Heterocyclics carbazoles

Coal tar is a complex mixture consisting almost exclusively of aromatic compounds. The main components of coal tar are naphthalene, phenanthrene, fluoranthene, pyrene, acenaphthene, anthracene, the heterocyclics carbazole, quinoline and isoquinoline, phenol and benzofuran-derivatives, as well as sulfur compounds such as thianaphthene. Olefinic compounds are also present in coal tar (Table 3.1). The total number of constituents is estimated at 10,000. 

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. 

Competitive metallation experiments with IV-methylpyrrole and thiophene and with IV-methylindole and benzo[6]thiophene indicate that the sulfur-containing heterocycles react more rapidly with H-butyllithium in ether. The comparative reactivity of thiophene and furan with butyllithium depends on the metallation conditions. In hexane, furan reacts more rapidly than thiophene but in ether, in the presence of tetramethylethylenediamine (TMEDA), the order of reactivity is reversed (77JCS(P1)887). Competitive metallation experiments have established that dibenzofuran is more easily lithiated than dibenzothiophene, which in turn is more easily lithiated than A-ethylcarbazole. These compounds lose the proton bound to carbon 4 in dibenzofuran and dibenzothiophene and the equivalent proton (bound to carbon 1) in the carbazole (64JOM(2)304). 

W. C. Sumpter and F. M. Miller, Heterocyclic Compounds with Indole and Carbazole Systems, p. 50. Wiley (Interscience), New York, 1954. 

Oddo reported that the organomagnesium derivatives of p3Trole, indole, skatole, and carbazole could be prepared in a single operation by mixing the parent heterocyclic compound with an alkyl halide and magnesium in anhydrous ether.The product formed was reported to be the same as that obtained by the more conventional procedure. However, this approach to the synthesis of the indole Grignard reagents does not seem to have been exploited in subsequent work. 

N-Bridged tellurium-containing heterocycles are exemplified by derivatives of l,4-tellurazine[2,3-g/i]carbazole 107 and heterocycle 108. One-pot reaction of A-propargylcarbazole with tellurium tetrahalides under conditions of the two-phase tellurohalogenation (Section n,B,l) leads to halogenomethylene derivatives 107 (90KGS126). The reaction proceeds as the spontaneous electrophilic cycliza-tion of initially formed tellurium trihalides 109. 

Syntheses of heterocycles, among them carbazole alkaloids, with participation of tricarbonyl(Ti -diene)iron complexes 99CSR151. 

The fourth chapter of this volume comprises the second part of an ongoing series by Professor A. P. Sadimenko (Fort Hare University, South Africa) dealing with organometallic compounds of pyrrole, indole, carbazole, phospholes, siloles, and boroles. This follows the review in Volume 78 of Advances covering organometallic compounds of thiophene and furan. The enormous recent advances in this area are summarized and classified according to the nature of the heterocycle and of the metals. 

These dienes are valuable for the Diels-Alder based synthesis of dibenzofurans, dibenzothiophenes, carbazoles and other classes of complex polycyclic heterocyclic compounds. Scheme 2.32 summarizes some of the cycloadditions [81] of 2-vinylbenzofurans (80). 

Bouaziz Z., Nebois P., Poumaroux A., Pillion H. Carbazole-l,4-Diones Syntheses and Properties Heterocycles 2000 52 977-1000... 

Pyrrole derivatives substituted in positions 1-, 3-, or 4- have also been electrochemically polymerized (positions 2- and 5- must be free for polymerization). Besides homopolymers, copolymers can also be prepared in this way. Other nitrogen heterocycles that have been polymerized by anodic oxidation include carbazole, pyridazine, indole, and their various substitution derivatives. 

A number of molybdenum-containing hydroxylases catalyzing the first hydrox-ylation step of N-containing compounds have been characterized thoroughly (e.g., carbazole [314], quinoline [327], and indole [350]). The enzyme s redox-active has been described as a molybdenum ion site coordinated to a distinct pyranopterin cofactor (two different [2Fe2S] centers) and in most cases, flavin adenine dinucleotide centers. This active center transfers electrons from the N-heterocyclic substrate to an electron acceptor, which for many molybdenum hydroxylases is still unknown [350],... 

The a-oxoketene dithioacetal 6.40 is derived from indoxyl (l,2-dihydroindol-3-one), a heterocyclic carbonyl precursor, and its reaction with simple allyl anions will also yield the corresponding Jl-annulation product. Thus when 6.40 was reacted with allyl anions 65 the corresponding carbinol acetals 66 formed insitu underwent smooth BF3.Et20 assisted cyclization to afford the corresponding carbazoles 67 in high yields <99T11563>. 

Intramolecular C-H arylations have been described and are mediated by a variety of palladium complexes, affording a host of heterocyclic products. Carbazoles can be synthesized by a sequential amination/C-H functionalization process (Equation (161)). 

The oxygen transfer to the -acetylenic carbon results in the very intense benzoyl cation, whereas the transfer to the a-carbon, via a series of fragments corresponding to the loss of OH , CO and CO2, respectively, leads to annelated heterocycles such as the radical cation of carbazole, as a result of elimination of CO81. The generation of the benzoyl cation was rationalized as shown in Scheme 1281. 

In the absence of any accurate isomer distribution studies the question of theoretical positional reactivities in dibenzothiophene becomes somewhat meaningless. However, when viewed within the context of the behavior of the related heterocycles dibenzofuran, carbazole, and fluorene, specific studies on dibenzothiophene are more valid. Such studies have been made by Eaborn and co-workers. The rates of cleavage of the four isomeric trimethylsilyldibenzothiophenes (Section VI, H, 2) were studied at 50° using a mixture of methanol and aqueous perchloric acid and compared with the rates of protodesilylation of 2- and 4-trimethylsilyldiphenyl sulfide. The reactivities shown in Fig. 3... 

Reduction of amides to aldehydes was accomplished mainly by complex hydrides. Not every amide is suitable for reduction to aldehyde. Good yields were obtained only with some tertiary amides and lithium aluminum hydride, lithium triethoxyaluminohydride or sodium bis 2-methoxyethoxy)aluminum hydride. The nature of the substituents on nitrogen plays a key role. Amides derived from aromatic amines such as JV-methylaniline [1103] and especially pyrrole, indole and carbazole were found most suitable for the preparation of aldehydes. By adding 0.25 mol of lithium aluminum hydride in ether to 1 mol of the amide in ethereal solution cooled to —10° to —15°, 37-60% yields of benzaldehyde were obtained from the benzoyl derivatives of the above heterocycles [1104] and 68% yield from N-methylbenzanilide [1103]. Similarly 4,4,4-trifluorobutanol was prepared in 83% yield by reduction of N-(4,4,4-trifluorobutanoyl)carbazole in ether at —10° [1105]. 

Tertiary amides derived from pyrrole, indole and carbazole were hydro-genolyzed to alcohols and amines by refluxing in ether with a 75% excess (0.88 mol) of lithiwn aluminum hydride. Benzoyl derivatives of the above heterocycles afforded 80-92.5% yields of benzyl alcohol and 86-90% yields of the amines [7704]. 

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