Reactions of mercury(II) acetate with nitrogen compounds. Part 9. Oxidations of the bis-(4-nitrophenylhydrazone)s of some 1,3-diketones with mercury(II) acetate and lead(IV) acetate

Abstract

Oxidation of bis-(4-nitrophenylhydrazone)s of 1,3-diketones containing an α-CH between the hydrazone chains resulted in cyclisation with fragmentation, giving pyrazoles. The reaction is considered to involve dehydrogenation of the conjugated tautomeric enamine form of the bis-hydrazone, giving a cis-azo-hydrazonoalkene intermediate. When the postulated azo-hydrazonoalkene intermediate had trans-stereochemistry, unfavourable for cyclisation, as in the oxidation of cyclohexa-1,3-dione bis-(4-nitrophenylhydrazone), the azo-alkene was stable and was the main product. When the conjugated enamine tautomeric form of the 1,3-bis-hydrazone could not exist, as with 3,3-dimethylpentane-2,4-dione bis-(4-nitrophenylhydrazone), pyrazoles were not formed and the hydrazone chains behaved as isolated monohydrazones.