New Insight into and Characterization of the Aqueous Metal-Enol(ate) Complexes of (Acetonedicarboxylato)copper.

Abstract

Nearly 50 years have passed since the classic studies by Larson and Lister [LarsonD. W.; ListerM. W.Can. J. Chem.1968, 46, 823]and Hay and Leong [HayR. W.; LeongK. N.J. Chem. Soc. A1971, 0, 3639]on the copper-catalyzed decarboxylation of acetonedicarboxylic acid (H3Acdica). Although the authors laid the foundations for what we know about this reaction; still very little information exists regarding the underlying aqueous metal-enol(ate)s of (acetonedicarboxylato)copper. In this study, UV–visible titrations revealed three pK values, pK[Cu(H2A)], pK[Cu(HA)], and pK[Cu(A)]. We associated the first two with ionization of α-carbon CH2 groups in [CuII(H2Acdica)keto]1+ and [CuII(HAcdica)keto]0 to form unstable metal-enolates, {[CuII(HAcdica)enolate]} and {[CuII(Acdica)enolate]}, which through β-carbonyl oxygen protonation can form metal-enols [CuII(H2Acdica)enol]1+ and [CuII(HAcdica)enol]0. The square-planar CuII center (electron paramagnetic resonance results) plays a dual role of stabilizing negative electron density at the β-carbonyl oxygen and as an electron sink in [[CuI(HAcdica)enolate]0]‡ and [[CuI(Acdica)enolate]1–]‡ (confirmed through cyclic voltammetry as two single 1e– transfers). The π → π* transition associated with [CuII(HAcdica)enol]0 was used to determine pK[Cu(A)] (deprotonation of enol OH) and enolization rate constant (stopped-flow spectroscopy) but also exhibited a time-dependent decrease in absorbance (on the order of min–1), suggesting a new method to possibly obtain experimental values for the estimated “kCuL” decarboxylation rate constant of metal-enolate [CuL]1– calculated by Larson and Lister. On the basis of our results, we postulate that decarboxylation takes place primarily through {[CuII(HAcdica)enolate]} and [CuII(HAcdica)enol]0. These results add to our understanding of aqueous metal-enol(ate)s, which contain underlying CuII/I redox chemistry, “active methylenes” and enol tautomers and enolate anions, which play roles in many catalytic reactions of interdisciplinary importance.