Regioselective synthesis of symmetrical pyridyl selenium compounds by bromine-magnesium exchange of bromopyridines using isopropyl magnesium chloride: X-ray crystal structure of 2,2',5,5'-tetrabromo-3,3'-dipyridyldiselenide

2,2'-Dipyridyl-3,3'-dipyridyl,5,5'-dipyridyl-diselenides have been synthesized by a convenient method employing non-cryogenic conditions. Various bromopyridines (2-Bromopyridine, 2,5-dibromopyridines and 2,3,5-Tribromopyridines) undergo selective monobromine-magnesium exchange to yield the corresponding pyridyl magnesium chlorides at room temperature upon treatment with (PrMgCl)-Pr-i. The resulting pyridyl magnesium chloride is quenched with elemental selenium, which upon further oxidation affords the above diselenides in good yields. The compounds prepared using this methodology have been characterized by elemental analysis, IR, NMR (H-1, C-13, Se-77) and mass spectral analysis. The molecular structure of 2,2,5,5'-Tetrabromo-3,3'-dipyridyldiselenide has been established by single-crystal X-ray diffraction analysis. It exists as a dimeric form due to the non-bonding interactions between the selenium of one pyridine moiety and the hydrogen of the other. Copyright (C) 2004 John Wiley Sons, Ltd.