Halogenopyridines halogen exchange

Halogenopyridines can undergo metal-halogen exchange when treated with butyllithium. The lithium derivatives then behave in a similar manner to arylithiums and Grignard reagents and react with electrophiles such as aldehydes, ketones and nitriles (Scheme 2.17). Thus, aldehydes and ketones form alcohols, and nitriles yield A -lithioimines, which on hydrolysis are converted into pyridyl ketones. 

Nucleophilic halogen exchange has been observed frequently with 2-and 4-halogenopyridines. The commonest reaction of this type is that of a 2- or 4-chloropyridine with hydriodic acid 819-20,... 

Some cases of halogen-halogen exchange accompanying quaternization of halogenopyridines have been mentioned already (p. 180). 

It should be noted that lithiopyridines alternatively and frequently are prepared by halogen-metal exchange reactions of halogenopyridines with organolithiums, e.g. ... 

Alternatively, lithiopyridines can be obtained by halogen-metal exchange from halogenopyridines and organoUthium compounds, as shown for 37 ... 

Nuclear alkylation by reaction of lithium alkyls with halogenopyridines is of consequence only as a side-reaction which diminishes the efficiency of halogen-metal exchange to produce pyridyl lithiums . This situation differs considerably from that found with some sodium reagents (see below). 

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