Untitled

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

“…Although Hay and Leong proposed that a “keto” complex (e.g., [CuL(H 2 O) 2 ] 1– as shown in Scheme 2) was responsible for the large increase in rate constant for copper-catalyzed decarboxylation of acetonedicarboxylate and has been supported by oxaloacetate studies pointing toward a highly reactive acidic keto complex, we propose instead that unstable {[Cu ( n –1) (HAcdica) enolate ]} (Scheme 4) provides the additional “push” for catalysis. 8,26 On the basis of this and information in Scheme 3, the reduction I and oxidation VI peaks of redox couple I – VI can be represented as eqs 5 and 6, respectivelyThe pH behavior of redox couple III – IV helped considerably in the assignment of the reaction associated with the second 1 e – copper center reduction. Peak splitting, Δ E p ( E pc 3 – E pa 4 = 0.067–0.091 V vs Ag/AgCl), was the smallest compared to that for redox couples I – VI and II – V .…”

“…26,27 There is no mention in the current scientific literature on the possible ( k E 0 , k E 1 , k E 2 ) and ( k K 0 , k K 1 , k K 2 ) values and p K a ’s (p K a3 , p K a4 ) for acetonedicarboxylate (shown in Scheme 1), but we mention them here because the enol and enolate forms of the ligand play a strong role in coordination to metal ions. 8…”

“…Naturally occurring 1,3- and 1,3,5-β-keto­(carboxylic) acids rely on β-carbonyl/α-carbon CH 2 resonance to form keto-enol tautomers and enolates with transition metals and have remained as an area of continued research due to their wide importance across chemical disciplines . Although fundamental research studies, such as the metal-ion-catalyzed decarboxylation of oxaloacetate, − have provided detailed information on reaction mechanisms, other studies seek to broaden our understanding of the biochemical significance of metal-enol­(ate)­s. For example, in active sites of enolases, Serratia endopeptidase, dioxygenase, and in β-keto acid cleavage enzymes, transition metals have been shown or postulated to induce enolization and stabilize reactive enolates, which otherwise do not exist at significant concentration at circumnetural pH. − At physiological pH, deprotonation of β-keto acid α-carbon CH 2 is generally unfavorable in the absence of metal ions or metalloenzymes, but in their presence, pathways for enol and enolate formation open up .…”

“…Although the keto forms are generally favored in aqueous solution, the keto-enol tautomer equilibria may be dependent on forward “enolization” ( k E 0 , k E 1 , k E 2 ) and reverse ( k K 0 , k K 1 , k K 2 ) “ketonization” or deprotonation at the β-carbonyl oxygen (p K a3 , p K a4 ) to form enolates (shown in Scheme ). Although UV–visible spectroscopy has been used to confirm the π → π* transition of the αβ carbonyl π-system associated with “H 3 Acdica (enol) ” (242 nm, ε = 294 M –1 cm –1 ) and enolate anion “Acdica 3– ” (268 nm, ε = 346 M –1 cm –1 ), a significant increase in the decarboxylation rate constant of H 2 Acdica 1– (2.90 × 10 –3 min –1 ) in comparison to that of H 3 Acdica (1.25 × 10 –3 min –1 ) and HAcdica 2– (0.10 × 10 –3 min –1 ) led the authors to suggest a six-membered enol transition state to explain the increase. , There is no mention in the current scientific literature on the possible ( k E 0 , k E 1 , k E 2 ) and ( k K 0 , k K 1 , k K 2 ) values and p K a ’s (p K a3 , p K a4 ) for acetonedicarboxylate (shown in Scheme ), but we mention them here because the enol and enolate forms of the ligand play a strong role in coordination to metal ions …”

Reactions of Aldehydes and Ketones and Their Derivatives

“…207 The kinetics of deuterium exchange have been measured for acetone over a range of solvents and temperatures, 208 and for acetone and 2-hydroxypropiophenone in deuteriomethanol. 209 Tautomeric equilibria in 2-, 4-, and 5-pyrimidones and their thio-and aza-analogues have been calculated.…”