Benzonitrile assisted enolization of the acetone and acetamide radical cations: proton-transport catalysis versus an intermolecular H+/· transfer mechanism

Abstract

The acetamide radical cation, CH3C(=O)NH2·+, can be induced to rearrange into its more stable enol isomer, CH2=C(OH)NH2·+, by an ion–molecule interaction with benzonitrile, C6H5C≡N, under conditions of chemical ionization. (This enolization does not occur unassisted because of a prohibitively high energy barrier: 26 kcal/mol, from a CBS-QB3 calculation.) The initially formed [C6H5C≡N ⋯ acetamide]·+ adduct ion isomerizes to a stable hydrogen bridged radical cation [C6H5C≡N ⋯ H-O-C(NH2)=CH2]·+ en route to its dissociation into the enol ion. Multiple collision and deuterium labeling experiments on the acetamide/benzonitrile and the previously reported acetone/benzonitrile systems, indicate that the acetone ion enolizes by way of a base-catalyzed 1,3-proton shift (“proton-transport catalysis”) but that a different mechanism must be operative in the acetamide system. Ab initio and density functional theory calculations at the PMP3//RHF/D95∗∗ and PMP3//B3LYP/D95∗∗ level of theory support a mechanism which can be described as a consecutive H+/H·transfer between the partners of the [C6H5C≡N·+ ⋯ acetamide] encounter complex. The calculations provide a rationale for the observed isotope effects and lead to a tentative explanation for the differences in interaction of the title ions with benzonitrile.