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

The acetamide radical cation, CH3C(=O)NH2.+, can be induced to rearrange in to its more stable enol isomer, CH2=C(OH)NH2.+, by an ion-molecule interact ion with benzonitrile, C6H5C equivalent toN, under conditions of chemical i onization. (This enolization does not occur unassisted because of a prohibi tively high energy barrier: 26 kcal/mol, from a CBS-QB3 calculation.) The i nitially formed [C6H5C equivalent toN . . . acetamide](.+) adduct ion isome rizes to a stable hydrogen bridged radical cation [C(6)H(5)equivalent toN . . . H-O-C(NH2)=CH2](.+) en route to its dissociation into the enol ion. Mu ltiple collision and deuterium labeling experiments on the acetamide/benzon itrile and the previously reported acetone/benzonitrile systems, indicate t hat the acetone ion enolizes by way of a base-catalyzed 1,3-proton shift (" proton-transport catalysis") but that a different mechanism must be operati ve in the acetamide system. Ab initio and density functional theory calcula tions 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 equivalent toN(.+) . . . acetamide] encounter c omplex. The calculations provide a rationale for the observed isotope effec ts and lead to a tentative explanation for the differences in interaction o f the title ions with benzonitrile. (C) 2001 Elsevier Science B.V.