Mechanistic studies on the free radical decomposition of some oxalic acid arylhydrazides: A source of aryl radicals in aqueous solution

Abstract

UV-Vis, 1H NMR and EPR (with spin-trapping) spectroscopy, MS and product studies have been used to study the reactions of dihydrazide and monohydrazide esters of oxalic acid in aqueous base (pH 10.5). In the absence of dioxygen, the dihydrazides are stable although the monohydrazide esters are rapidly hydrolysed in a reaction that gives monohydrazide oxalate anions, but not aryl radicals. However, in contrast both types of compound are rapidly degraded by dioxygen to give aryl radicals. The mechanisms involve an initial oxidation of the hydrazide anion by dioxygen followed by hydrolysis to give the corresponding aryl diazene anion. Further oxidation and loss of nitrogen yields aryl radicals. A mechanistic reaction scheme is proposed to account for the role of dioxygen, the fate of the aryl radicals and the products formed. The scope of these reactions as a non-photochemical source of aryl radicals under mild conditions is considered. ©ARKAT.