Behaviour of nitrile oxides towards nucleophiles. Part III. Dimerisation of aromatic nitrile oxides catalysed by trimethylamine

Abstract

Reactions of aromatic nitrile oxides (1) with trimethylamine in ethanol give the dimers (2)(3,5-diaryl-1,2,4-oxadiazole 4-oxides) among other products. Kinetic measurements of reaction rates show that the dimerisations are, at low trimethylamine concentrations, first order in both nitrile oxide and nucleophile; rate constants are much lower than those reported for the catalysed dimerisation of aromatic nitrile oxides to 3,6-diaryl-1,4,2,5-dioxadiazines (6). The production of two different kinds of dimer from aromatic nitrile oxides, according to the nucleophile employed as a catalyst, is ascribed to geometrical isomerisationof intermediate open-chain 1 : 2 adducts between the nucleophile and the nitrile oxide. These are thought to be produced in the anti-aryl configuration (leading to dioxadiazines), but with trimethylamine as nucleophile are isomerised to the more stable syn-aryl configuration (leading to oxadiazole oxides).