The copper‐catalysed oxidation of unsaturated carbonyl compounds: V. Kinetics and mechanism

Abstract

AbstractWe have investigated the kinetics of the oxidation of α,β‐ and β,γ‐unsaturated aldehydes or ketones by oxygen in methanol containing a base and a cupric pyridine complex as a catalyst. The results show that the mechanism comprises the following steps: a rate‐determining deprotonation of the substrate, yielding a dienolate anion, oxidation of this anion by the cupric complex to a dienoxy radical (or to a charge transfer complex of cupric and the dienolate anion), and oxygenation of the radical resulting in a peroxy radical, which is reduced by a cuprous complex to a γ‐hydroperoxide anion. The subsequent base‐catalysed reactions of this species were discussed in Part III of this series.A cupric ion‐initiated chain oxidation of the substrate does not occur under the conditions applied. In the absence of oxygen the cupric complex yields a stoichiometric amount of the dienoxy radicals which combine to give α,α‐, γ,γ‐, α,γ‐ and oxygen‐γ‐dehydrodimers.The active cupric complex contains a methoxide group and only three pyridine molecules.