Cyclization Borsche-Drechsel

Also known as Borsche carbazole synthesis, is the two-step conversion of phenyl hydrazine and cyclohexanone derivatives to the corresponding carbazole. Cf. Fischer indole synthesis. 

Name Reactions A Collection of Detailed Mechanisms and Synthetic Applications, DOI 10.1007/978-3-319-03979-4 33, Springer International Publislilng Switzerland 2014 

Borsche, W. Feise, M. Arm. 1908, 359, 49-80. Walther Borsche was a professor at Chemischen Institut, Universitat Gottingen, Germany when this paper was published Borsche was completely devoid of the arrogance shown by many of his contemporaries. Both Borsche and his colleague at Frankfurt, JuUus von Braun, suffered under the Nazi regime for their independent minds. 

Borsche Drechsel Cyclization, In Name Reactions in Heterocyclic Chemistry IT, Li, J. J., Ed. Wiley Hoboken, NJ, 2011, pp 91-101. (Review). 

Tetrahydrocarbazole synthesis from cyclohexanone phenylhydrazone. Cf. Fisher indole synthesis. 

Name Reactions, 4th ed., DOI 10.1007/978-3-642-01053-8 29, Springer-Verlag Berlin Heidelberg 2009 

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Borsche-Drechsel cyclization. Acid-catalyzed rearrangement of cyclohexanone phenylhy-drazone to tetrahydrocarbazole. Subsequent oxidation yields carbazole. 

M.W. Fultz (2011) Borsche-Drechsel cyclization, in Name Reactions in Heterocyclic Chemistry - II (eds. J.J. Li and E.J. Corey), Wiley Sons, Hoboken, New Jersey, pp. 91-101. 

The Borsche-Drechsel cyclization, also known as Borsche carbazole synthesis, is the two-step conversion of phenyl hydrazine and cyclohexanone derivatives to the corresponding carbazole. The first step, which is analgous to the Fischer Indole synthesis, converts the phenyl hydrazine 1 and cyclohexanone 2 to the tetrahydrocarbazole 3. The second step is the oxidation of the terahydrocarbazole to the corresponding carbazole 4. 

The first step of the Borsche-Drechsel cyclization proceeds through the same mechanism as the Fischer indole synthesis. The aryl hydrazine and cyclohexanone condense to form the iminium 5 and water. The imine nitrogen then coordinates to the Lewis acid, which causes a tautomerization of the hydrazone to the ene—hydrazine 6. This ene-hydrazine undergoes a [3,3]-sigmatropic rearrangement to form the iminium ion 7. Rearomitization through deprotonation of the benzylic proton releases the nucleophilic... 

Initially, sulfuric acid was used to initate the Borsche—Drechsel cyclization. However, Perkin found that acetic acid provided cleaner products. Recently, other reagents—such as cerium ammonium nitrate (CAN) and the more environmentally friendly acidic ionic liquids —have been used to catalyze the ring formation. It should be noted that people have found it difficult to promote the cyclization when there are substituents at the ortho-position of the phenylhydrazine. This difficulty becomes more pronounced when both the hydrazine and the cyclohexanone have a-substitution This difficulty usually leads to low yields compared with other methods of forming the carbazole. 

Borsche—Drechsel Cyclization Reaction of Cyclohexanone with (2,5-Dimethyl-4-Nitrophenyl)Hydrazine to Provide 1,4-Dimethyl-3-Nitro-9H-Carbazole ... 

Borsche—Drechsel Cyclization Reaction of 2,3,4,9-Tetrahydro-lH-Carbazole to Provide Carbazole ... 

Dowd-Beckwith Ring Expansion Reaction Drechsel (see Borsche-Drechsel Cyclization)... 

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