Hydrogen-bond basicity of push–pull α,β-unsaturated enaminoketones

A method is proposed for evaluating the hydrogen-bond basicity of certain conformers of two push–pull enaminoketones with the general formula RC(O)CHCHN(CH3)2, where R=CH3 (DMBN); R=CF3 (DMTBN). It has been shown, for both enaminoketones, that the (EE) conformer has a higher basicity than the (EZ) conformer. Moreover, the (DMBN) has one of the highest general pKHB values in the hydrogen-bond basicity scale of ketones. Substitution of the CH3-group in (DMBN) with the CF3-group in (DMTBN) reduces general pKHB sharply as a consequence of electron withdrawal of CF3-group. Hydrogen bond sites are also discussed; it is shown that, in both enaminoketones, carbonyl oxygen possesses the maximal basicity, whereas nitrogen of the dimethylamino group has less basicity, and the vinyl moiety is the least basic site. Enthalpies of hydrogen bond formation (−ΔH) in absolute values, as well as the respective pKHB values, are greater for the (EE) conformer than for the (EZ) as a consequence of greater contribution of resonance structure in the (EE) form.