Kinetic Studies on the Dehydrogenation of Flavanones with 2, 3-Dichloro5, 6-dicyano-p-benzoquinone

Abstract

In order to elucidate the mechanism of the dehydrogenation of flavanones with DDQ, the reactions have been conducted in dry benzene in the presence of acetic acid or its derivatives. The reaction rates were measured by means of HPLC analysis.
This reaction is second order with respect to the co ncentration of [1] and DDQ in all runs. The apparent second order rate constant, k_{2, a} is directly proportional to the square root of the initial concentration of acetic acid (Fig.3(b)), and its logarithm is also proportional to the acid strengths (pK_a) (Fig.4).
For twelve flavanone derivatives ((1)-(12)) which have various substituents on the C₂ phenyl group, k_{2, a} gave a satisfactory Hammett-type linear free energy relationship using Brown-Okamoto's σ⁺ constant, and it was increased by the electron-donating group and decreased by the electron-withdrawing group.
All experimental results strongly sugg est that this reaction proceeds along the path of two step ionic mechanism and rate-determining stage is a hydride abstraction with quinol cation of DDQ. From the data of activation parameters obtained by Arrhenius plot, it is concluded that the isokinetic relationship applies in this reaction, which proceeds under the control of enthalpy.