Contrasting reactions of 2,6-dichloro-4-trichloromethylpyridine, 2,6-dichloro-3-trichloromethylpyridine and their N-oxides with nucleophiles

Various nucleophiles react with 2,6-dichloro-4-trichloromethylpyridine and 2-chloro-6-trichloromethylpyridine as expected, by displacement of ring chlorine atoms. But in the reactions of nucleophiles with 2,6-dichloro-3-trichloromethylpyridine and 2,6-difluoro-3-trifluoromethylpyridine, displacement of ring halogen atoms is accompanied or followed by multiple attack at the trihalogenomethyl group. The reactivity of the trihalogenomethyl groups is modified by N-oxidation. A mechanism for the peculiar reactivity of the trihalogenomethyl groups is proposed, involving loss of halide ion from the CX3 group assisted by lone pairs on substituents or negative charge in Meisenheimer intermediates. Both geometrical isomers of 1,2-dichloro- 1,2-bis(2,6-dichloro-3-pyridyl)ethene are described.