Steric compression and twist in o-hydroxy acyl aromatics with intramolecular hydrogen bonding

A series of o-hydroxy acyl aromatics of the type 1,3-diacetyl-2,4,6-trihydroxybenzene (1), 6-methoxy-1,3-diacetyl-2,4-dihydroxy- (2), 2,4,6-trihydroxy-1,3,5-triacetylbenzene (3) and 1-acetyl-2-naphthol (4) have been investigated by means of single crystal X-ray diffraction, solution and solid state NMR spectroscopy and theoretical calculations. The structures of 1 and 2 exhibit interesting hydrogen bonds, planar structures and have, as a consequence, unexpected geometrical parameters (interatomic distances, bond lengths and valence angles) and depletion of electron density of the aromatic rings. Also significant packing effects are present. For 4, a twist of the carbonyl group is observed together with an out of plane bending of the C–Cdouble bond; length as m-dashO bond leading to the formation of a Cdouble bond; length as m-dashO⋯H–O–C hydrogen bond which is almost coplanar with the naphthalene rings. Solid state NMR spectra show lack of C3 symmetry for 3. Solution NMR spectra show-in the case of 1 and 2-quite different behaviour. A complex averaging-observed for 1 in solution-is unravelled at low temperature. Compounds 1–3 show large two-bond deuterium isotope effects, 2ΔC-2(OD), on 13C chemical shifts. This indicates strong hydrogen bonds. These can be understood in terms of an electronic effect caused by bond localisation of the benzene ring and a steric effect caused by either neighbouring CH3CO, OH or OCH3 groups leading to shorter OH⋯O and O⋯O distances and, consequently, stronger hydrogen bonds. A general scheme for distinguishing between steric twist (as seen in 4) and steric compression as seen in 1–3 is suggested. An experimental method based on isotope effects at the chelate proton of compounds deuteriated at the CD3CO groups is demonstrated. A Bader atom in molecules is done to investigate hydrogen bonding.