Carbazole, hydrogenation

Amino-9-methyl-carbazole hydrogenated in a stirring autoclave with 5%-rhodium-on-carbon in aq. HGl at 50-100° and 800 to 350 p. s. i. g. until no further pressure drop is observed crude 3-amino-9-methyldode-cahydrocarbazole. Y 72%. 

Two types of cycloaddition reactions have found application for the Synthetic elaboration of indoles. One is Diels-Alder reactions of 2- and 3-vinylindoles which yield partially hydrogenated carbazoles. The second is cycloaddition reactions of 2,3-indolequinodimethane intermediates which also construct the carbazole framework. These reactions arc discussed in the following sections. 

As might be anticipated from the behaviour of the parent heterocycles, C-2 of indole, benzo[i]furan and benzo[i]thiophene (Table 13) is shifted to lower field than C-3. However, the shifts for C-2 (O, 144.8 Se, 128.8 S, 126.1 NH, 124.7 Te, 120.8) and C-7a (O, 155.0 Se, 141.3 S, 139.6 NH, 135.7 Te, 133.0) in the benzo[i] heterocycles vary irregularly (80OMR(l3)3l9), and the sequence is different to that observed for C-2 in the parent heterocycles, namely 0>Se>Te>S>NH. Also noteworthy is the upheld position of C-7, especially in indole and benzofuran, relative to the other benzenoid carbons at positions 4, 5 and 6. A similar situation pertains in the dibenzo heterocycles (Table 14), where not only are C-1 and C-8 shifted upheld in carbazole and dibenzofuran relative to the corresponding carbons in dibenzothiophene and fluorene, but similar, though smaller, shifts can be discerned for C-3 and C-6 in the former compounds. These carbon atoms are of course ortho and para to the heteroatom and the shifts reflect its mesomeric properties. Little variation in the carbon-hydrogen coupling constants is observed for these dibenzo compounds with V(qh) = 158-165 and V(c,h) = 6-8 Hz. 

The crystal and molecular structure79 of a carbazole/tetracyano ethylene 1 2 complex reveals the presence of a hydrogen bond between the amino group of the carbazole and a nitrile nitrogen of tetracyano ethylene in the solid state, which resembles the stronger hydrogen bond observed between amines and the cyano group. 

Excitation of o-nitrodiphenylamine (111) and 3-nitro-2-phenylaminopyridine (112)64 causes the common intramolecular hydrogen abstraction, as the initial step, but subsequent steps involve the elimination of HNO2 and cyclization to give carbazole and a-carboline (113 and 114) in equation 55. 

In the latter case the structure was established by reduction with stannous chloride in acetic acid, which afforded the imidazole derivative (104), thus proving ortho substitution, followed by comparison of the ultraviolet spectrum of 104 with the two possible benzimidazole derivatives of carbazole. Deamination of 103 via diazotization in sulfuric acid afforded 1-nitrodibenzothiophene (105) (54%), being the only recorded route to this compound (Scheme 5). Initial attempts to hydrolyze 102 to the nitroamine (103) had been made with ethanolic hydrogen chloride... 

For dibenzofuran, no diphenyl was detected in the product, but phenylcyclohexane and bicyclohexane were detected. Therefore, hydrogenation of at least one of die rings was necessary before ring opening took place. A similar conclusion was found for the analysis of the product from hydrogenation of carbazole. There was evidence for the formation of the hexahydro-derivatives of the two compounds which could make both these compounds effective hydrogen donors. 

Carbazoles, too, can be reduced partially to dihydrocarbazoles by sodium in liquid ammonia [460], or to tetrahydrocarbazoles by sodium in liquid ammonia and ethanol [460] or by sodium borohydride [457]. Carbazole was converted by catalytic hydrogenation over Raney nickel or copper chromite to 1,2,3,4-tetrahydrocarbazole, 1,2,3,4,10,11-hexahydrocarbazole, and do-decahydrocarbazole in good yields [430]. 

Methoxymurrayanine (3) (3-formyl-l,6-dimethoxycarbazole) (15) was isolated from the roots of C. lansium (23). The roots of C. lansium were used to treat bronchitis and malaria. The UV (/Imax 227, 239, 251, 294, 335, and 349 nm) and IR spectra showed the presence of a 3-formylcarbazole framework. The H-NMR data were similar to murrayanine (9), with an additional methoxy group for one of the aromatic hydrogens at the C-ring of the carbazole. Since the signal for H-5 (5 7.90) is not ortho-coupled, an additional methoxy could be located at C-6. This structural conclusion was also supported by the C-NMR spectrum. 

The UV spectrum (/Imax 246, 251, 257, 307, and 341 nm) of cyclomahanimbine (151) indicates the presence of a carbazole framework with an ether oxygen at C-2. This assignment was supported by the IR spectrum [v ax 1602, 1615 (aromatic system), and 3425 (NH) cm ]. On hydrogenation, cyclomahanimbine yielded the corresponding dihydro derivative, which did not show any shift of the absorption... 

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