Regiochemically Flexible Substitutions of Di-, Tri-, and Tetrahalopyridines:  The Trialkylsilyl Trick
                                
                                    
                                        作者:Manfred Schlosser、Carla Bobbio、Thierry Rausis                                    
                                    
                                        DOI:10.1021/jo047962z
                                    
                                    
                                        日期:2005.4.1
                                    
                                    2,4-Difluoropyridine, 2,4-dichloropyridine, 2,4,6-trifluoropyridine, 2,4,6-trichloropyridine and 2,3,4,6-tetrafluoropyridine react with standard nucleophiles exclusively at the 4-position under halogen displacement. However, the regioselectivity can be completely reversed if a trialkylsilyl group is introduced in the 5-position of the 2,4-dihalopyridines or in the 3-position of the 2,4,6-trihalopyridines or 2,3,4,6-tetrahalopyridine. Then only the halogen most remote from the bulky silyl unit (at the 2-position in the case of the 2,4-halopyridines, at the 6-position with the other substrates) gets involved in the exchange process. After removal of the silyl protective group the nucleophile is invariably found to occupy the nitrogen-neighboring position.