Hydrolysis of azoesters via azoformate intermediates

Abstract

The alkyl- and aryl-azoformates (1) are hydrolysed in 4 : 1 water–dioxan solution at 25° by parallel acid, water, and base catalysed pathways. The initial products formed are the arylazoformates (5) which were detected spectrophotometrically and by their subsequent rates of (acid catalysed) decarboxylation. The azoesters (1) are highly reactive relative to benzoate esters (typically 10⁴-fold for HO^– catalysis, 10⁵-fold for H₂O reaction, and 165-fold for H₃O⁺ catalysis) indicating strong electron withdrawal by the azo function is dominant relative to mesomeric electron release. Substituent effects in the aryl ring of (1)(ρ_HO- 1.03, ρ_H2O 0.88, ρ_H3O⁺ 0.44) indicate a ‘transmission factor’ of 0.54 for the –NN– group. Decarboxylation of (5) is acid catalysed even at pH 10 and the carboxylate derived from the least basic azo-compound [5; Ar = 2,4-(NO₂)₂C₆H₃] shows an additional pH independent process at high pH, behaviour which is analogous to carbamates formed from weakly basic amines.