Polyhalogenoaromatic compounds. Part XIX. Metal–halogen exchange reactions of n-butyl-lithium with tetrabromo-4-pyridyl and tetrachloro-2-pyridyl derivatives

The reaction of n-butyl-lithium with tetrabromo-4-methoxypyridine, tetrabromo-4-dimethylaminopyridine, and tetrabromo-4-piperidinopyridine led to the corresponding tribromo-3-pyridyl-lithium derivatives. When heated in the presence of furan, tribromo-4-methoxy-3-pyridyl-lithium gave an adduct of 5,6-dibromo-4-methoxy-2-pyridine. The reaction of n-butyl-lithium with tetrachloro-6-methoxypyridine, tetrachloro-6-dimethylamino-pyridine, tetrachloro-6-pyrrolidinopyridine and tetrachloro-6-piperidinopyridine led to the corresponding trichloro-4-pyridyl-lithium derivatives. When heated in the presence of furan, each of these lithium compounds gave only one of the two possible pyridyne adducts; evidence is presented that the 2-substituted 3-pyridyne was involved in each case. Factors governing the position of metal–halogen exchange in polyhalogeno-benzenes and -pyridines are discussed.