Facile protic hydration of acetonitrile to protonated acetamide at oxygen mediated by chloroauric acid: insights from experimental and calculations

Abstract

Chemical transformations such as nitrile hydration or carbon–oxygen bond formation reactions under gentle conditions are important in the pharmaceutical industry because of the presence of potentially delicate functional groups. We present a non-catalytic hydration reaction of acetonitrile to the corresponding protonated acetamide gold(III) salt [CH3(OH)NH2]AuCl4 under ambient conditions in water using chloroauric acid H[AuCl4] for the first time. ATR-FTIR, Raman and 1H and 13C NMR spectroscopic data in addition to X-ray crystallography supported the isolation of protonated acetamide stabilized with [AuCl4]− anion. The protonation of N–C=O fragment of acetamide, O- versus N-protonation aptitude, was validated experimentally and theoretically. The X-ray crystal structure of the acetamide salt [CH3C(OH)NH2]AuCl4 in the triclinic Pī space group suggested the enolic form. However, the reaction of gold(III) trichloride AuCl3 with acetonitrile in water in the absence of a proton source formed the gold(I)/gold(III) salt [Au(CH3CN)2]AuCl4 without hydration as shown in X-ray structure in the monoclinic P21/c space group. Mapping of HOMO–LUMO energy gap using frontier molecular orbital theory and MESP surfaces of OH and NH conformers of acetamide from DFT calculations clearly shows subsequent changes in their profiles with the change in their protonation states. An energy gap of 56.4 kcal/mol in the optimized energies of OH and NH conformers of acetamide along with computed HOMO–LUMO energy difference represents the relative stability of OH conformer compared to NH conformer, thus leading to the conclusion that OH protonation site is more likely to exist in the acetamide structure as compared to the NH protonation state.