Pyridine, tetrachloro-4-substituted

An extensive study of the amination of halopyridines has been carried out by den Hertog and co-workers.A comparison of their results with studies in inert solvents using primary and tertiary amines should permit some evaluation of the postulated factors. 2,4-Dichloropyridine in concentrated aqueous ammonia (180°, 5 hr) resulted in the formation of 4-amino- (60% yield) and 2-amino-chloropyridines (20% yield). Under similar conditions, only 4-substitution of 3,4,6-trichloro- and of 2,3,4,5-tetrabromo- and -tetrachloro-pyridines was observed. However, in these and the other polyhalo pyridines, the appreciable and unequal mutual activation by the halogen substituents needs to be emphasized. 

Since the nitrogen in pyridine is electron attracting it seemed reasonable to predict that the trihalopyridynes would also show the increased electrophilic character necessary to form adducts with aromatic hydrocarbons under similar conditions to those employed with the tetra-halogeno-benzynes. The availability of pentachloropyridine suggested to us and others that the reaction with w-butyl-lithium should lead to the formation of tetrachloro-4-pyridyl-lithium 82 84>. This has been achieved and adducts obtained, although this system is complicated by the ease with which pentachloropyridine undergoes nucleophilic substitution by tetrachloro-4-pyridyl lithium. Adducts of the type (45) have been isolated in modest yield both in the trichloro- and tribromo- 58) series. 

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