Hydrogenation dibromine

Brick et al. have studied this bromination in more detail and showed that the extent of the bromination can be controlled by changing the ratio of the reagents. The first substitution was found to be in the para position but subsequent intramolecular rearrangements allowed the formation of 2-5-dibrominated species. Brick et al. also reported the functionalization of such species using Pd-catalyzed reactions such as Heck and Suzuki couplings to give fully substituted p-stilbenes, p-biphenyls, diarylamines, and methylcinnamates. Hydrogenation of... 

The mechanism of the orf/to-dibromination of phenol with NBS in the presence of amines is considered as follows. The hydrogen bonding between phenol and N-bromoamine which are generated from the reaction of NBS and amines (ref. 14), is the driving force, and causes the bromination at one o/t/io-position of phenol and regeneration of the amines. A catalytic amount of the amines is enough because of the regeneration of the amines. The repetition of the above process causes one more substitution at the other orf/io-position of 2-bromophenol. In the cases of 2-substituted phenols the orf/io-bromination can occur only once (Scheme 5). 

Chloroperoxidase derived from the microorganism Caldariomyces fumago combined with sodium chloride and hydrogen peroxide in buffered aqueous solution converts olefins to chlorohydrines374"376. Chloroperoxidase immobilized on a polymeric support catalyzes the bromination of barbituric acid in a continuous flow system377. Detailed mechanistic study on the mono- and dibromination of barbituric acid catalyzed by bromoperoxidase from Ascophyllum nodosum has been reported by Franssen and coworkers378. Pyrimidine bases are chlorinated and brominated by chloroperoxidase379. 

If a step of the sequence can be written as it proceeds at the molecular level, it is denoted as an elementary step (or an elementary reaction), and it represents an irreducible molecular event. Here, elementary steps will be called steps for short. The hydrogenation of dibromine is an example of a stoichiometrically simple reaction ... 

If this reaction would occur by H2 interacting directly with Br2 to yield two molecules of HBr, the step would be elementary. However, it does not proceed as written. It is known that the hydrogenation of dibromine takes place in a sequence of two steps involving hydrogen and bromine atoms that do not appear in the stoichiometry of the reaction but exist in the reacting system in very small concentrations as shown below (an initiator is necessary to start the reaction, for example, a photon Br2 + light — 2Br, and the reaction is terminated by Br 4- Br -f TB —Br2 where TB is a third body that is involved in the recombination process—see below for further examples) ... 

AcOH followed by ring closure with potassium hydroxide in methanol, which afforded the epoxide. The other coupling unit was made from the reaction of 3-(2,4,10-trioxatricyclo[3.3.1.13 7]dec-3-yl)propanal with tetrabromomethane and triphenylphosphine, affording the terminal dibrominated alkene which was subsequently transformed into the terminal alkyne with n-BuLi [40]. Coupling of the lithiated alkyne with the epoxide proceeded stereoselectively to the alcohol with no racemization. The resulting alkyne was then hydrogenated under... 

The double bond in 5-androsten-3jS-ol-17-one is first hydrogenated over paUadium/carbon. Androstan-3/ -ol-17-one is next oxidised with chromic acid, and the diketone selectively dibrominated in the A-ring. By a double elimination of HBr, androstadienedione is produced. Flash pyrolysis on quartz beadlets at 600 °C in tetralin or mineral oil gives then oestrone, which requires only hydrogenation to yield oestradiol. [29, 30] This remarkable achievement was a significant milestone on the long road to synthetic sex hormones. 

We will only consider the case of a closed system with constant volume and will start with a known concentration of dihydrogen and dibromine. Let us write the balance of the intermediates, which are the atoms of hydrogen, bromine and the hydrogen bromide formed ... 

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