The decarbonylation of the acetamide radical cation and the enolization of its dimer by self-catalysis

The acetamide radical cation, CH3C(=O)NH2o+, and its enol, CH2=C(OH)NH2o+, undergo several unimolecular reactions in the m-s time-frame of which decarbonylation is predominant. This reaction produces the ylid ion CH2NH3o+, rather than CH3NH2o+ [J. Am. Chem. Soc. 109, 4819 (1987)]. A previously proposed mechanism via ion-dipole complexes is confirmed by the present CBS-QB3 calculations. These calculations reveal the existence of a second mechanism which proceeds via the enol ion and the distonic ion oCH2C(=O)NH3+. Both mechanisms can account for previously reported isotopic labeling experiments. Tandem mass spectrometry based experiments do not provide evidence that the non-decomposing acetamide ions rearrange to any significant extent to the more stable enol form. However, this transformation occurs smoothly by interaction with a neutral acetamide molecule ("self-catalysis"). By integration of experimental data (MS/MS/MS and labeling experiments) and ab initio calculations [CBS-Q(RHF/DZP)] three mechanisms for this assisted tautomerization have been traced. In the first mechanism the neutral acetamide component of the dimer ion accepts a C-H proton from its ionic partner and then donates the proton back to the oxygen atom. This is an example of "proton-transport catalysis". In the second mechanism, isomerization takes place within the ionic partner via a conventional 1,3-H shift. The neutral partner serves only to lower the energy of the transition state by ion-dipole attractions. This is an example of the "Spectator" mechanism. In the third mechanism, proton transfer from the ionic partner to its neutral counterpart is followed by back-donation of a hydrogen atom. This is an example of the "Quid-pro-Quo" mechanism. The behavior of the acetamide dimer ion is compared to that of the acetone dimer ion which undergoes only proton-transport catalysis.