Halogen atoms, migration

Examples exist of rearrangements in which a halogen atom migrates from annular nitrogen to carbon, or from one ring carbon to another ( halogen-dance or transmetalation). Such processes occasionally have preparative importance, and they will be covered in the appropriate sections. 

In 1968, Tobey and Law reported on the intriguing reaction of furan, substituted furans, and cyclopentadiene with both tetrachloro- and tetrabromocyclo-propene (TBCP). The two unsaturated systems reacted smoothly to directly produce cycloheptanoid systems, the products of a formal [4-i- 3]-cycloaddition reaction. The authors proposed in their early manuscripts that the tetrahalocy-clopropenes underwent an initial thermal Diels-Alder cycloaddition to produce the cyclopropyl norbornene derivatives la and lb (Figure 2). However, these workers were unable to isolate or characterize the primary cycloadducts and instead observed a product which had spontaneously rearranged by way of a halogen atom migration to yield the bicyclo[3.2.1]octadiene nucleus 2a and 2b. 

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

In a slightly less convenient procedure, but one which has general versatility, carbonylation of aryl (or vinyl) palladium compounds produces aryl, heteroaryl, and vinyl carboxylic acids. As with the other procedures, immediate upon its formation, the carboxylate anion migrates to the aqueous phase. Consequently, haloaromatic acids can be obtained from dihaloarenes, without further reaction of the second halogen atom, e.g. 1,4-dibromobenzene has been carbonylated (90% conversion) to yield 4-bromobenzoic acid with a selectivity for the monocarbonylation product of 95%. Additionally, the process is economically attractive, as the organic phase containing the catalyst can be cycled with virtually no loss of activity and ca. 4000 moles of acid can be produced for each mole of the palladium complex used [4],... 

With the aid of density functional theory, the ZnCl2 acceleration of the Simmons-Smith reaction of ethylene and allyl alcohol has been investigated. A pathway involving direct Lewis acid acceleration of the leaving halogen atom (327) was found to be a more facile process than the more popular pathway involving 1,2-chlorine migration (328). 

The formation of these compounds is unusual in that it involves 1,2-migration of the halogen atom. 

Reported a-elimination studies of vinyliodonium salts are limited to the substrates shown in Table 10. While aryl migrations might be expected, a-elimination reactions of (/2-arylvinyl)iodonium salts have not been described. Thus far, migrations of /2-hydrogen atoms (equations 230,233 and 234)128, /2-halogen atoms (Cl, Br)104, /2-ArS(0) groups (n = 0,1,2)32 and the methyl group128 have been reported. 

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