Mechanism of reaction of isomeric nitrolic acids to nitrile oxides in aqueous solution

Abstract

Both E and Z isomers of acetonitrolic acids 15 and 16 can be prepared when the OH group is protected by acetylation. Photoisomerization of the E-isomer resulted in quantitative conversion into the pure Z-isomer 16. Hydrolysis of the E-isomer 15 produced the parent nitrolic acid 14 which undergoes loss of NO₂^– from the conjugate base at high pH. This reaction is however relatively slow suggesting base solubility and acidic reprecipitation as a method of purification of E-nitrolic acids. Deprotection of (Z)-O-acetylacetonitrolic acid by HO^– gives a highly reactive Z-nitrolic acid 17 which undergoes loss of NO₂^– at a rate which precludes its detection; however the subsequent reactions of acetonitrile oxide (CH₃CNO) formed were monitored. Rapid loss of NO₂^– therefore occurs when there is assistance from an antiperiplanar lone pair on the imino nitrogen of the oximate anion. Arylnitrolic acids were also examined; these were in the E configuration 26 and therefore underwent slow loss of NO₂^–. Since NMR and IR data are unreliable for the assignment of configuration of nitrolic acids (relative to other oximes) a single crystal diffraction study was carried out on E-acetonitrolic acid 14. The large difference in reactivity observed for the E- and Z-nitrolic acids now permits strong supporting evidence for structural assignments.