Simple Enols. 4. Generation of Some New Simple Enols in Solution and the Kinetics and Mechanism of Their Ketonization

Article abstract of DOI:10.1021/jo00219a026

The simple enols -2-hydroxypropene, -3-chloro-2-hydroxypropene, 2,2-dichlorovinyl alcohol, and hydroxypropadiene have been generated from reactive precursors in solution and characterized by NMR spectroscopy.The kinetics of ketonization of 3-chloro-2-hydroxypropene, hydroxypropadiene, and of the previously described (Z)-prop-1-en-1-ol and 2-methylprop-1-en-1-ol were studied by UV spectrophotometry at 15 deg C.It was found that k₀ varied with pH according to the equation k₀ = k_H2O + (k_H+10^-pH) + (k_HO-K_W)/10^-pH and values of k_H2O, k_H+, and k_HO- were evaluated for these four enols.Solvent isotope effects, k_H+/k_D+, were determined and the acid-catalyzed ketonization of 3-chloro-2-hydroxypropene and 2-methylprop-1-en-1-ol were studied in water-Me2SO4 mixtures.The kinetics of the acid-catalyzed hydrolyses of the methyl enol ethers that correspond to these enols were also investigated.It is concluded that the kinetic results were best explained by concerted mechanism for the hydronium ion catalyzed and spontaneous ketonization and by a stepwise mechanism for the hydroxide ion catalyzed ketonization.