The conformational analysis of push-pull enaminones using FTIR and NMR spectroscopy, and quantum chemical calculations. VI. β−N−Methyl-aminovinyl trifluoromethyl ketone and α−methyl−β−N−methylaminovinyl trifluoromethyl ketone

Abstract

X-Ray analysis, NMR and IR Fourier spectra of two enaminoketones 4-(N-methylamino)-1,1,1-trifluorobut-3-en-2-one F3CCOCHCHNH(CH3) (1) 4-(N-methylamino)-1,1,1-trifluoro-3-methylbut-3-en-2-one F3CCOC(CH3)CHNH(CH3) (2) were studied in solid state and solutions. It was shown that the substitution of proton at α-position of vinyl moiety with methyl group changes significantly electronic and spatial structure of enaminoketone. In solid state enaminoketone 1 is found to be exclusively as Z-s-Z-ap isomer with intramolecular hydrogen bond. At the same time molecules of enaminoketone 2 have E-s-E-ap stereoisomeric structure forming dimers due to intermolecular NH⋯OC hydrogen bonds. Exposure of Ar-matrix isolated 1 to UV-radiation results in appearance of various E forms with free methylamino- and hydroxy groups. The same effect was observed for 2 in solutions in CCl4 under addition of polar HMPA or DMSO during spontaneous E ⇌ Z isomerization. For enaminoketone 1 under similar conditions we observed reverse process of Z ⇌ E isomerization, E stereoisomeric forms with hydroxy group being trace quantity only.Study of solvent influence on FTIR spectra revealed that introduction of methyl group as substituent at α-position of vinyl moiety increases electron population of CO double bond. Consequently, in contrast with enaminoketone 1 where the main contribution to the ν˜ (CO) band shift to lower wavenumbers in all three stereoisomeric structures makes solvent polarity/polarizability term (π*), influence of solvent's hydrogen bond acceptor (HBA) basicity (β) on ν˜ (CO) band shift of 2 predominates in the E-s-Z-ap and Z-s-Z-ap isomers. Similarly to enaminoketone 1, negative shift of the ν˜ (CC) band to lower wavenumbers occurs due to hydrogen bond formation between π system of vinyl moiety and hydrogen bond donor solvents in all stereoisomeric forms of enaminoketone 2.