General Papers ARKIVOC 2001 (i) 242-268
2.5 O- Alkylation. 2.6 Nucleophilic substitution of 3-bromo-4-nitropyridine N-oxide. 2.7 Cycloaddition to dipolar N-O 3 Conclusion Introduction
The chemistry and applications of N-oxides have recently received much attention due to their usefulness as synthetic intermediates and their biological importance1. Heterocyclic N-oxides are also useful as protecting groups, auxiliary agents, oxidants1, ligands in metal complexes2 and catalysts1.
The N-O moiety of pyridine N-oxides possesses a unique functionality which can act effectively as a push electron donor and as a pull electron acceptor group. This strong push-pull property has an essential chemical consequence; it accounts for the equally easy synthesis of 4substituted derivatives of pyridine N-oxides with donor as well as acceptor groups. The contribution of the resonance forms I and II depends on the nature of the substituent at position
4. The strong electron-acceptor nitro group favors the charge transfer form II. 3-5 I II
Spectroscopic properties
For the single pyridine ring hydrogen atom of an isolated molecule three different vibrations are expected, e.g. =C-H stretch, in plane bend and out of plane deformation. The crystal data have shown that there are two different types of = C-H bonds in crystal.6 The =C-H stretch band is split into two components in the Raman spectrum, at 3066 and 3054 cm-1 , while IR spectrum shows just one band at 3052 cm-1 . The strongest band in the IR spectrum is observed at 1231cm1 together with adjacent absorptions at 1238 and 1250 cm-1; and the Raman band at 1252 cm-1 these are assigned to the N-O stretch, because this vibration is accompanied by a large change in dipole moment and polarizability.7-8 The blue shifted very strong IR band at 1258 cm-1 supports both the assignment to . (N-O) and existence of CH…O-N hydrogen bonding.
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