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-tetrafluoropyridine 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. This synthetically valuable regiocontrol can also be realized with other halopyridines such as 2, 4-dichloro-and 2, 4, 6-trichloropyridine.
