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-CO bond leading to the formation of a CO⋯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.