Bromine migration

Bromination can be conveniently effected by transfer of bromine from one nucleus to another. As the Friedel-Crafts isomerization of bromoaromatic compounds generally takes place through an intermolecular mechanism, the migrating bromine atom serves as a source of positive bromine, thus effecting ring brominations (161,162). 2,4,6-Tribromophenol, for example, has been prepared by bromination of phenol with dibromobenzene. 

Introduction of a 3-bromosubstituent onto thiophene is accompHshed by initial tribromination, followed by reduction of the a-bromines by treatment with zinc/acetic acid, thereby utilizing only one of three bromines introduced. The so-called halogen dance sequence of reactions, whereby bromothiophenes are treated with base, causing proton abstraction and rearrangement of bromine to the produce the most-stable anion, has also been used to introduce a bromine atom at position 3. The formation of 3-bromotbiopbene [872-31-1] from this sequence of reactions (17) is an efficient use of bromine. Vapor-phase techniques have also been proposed to achieve this halogen migration (18), but with less specificity. Table 3 summarizes properties of some brominated thiophenes. 

In 57 a bromine migration possibly competes with the generation of 4-ethoxy-2,3-pyridyne (58), induced by the abstraction of a hydrogen ion from C-2 of 57 (cf. the isomerization of 1,2,4-tribromobenzene to... 

Among the halogens, fluorine does not undergo rearrangement, and what evidence there is suggests that the rearrangement of chlorine is an intramolecular process (1,2 shift) whereas that of bromine appears to take place by both inter-and intramolecular routes. Less is known about iodine migration. 

RELATIVE RATE COEFFICIENTS FOR BROMINE MIGRATION IN THE BROMOTOLUENES180... 

We recognize redox reactions by noting whether electrons have migrated from one species to another. The loss or gain of electrons is easy to identify for monatomic ions, because we can monitor the charges of the species. Thus, when Br ions are converted into bromine atoms (which go on to form Br2 molecules), we know that each Br ion must have lost an electron and hence that it has been oxidized. When 02 forms oxide ions, 02-, we know that each oxygen atom must have gained two electrons and therefore that it has been reduced. The difficulty arises when the transfer of electrons is accompanied by the transfer of atoms. For example, is chlorine gas, Cl2, oxidized or reduced when it is converted into hypochlorite ions, CIO" ... 

Addition of bromine to 1 in chloroform solution at 10°C led in high yield to the formation of the exo-5-a/ih -7-dibromide 2. No other products were isolated. The formation of this rearranged product can be explained in terms of Wagner-Meerwein rearrangement where migration of the aryl group is involved (eqn. 1). 

However, most of the debate over the possibility of 1,3 migrations has concerned not methyl or bromine but 1,3 hydride shifts.There is no doubt that apparent 1,3 hydride shifts take place (many instances have been found), but the question is whether they are truly direct hydride shifts or whether they occur by another... 

Hydride ion can migrate. In example c, it was hydride that shifted, not bromine ... 

Nitronium fluoborate also reacts with silylallenes, as indicated in Eq. 13.40(38], Attack of the electrophile at the allene carbon atom distal to the silicon atom, 1,2-silicon migration and ring closure lead to a silyloxazole. Exposure of this material to bromine in a second step gives 124 in 72% overall yield from 123. This annula-tion is best performed with tert-butyldimethylsilylallenes. With trimethylsilyl compounds, protodesilylation is a competing side reaction. 

The electrophile E+ attacks the unhindered side of the still unsubstituted second aromatic ring. A proton (deuteron) is transferred from this ring to the second, originally substituted ring, from which it leaves the molecule. Thus, the electrophile enters, and the proton (deuteron) leaves the [2.2]paracyclophane system by the least hindered paths. Some migration of deuterium could be detected in the bromination of 4-methyl[2.2]paracyclophane (79). The proposed mechanism is supported by the kinetic isotope effects ( h/ d) found for bromination of p-protio and p-deuterio-4-methyl[2.2]paracyclophanes in various solvents these isotope effects demonstrate that proton loss from the a complex is the slowest step. 

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