A proton nuclear magnetic resonance study of hydrogen bonding between fluoride and the enol form of pentane-2,4-dione and other β-diketones

Abstract

The dependence of the hydroxy proton chemical shift of the enol form of pentane-2,4-dione upon the concentration of added fluoride has been determined in acetonitrile solution. Two methods involving tetra-n-butylammonium fluoride solvates and potassium fluoride–18-crown-6 complexes are described for carrying the fluoride anion into solution. Solutions containing mole ratios of pentane-2,4-dione: of up to 50 : 1 show ¹H n.m.r. spectra consistent with the diketone existing entirely in the enol form. The estimated value for the chemical shift of the 1 : 1 complex δ(OH), is ca. 13 p.p.m. upfield from that of the pure enol. Similar results are obtained for the enol forms of 1,3-diphenylpropane-1,3-dione and 2-acetylcyclopentanone. ¹H N.m.r. studies of competitive hydrogen bonding between acetic acid and pentane-2,4-dione for fluoride anions suggests that the former is the better hydrogen-bond electron acceptor.