Conformational analysis of push-pull enaminoketones using Fourier transform IR, NMR spectroscopy, and quantum chemical calculations. I. β-Dimethylaminovinyl methyl ketone, β-dimethylaminovinyl trifluoromethyl ketone and their deuterated derivatives

Fourier transform IR spectra of four enaminoketones with general formula R1CO–CHCH–N(R2)2 [R1 = R2 = CH3 (DMBN); R1 = CH3, R2 = CD3 (DMBN-d6); R1 = CF3, R2 = CH3 DMTBN; R1 = CF3, R2 = CD3 (DMTBN-d6)] were investigated in various pure solvents. On the basis of NMR and IR spectra it was shown that all studied enaminoketones are present in solutions as an equilibrium of two conformers, (E-s-Z) ⇌ (E-s-E). Quantum chemical calculations were carried out by semiempirical AM1 and ab initio methods, HF/6-31G(d,p) and H/F 6-311++G(2d,2p), to evaluate the relative energy and dipole moment of each conformer. The equilibrium constant, Keq = c(E-s-E)/c(E-s-Z  ), depended strongly on total enaminoketone concentration according to equation: ln Keq=ln Keq0+a(1−e−bctotal)ln Keq=ln Keq0+a(1−e−bctotal). Besides, the (E-s-Z) conformer of DMBN and DMBN-d6 was more polar and more stable than the (E-s-E) conformer, whereas for DMTBN and DMTBN-d6 enaminoketones the stability and polarity of the conformers were opposite as a consequence of additional Cβ–H⋯F–CF2 interaction in the (E-s-E) conformer. Correlation of the out-of-phase ν(CO) and in-phase ν(CC) vibrations with solvatochromic parameters of Kamlet, Abboud, and Taft revealed that the main feature of the enaminoketones DMBN and DMBN-d6 was the primary contribution of structure C, whereas contribution of the canonical structure B prevailed in the DMTBN and DMTBN-d6 enaminoketones. The increase of solvent's polarity/polarizability (π*) and hydrogen bond donor (HBD) acidity (α) assisted in stabilization of structure C in DMBN and DMBN-d6 enaminoketones whereas this increase promoted stabilization of structure B in enaminoketones DMTBN and DMTBN-d6.Figure optionsDownload full-size imageDownload as PowerPoint slide