Structures of the Intermediates Formed in the Ring-Opening Reaction of 2-Chloro-3-nitropyridine

The reaction of 2-chloro-3-nitropyridine with two equivalents
of hydroxide ion was studied by NMR and X-ray crystallography. On the basis of NMR coupling constants, the
originally formed ring-opened intermediate is the pseudo-cis form, as predicted by the SN(ANRORC) mechanism.
However, the first intermediate is unstable and isomerizes
to a second intermediate, which was isolated. The pseudo-trans geometry of the second intermediate [3-pentenenitrile,
2-nitro-5-oxo, ion(−1), sodium] explains why additional base
does not lead to the ring-closing reaction as observed with
2-chloro-5-nitropyridine.