DIRECT MEASUREMENT OF THE RATES OF REKETONIZATION OF DIENOLATES PRODUCED BY PHOTOCHEMICAL ENOLIZATION OF Β-ALKYL Α,Β-UNSATURATED KETONES IN AQUEOUS BASIC SOLUTION

Article abstract of DOI:10.1021/ja00242a038

The transient dienols produced by irradiation of five α,β-unsaturated ketones possessing γ-hydrogen atoms were observed as the corresponding dienolates in aqueous basic solution by using the technique of flash photolysis.The transients reketonize by protonation of the dienolate at carbon or via a 1,5-hydrogen shift in the dienol.The rates of both processes were measured; protonation by water at carbon of the dienolates occurs at room temperature at pseudo-first-order rates at ambient temperature in the range 375-1400 s^-1, depending upon the structure of the dienolate.The temperature dependencies of these rates for two of the ketones were measured and indicate that the activation enthalpy, ΔH*, for the protonation step is ca. 50 kJ mol^-1 and the activation entropy, ΔS*, is ca. -26 Jmol^-1K^-1.Reketonization of the dienols via the 1,5-shift mechanism occurs at room temperature at a rate in water of ca. 50 s^-1 or slower, depending upon the structure of the dienol.The pK_a for the dienol-dienolate equilibrium was measured for each ketone and was in the range 10.42-12.01 for the five systems examined.