Brominations monobromination

The novel bromination method also works well for carbazole (Fig. 9). Because there is little influence on the reactivity of the second benzene ring when the first is brominated monobromination is not particularly selective. However, dibromination is highly selective, as is tribromination. Tetrabromination is very slow, even with excess NBS. The behaviour of N-ethylcarbazole is similar, except that monobromination is slightly less selective (50% 3-bromo-lV-... 

Br bromine (general) Br, bromine) ), monobromine —Br, bromo bromine (general) Br+, bromine(l+) bromide (general) Br-, bromide(l—) bromide bromido (general) Br-, bromido(l—) bromido... 

Bromination. Monobromination of the highly reactive resorcinol dimethyl ether (la) and 2-methylresorcinol dimethyl ether (lb) can be effected in 80-90% yield with dioxane dibromide. ... 

Bromination. Monobromination of arenes is achieved with this reagent. 

Bromination of 2,4-dimethylselenazole with cold bromine gives an unstable monobrominated derivative initially (m.p. 168°C). which is easily converted to a product [m.p. 205°C (decomp.)] considered by Haginiwa to be 5-bromO-2,4-dimethylselenazole hydrobromide (19). 

Bromoresorcinol has been prepared by the monobromination of resorcinol monobenzoate and subsequent hydrolysis, from 2-bromo-5-aminophenol by the diazo reaction, by treating resorcinol with dichlorourea and potassium bromide, and by the bromination of 2,4-dihydroxy benzoic acid followed by decarboxylation. The above procedure is based particularly upon the observations of Rice. ... 

It has already been mentioned that monobromination of 5a-3-ketones gives 2a-bromo compounds (18). Further bromination occurs under acidic or distinctly basic conditions (the reaction is not satisfactory in weakly basic... 

Furthennore, only one monobromination product of benzene was ever obtained, which suggests that all the hydrogen atoms of benzene are equivalent. Substitution of one hydrogen by bromine gives the sane product as substitution of any of the other hydrogens. 

A hydroxyl group is a very powerful activating substituent, and electrophilic aromatic substitution in phenols occurs far- faster, and under milder conditions, than in benzene. The first entry in Table 24.4, for exfflnple, shows the monobromination of phenol in high yield at low temperature and in the absence of any catalyst. In this case, the reaction was carried out in the nonpolar- solvent 1,2-dichloroethane. In polar- solvents such as water it is difficult to limit the bromination of phenols to monosubstitution. In the following exfflnple, all three positions that are ortho or para to the hydroxyl undergo rapid substitution ... 

This method of bromination has been employed in the selective bromination (777) of the ketone (167). While direct bromination results in bromination not only in the position alpha to the ketone but also in the aromatic ring, bromination of the enamine (168) and subsequent hydrolysis gave only the monobrominated product (169). 

Monobromination at the /1-position can be achieved by using polymer-bound bromine (amberlyst Br2).26... 

Treatment with NCS in carbon tetrachloride converted the parent into the 2,3-dichloro derivative. The 2,3-dibromo compound was made similarly with NBS (74BSF2239), or with bromine in chloroform in the presence of sodium acetate (72CHE13). Monobromination is possible, but generally mixtures form with 2- and 3-bromo products in ratios of the order of 1 3 (72CHE13). It was possible to prepare 3-bromobenzo[h]sele-nophene by reaction of the 2,3-dibromo derivative with butyl lithium followed by hydrolysis. Four moles of bromine gave the 2,3,6-tribromo derivative from benzo[h]selenophene (74BSF2239). 

Monobromination of 4-methoxy-3-methyl-2,l-benzisoxazole occurred at C-7, but side-chain bromination was evident with excess reagent (74RTC139). 

The bromination behavior of isomeric thienothiophenes (53-55) has been studied in detail. Both the [2,3-b] (53) and the [3,2-h](54) isomers reacted with one equivalent of NBS in acetic acid to give an a-monobrominated product, with some evidence of 2,5-dibromo species also being formed. With 2 mol of NBS these latter products were formed in good yield three molar equivalents led to 2,3,5-tribromothieno-[2,3-h]-and -[3,2-6]-thiophenes (Scheme 28). The monobromo compounds can also be prepared from lithium derivatives quenched with bromine [76AHC( 19) 123]. Apparently the [2,3-c]isomer (55) also reacted initially in the 2-position. 

The dienone, which is prepared essentially as described by Benedikt7 and Calo,8 monobrominates a wide range of primary, secondary, and tertiary aromatic amines almost exclusively in the para-position. The procedure described is of general synthetic utility for the preparation of para-brominated aromatic and heteroaromatic amines in high yields and frequently in a high state of purity. The submitters have used this technique to para-brominate many compounds in quantities... 

Monobromination with bromine leads to exclusive 4-bromophenol, and dibromination with the same reagent gave predominant 2,4-dibromophenol. In the case of monobromination with NBS, the main product was 2-bromophenol, but no selectivity appeared in the bromination using two molar amounts of NBS. 

With 3-methylphenol (meto-cresol), around 8 % of dibromo products result when one equivalent of brominating agent is used, and this rises to 23 % of dibromo products on attempted monobromination of 3,5-dimethylphenol with 1 molar equivalent of resin. Nevertheless, the good yields of products obtained from the mono-substituted phenols tried demonstrate that this is a powerful new synthetic method for the organic chemist. 

Finally we mention in this section the non-catalytic selective bromination of aniline by the application of a zeolite pre-loaded with Bt2 as a slow release reagent (ref. 27). Aniline, dissolved in CCI4 was treated with Br2 adsorbed onto various zeolites and zeolite CaA was found to be most selective for monosubstitution (92%). The addition of organic bases improved the performance, probably due to scavenging of HBr. Also the toluidines could be monobrominated with this system with >95% selectivity. 

Potential hepatotoxic effects of various brominated benzenes, however, should be considered. Bromobenzene which is a monobrominated compound, is used in various experiments as a model hepatotoxic compound (refs. 9-11). 

These redistribution reactions are possible at atmospheric pressure under the action of MW irradiation is performed for a few minutes in the presence of the same catalysts [57]. These reactions with the less volatile germanium tetrabromide (44b) (b.p. 184 °C) have also been performed by use of the GS/MW process, without added catalyst (Tab. 7.4, entries 1 and 3) [15, 16]. In this instance, despite the use of weaker incident power, the temperature reached 420 °C, very much higher than that obtained under the action of MW irradiation of a reaction mixture containing AlBr3 (200 °C to 250 °C) (Tab. 7.4, entries 2 and 4). The presence of this catalyst considerably favors redistribution towards the dibrominated products (46b) (84% for R = nBu, 85% for R = Ph) relative to the monobrominated compounds (46a), which are the major products of the GS/MW process (78% and 43% respectively). The tri-brominated products (46c), the most difficult to prepare, have been obtained with a rather high selectivity (73 to 80%) by use of the catalytic process under the action of MW [57]. In this reaction, therefore, the GS/MW process seems less effective than the MW-assisted and AlX3-catalyzed process. 

This procedure is described by Lock 2 a modification using a small amount of phenol has been published.3 The patent literature discloses the use of a tertiary amine, such as pyridine, and its combination with other solvents for the monobromination of pyrene with elemental bromine.4 Brominating agents, such as N-bromosuccinimide5 and N-bromohydantoins,6 have also been used. 

Highly functionalized imidazo[2,l-A][l,3,4]thiadiazoles 166 react with bromine in acetic acid to give the corresponding monobrominated derivatives 167 (Equation 6) <2003IJC(B)1463, 2001IJC(B)303, 2003IJH33, 2002IJH125>. 

When bromination of the enolizable ketone is conducted in glycol, simultaneous monobromination and acetalization is observed [2]. 1,5-Dicarbonyl compounds react with benzyltrimethylammonium dichloroiodate to yield initially a monochloro derivative, which undergoes ring closure to produce a 2-substituted 5-acylfuran [26],... 

Bromination of (1) leads to the tetrabromide which, when treated with base, forms 4,7-dibromo-2,1,3-benzothiadiazole <70RCR923>. It was subsequently demonstrated that the 4,6-dibromo (20%) and a trace of the 4,5-dibromo derivatives are also produced in this reaction <70JHC629>. In the presence of iron, bromination of a melt of (1) yields almost exclusively the 4,7-dibromo derivative even when equimolar quantities of bromine are used. As in the chlorination of (1), the process under these conditions cannot be limited to monosubstitution <70RCR923>. Monobromination and subsequent dibromination can be achieved, however, when the reaction is carried out by dropwise addition of bromine to (1) in 47% hydrobromic acid at 130°C <70JHC629>. 

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