Strong intramolecular hydrogen bonding within the dimedone–aldehyde adducts

Ionization of the well-known 2:1 dimedone–aldehyde adducts in polar solvents leads to the respective monoanions involving the intramolecular ionic hydrogen bond. These anions feature the short O···Ha···O distance (2.403 Å for 4, Rp-ClC6H4) and the strong downfield shift of the corresponding signal (δHa = 17.0 ppm, in dichloromethane-d2). The negative charge is delocalized over the two dimedone moieties. This phenomenon is also reproduced by quantum mechanical calculations. The dynamic NMR spectra provide an estimate of the hydrogen bond energy as high as 16 kcal/mol.