Dehydrogenating agents chloranil

A large rtumber of carbazole syntheses involve the preparation and dehydrogenation of hydrocarbazoles, mainly 1,2,3,4-tetrahydrocarbazoles. They are usually prepared by either the Fischer-Borsche synthesis or the Japp-Klingemann reaction. The most commonly used dehydrogenating agents are palladium on charcoal or chloranil (495,496). 

Chloranil in boiling xylene has been shown to be an excellent dehydrogenation agent for the preparation of carbazoles from 1,2,3,4-tetrahydro-carbazoles (75-95%). By this procedure, carbazoles substituted in the 1-, 2-, and 3-positions with alkyl, halo, alkoxyl, carboxyl, or nitro groups are readily made. The starting materials are available by ring closures of cyclohexanone-OT-phenylhydrazones. A palladium-carbon catalyst for the same purpose is also noteworthy, the yields of alkylcarbazoles being 86-100%. ... 

The micFobiologically important 4-oxoquinoline-3-carboxylic acid skeleton is formed by applying Dieckmann conditions to the diester (70.9) in which the NH is protected as its acyl derivative. Dehydrogenating agents such as chloranil do not always give good results when applied to the dihydroquinolinones thus produced. Better yields are obtained by electrolysis in acetic acid-t-butanol with a quaternary ammonium salt as electrolyte. With careful choice of conditions, a high yield of the deacylated quinolinone may be obtained. 

Chloranil is a mild dehydrogenating agent.128 Xylene (b.p. 140°) is used as solvent, so that dehydrogenation occurs at a relatively low temperature. 

Closely related to the vinylbenzo[b]thiophene reactions are the condensations of the bisbenzo[b]thienyls 108 and 110 with maleic anhydride in the presence of the dehydrogenating agent chloranil to form 419 and 420, respectively (55-78%), which are hydrolytically decarboxylated (37-62%) to the parent pentacycles 56 and 54 by means of soda lime at 400°C.146... 

When a /3-aminoenethione, which can be produced from an isoxazole via hydrogenolysis then reaction of the / -aminoenone with a thionating agent, is treated with a dehydrogenating agent such as chloranil " or sulfur, ring closure to an isothiazole results. 

Scheme 9.13). The process starts with a Michael reaction of a keto-thiol with a vinylphosphonium salt. The Michael adduct is then reacted under the usual conditions of the Wittig synthesis to form a dihydrothiophene. This is easily oxidized (dehydrogenated) to the thiophene with the mild agent chloranil, which is reduced to tetrachlorohydroquinone. 

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