Deuterium Isotope Effects on13C Chemical Shifts ofo-Hydroxyacyl Aromatics. Intramolecular Hydrogen Bonding

The interesting deuterium isotope effects of gossypols have been reinvestigated and the very large two-bond isotope effect, 2ΔC-6(OD), is ascribed to electric field effects. Common to the investigated compounds is the presence of intramolecular hydrogen bonds. A feature strongly related to the strength of the intramolecular hydrogen bond is intermolecular OH exchange. Electron-attracting substituents at the 3- and 5-positions of 2-hydroxyacyl aromatics increase the acidity of the 2-OH proton and therefore the intermolecular exchange, but not the hydrogen bond strength, whereas alkyl groups ortho to the intramolecularly hydrogen bonded OH prevent the OH group from swinging out and therefore prevent intermolecular exchange. Conformational equilibria were studied in 2-hydroxy-3-nitro-6-methoxyacetophenone. Surprisingly, the form with the weaker intramolecular hydrogen bond to the nitro group is dominant at ambient temperature, whereas it is the opposite at 160 K. For 2-hydroxy-5-methyl-3-nitroacetophenone a similar pattern is seen, but with much less of the form having hydrogen bonding to the nitro group at ambient temperature. 2-Acetyl-1, 8-dihydroxy-3,6-dimethylnaphthalene is involved in tautomerism of the enolic β-diketone type and large deuterium isotope effects on the 13C and OH chemical shifts are observed.