Correlation of kinetic and thermodynamic parameters of conformational transformations of α-substituted β-ethoxyvinyl trifluoromethyl ketones under ultraviolet irradiation in an argon matrix

Comparison of kinetic and thermodynamic parameters of conformational transformations of α-substituted β-ethoxyvinyl trifluoromethyl ketones under exposure to UV-irradiation (λ = 275 nm (I), 290-300 nm, (II), 300-320 nm (III)) was studied by the FTIR spectroscopy and quantum chemical calculation (DFT method). These are modeling drug compounds that contain vinyl trifluoromethyl ketone motif such as C2H5O–CH=C(R)–COCF3 (I-III) (I: R = H; II: R = F; III; R = CH3) in Ar-matrix. It was found that all conformational changes of enone (I) occur exclusively due to hindered rotation around CH2–O single bond. Contrary to enone (I), interconversion of conformers in enone (II) occurs predominantly due to synchronous rotation of the trifluoromethyl carbonyl group around O=C–C=C single bond and ethoxy group around EtO–C= single bond as a result of the mesomeric effect of the α-fluorine substituent. Introduction of methyl group as α-substituent reduces the ability of ethoxy group to hinder rotation around EtO–C= single bond. It was found that experimental kinetic parameters (k1 and k-1 rate constants) and calculated thermodynamic parameters (changes of zero-point corrected relative energies Δ(ΔE+ΔZPE)) of all studied processes form a single good correlation. Comparison with similar correlation for α-substituted β-diethylaminovinyl trifluoromethyl ketones reveals that conjugation in enaminoketone molecules is much weaker than that in appropriate α-substituted β-ethoxyvinyl trifluoromethyl ketones