Rerouting Nucleophilic Substitution from the 4-Position to the 2- or 6-Position of 2,4-Dihalopyridines and 2,4,6-Trihalopyridines: The Solution to a Long-Standing Problem

2,4-Difluoro-, 2,4,6-trifluoro-, and 2,3,4,6-tetra(fluoro)pyridine undergo nucleophilic substitution preferentially if not exclusively at the 4-position. However, after the introduction of a trialkylsilyl group at C-3 or C-5, the halogen at the 6-(2-)position is displaced selectively. Bulky trialkylsilyl groups were found to suppress the nucleofugal mobility of neighboring halogen atoms. This synthetically valuable regiocontrol can also be realized with other halopyridines such as 2,4-dichloro- and 2,4,6-tri(chloro)pyridine. [on SciFinder (R)]