Stabilizing the C−N Double Bond Character in Fumaramide with the Aid of Superacids

Abstract

AbstractFumaramide was reacted with the superacidic systems XF/SbF5 and XF/BF3 (X=H, D) leading to the formation of the O‐diprotonated species. Using an equimolar amount of the Lewis acids relating to fumaramide, a mixture of the diprotonated salt and the diadduct with O‐coordinated HF was obtained. The salts [C4H2X6N2O2]2+[(BF4)−]2 and [C4H2X6N2O2]2+[(SbF6)−]2 (X=H, D) were characterized by low‐temperature vibrational spectroscopy. Single‐crystal X‐ray structure analyses were carried out for the compounds [C4H8N2O2]2+[(BF4)−]2, C4H6N2O2⋅⋅⋅2HF, and fumaramide. To discuss the experimental results, quantum chemical calculations were executed at the B3LYP/aug‐cc‐pVTZ level of theory. To investigate the impact of the protonation on the resonance +M effect and the electron distribution concerning the conjugated system ESP maps, NPA charges, and NBO analyses were consulted. Due to the protonation, the nitrogen lone pair contributes completely to the formation of the C=N π‐bond, stabilizing the C=N double bond character. Since no monoprotonation of fumaramide is observed, amide hydrolysis is possible simultaneously on both amide groups.