Rearrangement halogen migration

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. 

Diiodoanisole is also formed in this reaction, which has been taken178 as evidence that the reaction is intermolecular. Halogen migration during sulphonation (Jacobsen rearrangement) is also well-known163. 

The first step of the reaction is a rearrangement, with the halogen migrating from the nitrogen to the 4 or 5 position of the alkyl group and the second step, the ring closure takesplace. 

Since then numerous thermal rearrangements of dihalocyclopropanes have been observed and the isomerization has been reviewed several times [11, 186]. It therefore suffices to discuss some special features of these reactions here. The thermal isomerization of dihalocyclopropanes is often accompanied by a halogen migration and then leads to 2,3-dihalogenated propenes. 

Bromine trifluoride is used to selectively substitute fluorine for bromine in brominated alkanes and esters. The reactions are carried out by gradual addition of bromine trifluoride to a solution of the substrate in CFC-113 or CFC-112 at 10-20nC. The bromine-fluorine exchange in mono-bromohaloalkanes is nonstereoselective and accompanied, in some eases, by skeletal rearrangements, hydride shifts, and halogen migrations. All three fluorine atoms in bromine trifluoride are involved in the fluorination reaction. Chlorine atoms in the substrate molecules remain intact.109... 

Chloroketones are not susceptible to halogen migration this is emphasised by the specific rearrangement [3s] of 2"bromo-2"Chloro 5a"Cholestan-3 One (4) with HBr (but not with HCl) to give 2a"Chloro-4a-bromo-5a-cholestan"3"One (5),... 

In radical rearrangements, the migrating groups are those that can accommodate electrons in a tt system (vinyl, aryl, carbonyl) or atoms that can expand their valence shell, i.e., all halogens but fluorine. Hydrogen and alkyl do not migrate to... 

Silver perchlorate N C-Halogen migration with skeletal rearrangement... 

Thts chapter includes migration of halogen atoms, catalyst assisted reatrange ments, and thermal and photochemical rearrangements... 

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. 

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