Base-Catalyzed-Halogen-Dance

Schlosser and co-workers have reported the shift of lithium in lithiated l-bromo-3-(tri-fluoromethyl)benzene 2,fa Quenching at — 100 C gives exclusively the product derived from 2, whereas after 2 hours at — 75 C, arene 2 is completely converted into less basic 3. A lithium-iodine exchange takes place in lithiated 2-chloro-3-iodo-6-(trifluoromethyl)pyridine 4, which at —85 C is totally converted into the less basic isomer 5.7 These rearrangements have been discussed in terms of a base-catalyzed halogen dance or halogen-shuffling mechanism. 

In studying the base-catalyzed halogen dance, Bunnett has made the following observations. When IV is treated with QH5NHK/NH3, it is isomerized to V. There is... 

Suggest a complete mechanism for the base-catalyzed halogen dance, and show how it accounts for all the facts. It may help to go at the problem in this way. First, start with V and the base, in the presence of VI, and show how IV can be formed. Show how, under the same conditions, V can be formed from IV. 

J. F. Bunnett, The Base Catalyzed Halogen Dance, and Other Reactions of Aryl Halides, Accounts Chem. Res., 5, 139 (1972). 

Rearrangement of polyhalobenzenes can also be catalyzed by very strong bases for example, 1,2,4-tribromobenzene is converted to 1,3,5-tribromobenzene by treatment with PhNHK." This reaction, which involves aryl carbanion intermediates (Sgl mechanism), has been called the halogen dance. 

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