A free-radical analogue of the Chapman rearrangement; conversion of arylhydrazonates into N′N′-diarylhydrazides

Abstract

Aryl N-(aryl)benzohydrazonates with substituents in the C-, N-, and O-aryl rings have been prepared from the corresponding hydrazonyl halides and substituted sodium phenoxide in benzene. The hydrazonates were shown to undergo rearrangement to N′N′-diarylbenzohydrazides on heating either alone or in the presence of a solvent. Radical initiators and oxidizing agents (e.g. manganese dioxide) catalyse the rearrangement. The intramolecularity of the rearrangement was demonstrated by the absence of cross-over products and the observation that the configuration of the migrating group is retained. Kinetic experiments have shown that electron-withdrawing substituents in each of the aromatic rings reduce the rate of rearrangement, the effect being greatest in the N-aryl ring where ρ=–2·1. A mechanism involving radical initiated hydrogen abstraction followed by a 1,4-group migration to give a more stable hydrazyl free radical is proposed. O-Alkyl hydrazonates failed to isomerise under these conditions as did the S-aryl thio-analogues and the N-aryl nitrogen analogues. The N′N′-diarylhydrazides were hydrolysed in concentrated acid to the corresponding NN-diarylhydrazines, providing a route to these materials which are otherwise difficult to obtain, particularly when the aryl groups are dissimilar.