Reactions of lead tetra-acetate. Part XVIII. Oxidation of aldehyde hydrazones: a new method for the generation of nitrilimines

Abstract

A number of aldehyde arylhydrazones have been oxidised with lead tetra-acetate. In many cases diacylhydrazines, RCO·NH·NAcAr, can be isolated in good yield. With benzaldehyde phenylhydrazone, provided that precautions are taken to avoid autoxidation, α-phenylazobenzyl acetate can be isolated in up to 27% yield. Evidence has been obtained that this compound is not the main precursor of the diacyl derivative PhCO·NH·NAcPh or of further oxidation products, but that these arise via the nitrilimine PhC[graphic omitted]Ph; this 1,3-dipolar compound has been trapped with acrylonitrile, to form 1,3-diphenylpyrazole-5-carbonitrile together with a small quantity of the isomeric 4-carbonitrile. Analogous nitrilimines have been similarly trapped during the oxidation of the p-nitrophenylhydrazones of benzaldehyde, propionaldehyde, benzoin, and benzil. Oxidations with lead tetrabenzoate, and with lead tetra-acetate in methanol, have also been carried out; they lead to azo-dibenzoates and azo-dimethyl ethers, respectively, and benzaldehyde phenylhydrazone also forms an azo-benzoate.

The acylhydrazones PhCH:N·NH·COR (R = Ph or NPh₂) undergo oxidative cyclisation with lead tetra-acetate to give high yields of the corresponding oxadiazoles. Likewise, the phenyl- and p-nitrophenyl-hydrazones of o-nitrobenzaldehyde are cyclised to 3-arylazoanthranil 1-oxides. Nitrilimines, by implication, mediate in each case.