RESUMEN
Although it is well known that reactivity of α,ß-unsaturated enaminoketones is closely associated with spatial and electronic structure but until now little attention was devoted to quantitative investigation of interconversion of different stereoisomeric forms of enaminoketones. In present work we studied peculiarities of kinetics of ZâE isomerization of enaminoketone 4-(N-methylamino)-1,1,1-trifluorobut-3-en-2-one F3C-COCHCHNH(CH3) (1) in Ar-matrix exposed to UV-radiation (λ=340nm) with IR Fourier and 2D correlation spectroscopy and we found that Z-s-Z-s-trans isomer transforms primarily into two E-isomers, E-s-E-s-trans and E-s-Z-s-trans which further turn into the E-s-E-s-cis and E-s-Z-s-cis conformers all interconversion rate constants being comparable in magnitude. Along with this process long-term exposure to the UV-radiation results in proton transfer from nitrogen of methylamino group to carbonyl oxygen with simultaneous isomerization of 'cyclic' iminoenol form into 'linear'one. In solution of enaminoketone 4-(N-methylamino)-1,1,1-trifluoro-3-methylbut-3-en-2-one F3C-CO-C(CH3)CH-NH(CH3) (2) we observed reversed process, namely, spontaneous interconversion of the E-s-E-s-trans and E-s-Z-s-trans conformers into the Z-s-Z-trans isomer. It was found that rate constants of the dimeric forms of the E-s-E-s-trans and E-s-Z-s-trans conformers are higher than those of the monomers and are independent on total enaminoketone concentration. Addition of highly polar HMPA promotes proton transfer from nitrogen to oxygen in the Z-s-Z-s-trans isomer of 2 with subsequent isomerization into the linear imino-enol product but the rate constant of this transformation is ten-fold smaller than that for 1 in the Ar matrix exposed to UV radiation.
RESUMEN
IR Fourier spectra of two enaminoketones with general formula F3CCOCRCHN(CH3)2, R=F (DMTFBN); R=CH3, (DMTMBN) were studied in various pure solvents. For comparison results of earlier investigated enaminoketone R=H (DMTBN) was also presented. On the basis of NMR and IR spectra it was shown that enaminoketones DMTBN, DMTFBN and DMTMBN presented in solutions as equilibrium of two conformers, (E-s-Z)â(E-s-E) (for DMTFBN these conformers are denoted as (Z-s-Z) and (Z-s-E), respectively). DFT calculations were carried out to evaluate relative energy and dipole moment of each spatial form. It was shown that 'closed-ring' complex formation between (E-s-Z) and (E-s-E) conformers of DMTBN accounts for discrepancies between DFT calculations of conformer relative energies and experimentally evaluated enthalpies of (E-s-Z)â(E-s-E) equilibrium. In α-substituted DMTFBN and DMTMBN, where formation of 'closed-ring' complex was impossible we did not observe such discrepancies. For both (E-s-Z) and (E-s-E) conformers of the DMTBN and DMTMBN the main influence on the νÌ(CO) vibrations has the solvent's hydrogen bond donor (HBD) acidity, whereas for the DMTFBN an influence of the solvent's polarity/polarizability dominated.