponding carbazoles are formed by a phosphorus analogue of the Graebe-Ullmann reaction. The irradiation of 2,2-dimethoxy-3-(4-methoxyphenyl)-2-phenyl-2,3-dihydrobenz-l,3,2-oxazaphospholine (22) in benzene gives 3-methoxycar-... 

Dediazoniation of 1-phenylbenzotriazole leads via a diradical intermediate almost quantitatively to the formation of carbazole (Graebe-Ullmann reaction) ... 

The preparation of carbazoles involving the pyrolysis of 1-phenyl-1,2,3-benzotriazole prepared from ort/io-amino-diphenylamine and nitrous acid is known as the Graebe-Ullmann reaction. It is known that this reaction involves the cyclization of a diradical or iminocarbene intermediate, which isomerizes to carbazole via a [l,3]-hydrogen shift. 

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

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. 

Gabriel-Isay synthesis (pteridine) Gabriel synthesis (thiazole) 207 Gassmann synthesis (indole) 135 Gattermann synthesis, HCN-free 509 Gewald synthesis (thiophene) 97 Graebe-Ullmann reaction 266 Guareschi synthesis (2-pyridone) 384... 

Pyrolysis of 3-(o-azidophenyl)pyridine gives a- and y-carboline. Although the process does not formally involve a free radical ( N.GeH4. C5H4N), it is reminiscent of a radical substitution. The mechanism of the Graebe-Ullmann reaction, the formation of carbazoles by the pyrolysis of 3-arylbenzotriazoles, has not been elucidated, but some examples of it, notably the formation of carbolines from 3-pyridylbenzotriazoles, suggest the character of a radical substitution. 

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