Pyridynes halogenopyridines

Whereas only one dehydrobenzene, benzyne, has been detected, two pyridynes are possible. Thus, the scheme we can write ab initio for the action of a nucleophile on the isomeric monosubstituted derivatives of pyridine involving 2,3- (26) and/or 3,4-pyridyne (31) is more complicated than that for the analogous reaction of the corresponding benzene derivative. The validity of this scheme can be checked using data available in the hterature on reactions of halogenopyridines with potassium amide and hthium piperidide involving pyridynes. 

Those reactions of halogenopyridines with potassium amide and lithium piperidide which proceed via 3,4-pyridyne form the 3- and 4-substituted pyridine derivatives in ratios of 1 2 and 1 1, respectively (see Section II, A, 1). It appears that the ring nitrogen atom has an orienting effect on these additions, but the quantitative divergence of the addition of ammonia and piperidine is not understood at present. 

Why, then, was 2,3-pyridyne not found to play a role in the amination of 2- and 3-halogenopyridines ... 

While awaiting the results of tracer experiments, the present authors prefer to desist from assuming that the route from 24 to 25 via the addition product shown is the only pathway followed in the amination of 2-halogenopyridines. This is the more so since it seems probable that in the experiments described below derivatives of 2,3-pyridyne occur as intermediates. 

Pyridynes are also commonly generated from orf/io-dihalogenopyridines by lithium exchange processes, e.g. Scheme 138, and processes not involving halogenopyridines, e.g. thermal decomposition of pyridinediazonium carboxylates. 

Those reactions of halogenopyridines with potassium amide and lithium piperidide which proceed via 3,4-pyridyne form the 3- and... 

Pyridynes also arise in the amination of halogenopyridine l-oxides ... 

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