Studies of azo and azoxy dyestuffs. Part 17. Synthesis and structure determination of isomeric α and β phenylazoxypyridines, N-oxides, and methiodides. A reexamination of the oxidation of phenylazopyridines and X-ray structure analyses of 4-(phenyl-α-azoxy)pyridinium methiodide and 4-(phenyl-β-azoxy)pyridine-N-oxide

Abstract

In an extension of Wallach rearrangement studies into the phenylazoxypyridine series, an investigation of 4-, 3-, and 2-phenylazoxypyridines, the N-oxides, and methiodides is reported. Oxidation of 4- and 3-phenylazopyridine with peracetic acid gives rise to the α and β phenylazoxypyridine-N-oxides, contrary to previous literature reports on the obtention solely of the α isomers. 2-Phenylazopyridine, however, yields only the 2-(phenyl-α-azoxy)pyridine-N-oxide. These results are rationalized on the basis of field, resonance, and steric effects. An unprecedented reactivity difference has been observed in the reactions of the α,β isomers of phenylazoxypyridines under conditions of the Wallach rearrangement. This reactivity difference permits the isolation of the α-azoxy isomers from the α,β mixtures. Unequivocal confirmation of the structures has been obtained from X-ray crystal structure determinations of two representative compounds in this series, viz. 4-(phenyl-β-azoxy)pyridine-N-oxide (11) and 4-(phenyl-α-azoxy)pyridinium methiodide (12), which itself was formed by deoxygenation of 4-(phenyl-α-azoxy)pyridine-N-oxide, followed by methylation with methyl iodide. The crystal structure of 11 was solved by direct methods and refined by block-diagonal least squares to R = 0.041 for 2479 observed reflections. The asymmetric unit contains two independent molecules, both of which are planar. The structure of 12 was determined by the heavy-atom method and refined by full-matrix least squares to R = 0.043 for 1718 observed reflections. The molecules are not planar, the pyridine ring being rotated by 36.5° from the phenylazoxy plane. Evidence is presented for the existence of intramolecular [Formula: see text] and [Formula: see text] bonds in crystal structures of trans-azoxyarenes. The carbon atoms involved in these hydrogen bonds are ortho to the azoxy group and can act as proton donors as a result of the inductive effect of the adjacent nitrogen.