Halogen “shuffling

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. 

Mongin, F Tognini, A. Cottet, F. Schlosser, M. Halogen shuffling in pyridines site selective electrophilic substitutions of 2-chloro-6-(trifluoromethyl)pyridine. Tetrahedron Lett. 1998, 39(13), 1749-1752. 

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