Revisiting the Formation Mechanism of 1,3,4-Oxadiazole-2(3 H)-ones from Hydrazonyl Chloride and Carbon Dioxide.

The reaction mechanism for the synthesis of 1,3,4-oxadiazole-2(3 H)-ones from hydrazonyl chloride and CO2 in the presence of CsF/18-crown-6 and toluene, is revisited via density functional theory computations. Although this reaction was earlier classified as a 1,3-dipolar cycloaddition, we found some competing pathways involved therein. The mechanisms including the (F-CO2)- anion and the nitrile imine intermediate are some options. The dimerization of nitrile imine is another competing mechanism in this reaction. Our results show that the most favorable mechanism proceeds via a stepwise pathway without involving any nitrile imine intermediate or the (F-CO2)- anion. The F- anion, resulting from the formation of a complex between 18-crown-6 and Cs+ cation, deprotonates the nitrile imine precursor easily, which acts then as a nucleophilic anion, enhancing the reactivity of CO2 toward it. The mechanism for the reaction with COS, an isoelectronic analogue of CO2, is also explored.