Carbazole

Production

Several routes exist beginning with extraction from coal tar by distillation. Carbazole concentrates in the anthracene distillate and is removed before anthraquinone production; that waste product is the major industrial carbazole source. Polar compounds (e.g., ketones) selectively precipitate it from the anthracene; a more modern technique is simply selective crystallization from molten coal tar at high temperature or low pressure (70 mmHg).

Preparative methods:

• Tetrahydrocarbazole, readily available via the Borsche–Drechsel cyclization, is oxidized by red lead to carbazole.

• Another classic is the Bucherer carbazole synthesis, which uses a naphthol and an aryl hydrazine.

• A third method for the synthesis of carbazole is the Graebe–Ullmann reaction.

In the first step, an N-phenyl-1,2-diaminobenzene (N-phenyl-o-phenylenediamine) is converted into a diazonium salt which instantaneously forms a 1,2,3-triazole. The triazole is unstable and at elevated temperatures, nitrogen is released and the carbazole is formed.

Diphenylamine derivatives, being electron rich, are naturally oxidized to carbazoles when heated in air; a similar reaction is the Mallory reaction.

Substituted carbazoles are most easily synthesized with transition metal coupling reactions. For applications that transition-metal impurities in the final product might inhibit, an alternative is nucleophilic aromatic substitution on dibenzothiophene dioxide.

Natural Occurrence

Carbazoles occur naturally in carbazole alkaloids. Carbazole alkaloids with unsubstituted benzene rings occur rarely. Olivacin has been found in the bark of Aspidosperma olivaceum and ellipticin in Ochrosia elliptica. Some carbazole alkaloids, especially glybomin B, have been isolated from Glycosmis pentaphylla.

Applications

As carbazoles have a relatively rich UV-visible light spectrum, they see application as pigments and photocatalysts. The parent carbazole is used in Hydron Blue production and aminoethylcarbazole is used in pigment violet 23 production.

Carbazoles stabilize triplet emitters in certain light-emitting diodes; in general, they are electron photodonors (hole acceptors).

Carbazole electrochemically oxidizes to a conductive polymer, which has not achieved substantial industrial use. Polyvinylcarbazole is useful in the electrical and electronic industries, and certain carbazole novolaks are extremely heat resistant.

In organic chemistry, carbazole proper is also an ingredient for several bioactive molecules. The insecticide Nirosan, the cocaine overdose antidote Rimcazole, and the veterinary NSAID Carprofen are all made from carbazole. The topoisomerase II inhibitor ellipticine fuses carbazole to a pyridine ring.