Ortho-dibromination

Addition of every primary amine was effective for obtaining the ortho-dibromide selectively. In the absence of amines, the yield of 2,6-dibromophenol was much lower than that of 2,4,6-tribromophenol. The selective ortho-dibromination of phenols was also observed when secondary amines were added. 

Sulfonamides in mixtures have been determined by titration with 0.02 M chloramine-T after bromination. The ortho dibromination is prevented by acetylation of the aromatic amino group but the reaction of side-chain substituents remains unaffected. The relative standard deviations are about 1%. Interferences even in small amounts, from barbituric acid, isoniazid, ascorbic acid, methionine, thymol and penicillins are observed (33). 

The X-ray structure of the dibromine complex with toluene (measured at 123 K) is more complicated, and shows multiple crystallographically independent donor/acceptor moieties [68]. Most important, however, is the fact that in all cases the acceptor shows an over-the-rim location that is similar to that in the benzene complex. In both systems, the acceptor is shifted by 1.4 A from the main symmetry axis, the shortest Br C distances of 3.1 A being significantly less than the sum of the van der Waals radii of 3.55 A [20]. Furthermore, the calculated hapticity in the benzene/Br2 complex (x] = 1.52) is midway between the over-atom (rj = 1.0) and over-bond (rj = 2.0) coordination. In the toluene complex, the latter varies from rj = 1.70 to 1.86 (in four non-equivalent coordination modes) and thus lies closer to the over-bond coordination model. Moreover, the over-bond bromine is remarkably shifted toward the ortho- and para-carbons that correspond to the positions of highest electron density (and lead to the transition states for electrophilic aromatic bromination [12]). Such an experimental location of bromine is in good agreement with the results of high level theoretical... 

Also surprisingly, the 6-(3 -hydroxyphenyl) and l,4-dimethyl-6-(3 -hydro-xyphenyl) analogs give not only 6-(4 -bromo-3 -hydroxyphenyl) derivatives but also dibrominated products in which a bromine atom has been substituted at a position ortho (6 -) to the dihydrodiazepinium ring (86LA1380). 

In 2003, Kulkarni and coworkers presented a method for the -selective oxybromi-nation of a variety of substituted phenols employing a novel heterogeneous catalytic system, the CrZSM-5 as catalyst, H2O2 as oxidant and KBr as bromine source ". Next to CrZSM-5 also other zeolites have been tested as catalysts, but although MoZSM-5 showed the highest conversion after 5 hours (89%), para-selectivity was lower (para 36% ortho 31% dibromination 22%) than observed with the CrZSM-5 material (83%... 

Examjdes.—Aniline is rapidly tribrominated at room temperature, the end point is very sharp, and this estimation of aniline is preferable to the nitrous acid method. At ordinary temperature ortho- and para-toluidines are dibrominated and meta-toluidine is tribrominated. Dimethylaniline is monobrominated at 0°—5°, dibrominated at 40°—50° and tribrominated at 60°—70°, an interesting example of the effect of temperature. Sulphanilic acid at 60°—70° yields tribromoaniline, and at this temperature p-nitroaniline reacts quantitatively. Phenol at about 22° yields tribromophenol. For reference to the estimation of m-diamines, m-dihydroxy compounds, R salt, cresols, p nitroaniline, diphenylamine, p-nitrophenol, and other notes, see J. S. C. I., 41, 161. 

Electrophilic substitution reactions of 5-hydroxybenzo[6]thiophene have been investigated in some detail. The 4-position is the most reactive toward nitration,152 nitrosation,497 bromination,422 and formylation (Duff procedure).338 Dibromination in the presence of acetate ion affords 4,6-dibromo-5-hydroxybenzo[6]thiophene,421,422, 497 and not the 3,4-dibromo derivative, as previously believed.542 Dichlorination similarly affords the 4,6-dichloro derivative,421 and not 4,4-dich loro-4,5-dihydrobenzo[6]thiophen-5-one, as reported earlier by Fries d al.542 An interesting comparison can be made between the behavior of 5-hydroxy benzo[6]thiophene and 2-naphthol in electrophilic substitution reactions. It is clear that both positions ortho to the hydroxyl group in 5-hydroxybenzo[6]thiophene are attacked, in contrast to 2-naphthol, where only the 1-position is attacked even in the presence of an excess of the reagent. Disubstitu-... 

The methyl protons were affected in several ways by bromine. The methyl peaks were shifted upfield (0.1 ppm) by shielding bromines of adjacent dibrominated rings. In addition, a bromine in the same ring shifted an ortho methyl downfield (0.2 to 0.3 ppm) and shifted a para methyl upfield (<0.1 ppm). This latter effect accounted for the two methyl peaks between 2.0 and 2.1 ppm in polymer XI. 

There are several stereoisomers in disubstituted [2.2]paracyclophane, such as ortho-, meta-, para-, pstudo-ortho-, pseudo-m fa-, pseudo-para-, and pseudo-g mma/-disubstituted [2.2]paracyclophanes. Iron-catalyzed dibromination of commercially available [2.2]paracyclophane yields the mixtures of the dibromo[2.2]paracyclo-phane isomers, and pseudo-para-dibromo[2.2]paracyclophane 9 can be obtained by recrystallization as it has the poorest solubility in common organic solvents [76]. Thermal isomerization of 9 in triglyme afforded the psowdo-ortho isomer 38, as shown in Scheme 10. This transformation is an equilibrium reaction when the reaction was allowed to cool to room temperature, pseudo-para isomer 9 was precipitated as a result of its low solubility and was readily separated by simple filtration to obtain pseudo-arf/za isomer 38 in moderate isolated yield [76]. V ondo-ortho-dibromo[2.2]paracyclophane 38 was converted into pseudo-arf/ a-diethynyl[2.2] paracyclophane 40 via pseudo-arf/ a-diformyl[2.2]paracyclophane 39 (Scheme 10) [77]. Sonogashira-Hagihara coupling polymerization of 40 with 12 afforded the corresponding jc-stacked polymer 41 in 23 % isolated yield with the of 3,800 [78]. 

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