Pyridyne

Pyridyne in chemistry is the pyridine analogue of benzyne. Pyridynes are the class of reactive intermediates derived from pyridine. Two isomers exist, the 2,3-pyridine (2,3-didehydropyridine) and the 3,4-pyridyne (3,4-didehydropyridine). The reaction of 3-bromo-4-chloropyridine with furan and lithium amalgam gives 1,4-epoxy-dihydroquinoline through the 2,3-pyridyne intermediate. The reaction of 4-bromopyridine with sodium in liquid ammonia gives both 3-aminopyridine and 4-aminopyridine through the 3,4-pyridyne intermediate and an E1cB-elimination reaction.

History
Pyridynes were first postulated by Levine and Leake in 1955. In 1969 Zoltewicz and Nisi trapped 3,4-pyridyne in a reaction of 3-bromopyridine with methylmercaptan and sodium amide in ammonia. The methylthio and amino pyridines were found to be formed in the same ratio.


 * Pyridyne intermediate.svg

In 1972 Kramer and Berry inferred the formation of 3,4-pyridyne in gas-phase photolysis of pyridine-3-diazonium-4-carboxylate via time-of-flight mass spectrometry. The dimer compound diazabiphenylene was detected. In 1988 Nam and Leroy reported the matrix isolation (13K, Ar) of 3,4-pyridyne by photolysis of 3,4-pyridinedicarboxylic anhydride with the IR-spectrum revealing an acetylenic bond in the same way as ortho-benzyne.

Scope
Strategies involving pyridynes have been employed in the total syntheses of ellipticine and (S)-Macrostomine.