Dibrominated products

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. 

However, it is interesting that the same reaction in CHC13 afforded a mixture of dibromination products of both C=C bonds [212]. 

Similar oxidations also occurred with enamine substrates. In the reaction outlined in Scheme 8 [18], the formation of a dibrominated product arose because of the initial oxidation of an enamine... 

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). 

Substituents at the 3-position, if capable of stabilizing an adjacent positive charge by resonance and/or inductive effects, direct strongly to the adjacent (i.e. 2) position (equation 3). Stabilization of the allylic carbonium ion intermediate is obviously involved. This effect can be quite pronounced for R = Ph in equation (3) the ratio of 2- to 5-bromo derivatives for electrophilic bromination is ca. 660 (68JOC2902). Even more striking is the exclusive formation of the 2,2 -dibrominated product (19) from 3,3 -dithienyl (equation 4) (69JOC343). As expected, substituents not capable of stabilizing the cationic intermediate direct substitution to the 5-position. 

When 6-methyl-3-phenyl-2-pyrone was treated with bromine in trifluo-roacetic acid containing aluminium chloride catalyst, bromine entered the 5- and the para-positions giving an 80% yield of dibrominated product. When both 3- and 5-positions were occupied, as in 5-ethoxycarbonyl-6-... 

Three dibromination products are possible from a halogenation of 2-butanone. 

To explore the possibility of halogenation at the C-4 position, deprotonation of iV-TBDMS /3-sultam with equimolar amounts of butyllithium followed by reaction with bromine leads to a stable compound 144 formed by a fra/w-silylation followed by bromination of the intermediate silylated /3-sultam. When an excess of butyllithium is used, mixtures of mono- and dibrominated products 145 and 146 are isolated (Scheme 43) <2004HCA1574>. 

Dimethylbutane reacts with bromine in the presence of light to give a monobrominated product. Further reaction gives a good yield of a dibrominated product. Predict the structures of these products, and propose a mechanism for the formation of the monobrominated product. 

The free alcohol in 44 was protected as the silyl ether, and the less hindered C-3 acetate selectively hydrolysed and subjected to Jones oxidation (Scheme 20). The ketone was a-brominated to give 45 in 76 % yield, together with 14 % of the C-26 desilylated monobromide and 7 % of the 2,2-dibrominated product. 

The direct bromination of -alkoxylactones at the p position initially generates the a. -unsaturated lactones (eq 14) however, the required radical abstraction is not so facile and further bronii-nation of the Q , -unsaturated lactone proceeds competitively to afford the mono- and dibrominated products. NBS is also used for the oxidative aromatization of polycyclic compounds, including steroids and anthraquinone precursors (eq 15). ... 

Moreover, this catalytic oxidative bromination using cat. NHiVOs/AlBrs/Oa system can be applied to the a-bromination of ketones (Table 2.2) [188]. The hromina-tion reaction of p-keto esters such as ethyl benzoylacetate occurs to give mouo- or dibromination products depending on the amount of AlBr3 (entries 1 and 2). Substitution at a-position leads to the formation of the monobrominalion product (entry 3). 4 -Methoxyacetophenone is converted to the monobromide together with the dibromide. Bromination reaction of 4 -chloroacetophenone affords the monobromide as a major product (entry 4). The trihalo compound is obtained from... 

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