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

Abstract

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.