Aromatization with chloranil

After aromatization with chloranil [31], relatively short PPP sequences of about 7 units were obtained. The PSt-PPP copolymer remained soluble in THE, and the PPP sequences could be n-doped by potassium [35]. 

Some tetrahydro azoles can be aromatized, but this is more difficult than in the corresponding dihydro series. Thus the conversion of pyrazolidines into pyrazoles is accomplished with chloranil. Imidazolidines are aromatized with great difficulty. 

In the synthetic scheme, 3-amino-9-ethylcarbazole 138 (2 mol) is condensed with chloranil 137 (1 mol) to form the intermediate 139. This intermediate is then converted into the dioxazine pigment 136 by oxidative cyclisation at around 180 °C in an aromatic solvent and in the presence of a catalyst such as aluminium(m) chloride or benzenesulfonyl... 

Both 3-amino-N-ethylcarbazole and l,4-diethoxy-2-amino-5-benzoylamino-benzene are suitable aromatic amines. Chloranil may also be replaced by 2,5-di-chloro-3,6-bisacetylamino-l,4-benzoquinone to react with appropriately substituted o-alkoxyanilines. 

More recently, an environmentally benign method using air as oxidant has been developed for the oxidative cyclization of arylamine-substituted tricarbonyl-iron-cyclohexadiene complexes to carbazoles (Scheme 19). Reaction of methyl 4-aminosalicylate 45 with the complex salt 6a affords the iron complex 46, which on oxidation in acidic medium by air provides the tricarbonyliron-complexed 4a,9a-dihydrocarbazole 47. Aromatization with concomitant demetalation by treatment of the crude product with p-chloranil leads to mukonidine 48 [88]. The spectral data of this compound are in agreement with those reported by Wu[22j. 

Under forcing conditions, Grignard reagents add to certain aromatic compounds. For example, the addition of PhMgBr to naphthalene takes place at 200 °C (Scheme 83) °. After treatment of the crude product with chloranil (for dehydrogenation), 1-phenylnaph-thalene is obtained in 34% yield. 

Aromatizations play a particularly prominent role in the synthesis of carbazoles since both the Fischer cyclization (Borsche s method) of cyclohexanones (Section 3.06.3.4.2) and the cycloaddition of 2-vinylindole (Section 3.06.6.1) yield tetrahydrocarbazoles. Both catalytic dehydrogenation over palladium/carbon catalyst and dehydrogenation with chloranil have been employed to effect aromatization (80JA4772,79JOC4402). 

Cycloaddition of ylide 90, formed from 3-oxopyrido[2,l-6][l,3]thiazin-5-ium iodide (89) on the action of triethylamine, with acrylonitrile or methyl acrylate gave [l,3]thiazino[4,3,2-cd]indolizines (91), which were subsequently aromatized by treatment with chloranil (80CL947). 

Reactions on or close to solid surfaces maybe inhibited by deposition ofinsoluble or poorly soluble products on the reactant surface, a phenomenon referred to as overgrowth. Examples include the reaction of amines with chloranil [7], the diazotisation of poorly soluble aromatic amines in which the product diazonium salt is also insoluble and halogen exchange reactions of chloroaromatic compounds using potassium fluoride in dipolar aprotic solvents where the potassium chloride product may coat the potassium fluoride [8]. 

In Stark contrast, a biomimetic approach [76] that was reported shortly after Weinreb s classical synthesis gave access to 70 in three steps (Scheme 9.8) from the commercially available 4,5-dibromo-2-formylpyrrole 80 and 2-aminohistidine 81. Protection of the pyrrole 80 followed by Lewis acid-catalyzed Pictet-Spengler condensation with 81 gave tetrahydroageladine A 83. Treatment of 83 with chloranil in refluxing chloroform effected aromatization of the imidazopyridine ring system... 

Thienylpyridazines were prepared from 2- or 3-thienyllithium and py-ridazine. In diethyl ether, the thienyl ring is attached at position 3 to give first the corresponding 2,3-dihydro derivatives accompanied by a small amount of 1,4-dihydro isomers, whereas in tetrahydrofuran and at low temperature, position 4 of the pyridazine ring is attacked. Aromatization can be achieved with various oxidants, preferentially with chloranil [81JCR(S)104 82CJC2668],... 

The elements sulfur and selenium, which combine with the hydrogen evolved to give, respectively, H2S and H2Se. Little is known about this mechanism either. Quinones, which become reduced to the corresponding hydroquinones. Two important quinones often used for aromatizations are chloranil (2,3,5,6-tetrachloro-l,4-benzoquinone) and DDQ (2,3-dichloro-5,6-dicyano-l,4-ben-zoquinone). The latter is more reactive and can be used in cases where the substrate is difficult to dehydrogenate. It is likely that the mechanism involves a transfer of hydride to the quinone oxygen, followed by the transfer of a proton to the phenolate ion ... 

Several dehydrogenations forming double bonds conjugated with aromatic rings are achieved by heating with quinones. Acenaphthene gives a 75% yield of acenaphthylene when refluxed with chloranil for 16 h in xylene at 140 °C [967], and p,p -dimethoxybibenzyl affords an 83-85% yield of p,p -dimethoxystilbene when refluxed with 2,3-dichloro-5,6-dicy-ano-p-benzoquinone in dioxane at 105 °C [968]. 

Some aromatizations are accomplished with agents other than those previously mentioned. Thus tetralin is converted quantitatively into naphthalene by refluxing with 2,3-dix hloro-5,6-dicyano-p-benzoquinone [970], and 1,2,3,4-tetrahydrocarbazoles are transformed into carbazoles in 50-95% yields by refluxing with chloranil in xylene [969], Anhydrous aluminum chloride in refluxing carbon disulfide is used in the preparation of coronene [408]. 

Many years after the classical investigations of the production of minute amounts of aromatic hydrocarbons by selenium dehydrogenation of steroids, Dannenberg found (1956-63) that steroids can be dehydrogenated with chloranil in refluxing xylene or anisole to give optically active aromatized hydrocarbons in yields up to 50%. Cholesterol affords 3-methyl-3 -isooctyl-A -l,2-cyclopentadienophenanthrene (13) and its methyl derivative (14). 

The highest conductivities thus far observed in organic molecules are found among the donor-acceptor complexes. The activation energies for conduction are small, on the order of 0.1 eV ( 2 kcal) or less. Examples are the complexes of aromatic hydrocarbons with iodine and complexes of amines with chloranil or related quinones. The best conductors have a high free-radical content and are usually not of simple stoichiometry. The complex salts of TCNQ ° (I) are the best of the known organic conductors. The... 

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