Studies of intramolecular hydrogen bonds: protonation of keto and enol forms of β-diketones by hydrogen bromide in dibromodifluoromethane

Abstract

N.m.r. spectroscopic studies over the range 158–258 K show that 1-phenylbutane-l,3-dione, 2-methyl-l,3-diphenylpropane-1,3-dione, and 2,2-dimethyl-1,3-diphenylpropane-1,3-dione are monoprotonated by HBr in CBr₂F₂. In the ¹H n.m.r. spectrum of 1-phenylbutane-1,3-dione in solutions of HBr-CBr₂F₂at 158 K, distinct signals are found at δ 14.31 and 12.51 for the hydroxy protons in the protonated enol tautomer and at –2.23 for HBr. Proton exchange with HBr leads to collapse of the peak at S 14.31 at temperatures above 168 K, and above 208 K the peak at 12.51 and that of HBr coalesce. For solutions of 2-methyl-1,3-diphenylpropane-1,3-dione and 2,2-dimethyl-1,3-diphenylpropane-1,3-dione in HBr–CBr₂F₂ at temperatures below 180 and 158 K respectively, distinct signals are found for HBr at δca. –2 and for the acidic protons in the protonated keto forms at 21.44 and ca. 14.1, respectively. The measured negative values of the isotope effect on the chemical shift [Δδ(¹H–²H)] of the added protons in, the protonated diketones show that the proton is located between the keto groups in a very strong intramolecular hydrogen bond.