7-Bromo pyridine nucleophilic displacement reactions

Intramolecular nucleophilic displacement of the bromo group by an azine-nitrogen occurs in the cyclization of A-2-quinaldyl-2-bromo-pyridinium bromide (248) to give the naphthoimidazopyridinium ring system. The reaction of 2-bromopyridine and pyridine 1-oxide yields l-(2-pyridoxy)pyridinium bromide (249) which readily undergoes an intramolecular nucleophilic substitution in which departure of hydrogen as a proton presumably facilitates the formation of 250 by loss of the JV-oxypyridyl moiety. 

Nucleophilic halogenations tend to favor the pyridine moiety. The Meisenheimer reaction of thieno[3,2-6]pyridine N-oxide (125) gave only a 24% yield of a 1.4 1 mixture of the 5- and 7-chloro derivatives. Nucleophilic displacement of a 7-nitro group provided a more satisfactory route to the 7-chloro (73%) and 7-bromo (39%) derivatives (85JHC1249). 

Direct lithiation of l,2,3-triazolo[l,5-a]pyridines occurs with ease, at C-7, subsequent reaction with electrophiles being unexceptional, for example conversion into the 7-bromo derivative then allows nucleophiles to be introduced via displacement of bromide, thus providing, overall, a route to 2,6-disubstituted pyridines. ... 

In some, apparently straightforward, displacements, more detailed mechanistic study reveals the operation of alternative mechanisms. For example the reaction of either 3- or 4-bromopyridine with secondary amines in the presence of sodamide/ sodium t-butoxide, produces the same mixture of 3- and 4-dialkylaminopyridines this proceeds via an elimination process (Sn(EA) - Substitution Nucleophilic Elimination Addition) and the intermediacy of 3,4-didehydropyridine (3,4-pyr-idyne)." That no 2-aminated pyridine is produced shows a greater difficulty in generating 2,3-pyridyne, it can however be formed by reaction of 3-bromo-2-chloropyridines with butyllithium" or via the reaction of 3-trimethylsilyl-2-trifluoromethanesulfonyloxypyridine with fluoride." ... 

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