Imine ⇌ Enamine Tautomerism

Acetaldimine ⇌ vinylamine tautomerism and their conformational preferences are studied using MP2-(full)/6-31G∗ geometries and Gaussian-2 energies. Geometrical parameters, harmonic vibrational frequencies, relative stabilities, conformational stabilities, the amine inversion barrier, ionization energies, atomization energies, and equilibrium constants are compared with experimental gas-phase data when available. Their less than satisfactory agreement results mainly from a lack of definitive experimental data. The imine ⇌ enamine tautomerism is also compared with the keto ⇌ enol and nitro ⇌ aci-nitro tautomeric processes. Solvent effects are estimated by Onsager's reaction field model (SCRF) at HF/6-31G∗ for structural parameters and vibrational frequencies, both of which are minimal, and at QCISD for energies. The anti-acetaldimine tautomer is energetically favored over vinylamine by 3.9 kcal/mol at G2 and by 4.3 kcal/mol after extrapolation of the acetonitrile solvent effect. The theoretical (gas-phase) proton affinity of the enamine anion amounts to 377 kcal/mol, and the atomization energy of acetaldimine is 697 kcal/mol. The anti conformation of acetaldimine is energetically preferred over the syn form by <1 kcal/mol, which becomes even less after inclusion of solvent effects. Similarly, the energetic preference of the syn conformer over the anti form of the enamine anion becomes negligible after inclusion of solvent effects. The effect of charges in Coulombic versus resonance delocalization in the allylic enamine anion is discussed in detail.