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

Nearly 50 years have passed since the classic studies by Larson and Lister [Larson, D.W.; Lister, M.W. Can. J. Chem. 1968, 46, 823] and Hay and Leong [Hay, R. W.; Leong, K. N. J. Chem. Soc. A 1971, 0, 3639] on the copper-catalyzed decarboxylation of acetonedicarboxylic acid (H(3)Acdica). 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 alpha-carbon CH2 groups in [Cu-II(H(2)Acdica)(keto)](1+) and [Cu-II(HAcdica)(keto)](0) to form unstable metal-enolates, {[Cu-II(HAcdica)(enolate)]} and {[Cu-II(Acdica)(enolate)]}, which through beta-carbonyl oxygen protonation can form metal-enols [Cu-II(H(2)Acdica)(enol)](1+) and [Cu-II(HAcdica)(enol)](0). The square-planar Cu-II center (electron paramagnetic resonance results) plays a dual role of stabilizing negative electron density at the beta-carbonyl oxygen and as an electron sink in [[Cu-I(HAcdica)(enolate)](0)](double dagger) and [[Cu-I(Acdica)(enolate)](1-)](double dagger) (confirmed through cyclic voltammetry as two single 1e(-) transfers). The pi -> pi* transition associated with [Cu-II(HAcdica)(enol)](0) was used to determine pK([Cu(A)]) (deprotonation of enol OH) and enolization rate constant (stoppedflow 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 k(CuL) 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 {[Cu-II(HAcdica)(enolate)]} and [Cu-II(HAcdica)(enol)](0). These results add to our understanding of aqueous metal-enol(ate) s, which contain underlying Cu-II/I redox chemistry, active methylenes and enol tautomers and enolate anions, which play roles in many catalytic reactions of interdisciplinary importance.