Mechanistic Studies of the O2-Dependent Aliphatic Carbon−Carbon Bond Cleavage Reaction of a Nickel Enolate Complex

The mononuclear nickel(II) enolate complex [(6-Ph2TPA)Ni(PhC(O)C(OH)C(O)Ph]ClO4 (I) was the first reactive model complex for the enzyme/substrate (ES) adduct in nickel(II)-containing acireductone dioxygenases (ARDs) to be reported. In this contribution, the mechanism of its O2-dependent aliphatic carbon−carbon bond cleavage reactivity was further investigated. Stopped-flow kinetic studies revealed that the reaction of I with O2 is second-order overall and is ∼80 times slower at 25 °C than the reaction involving the enolate salt [Me4N][PhC(O)C(OH)C(O)Ph]. Computational studies of the reaction of the anion [PhC(O)C(OH)C(O)Ph]− with O2 support a hydroperoxide mechanism wherein the first step is a redox process that results in the formation of 1,3-diphenylpropanetrione and HOO−. Independent experiments indicate that the reaction between 1,3-diphenylpropanetrione and HOO− results in oxidative aliphatic carbon−carbon bond cleavage and the formation of benzoic acid, benzoate, and CO:CO2 (∼12:1). Experiments in the presence of a nickel(II) complex gave a similar product distribution, albeit benzil [PhC(O)C(O)Ph] is also formed, and the CO:CO2 ratio is ∼1.5:1. The results for the nickel(II)-containing reaction match those found for the reaction of I with O2 and provide support for a trione/HOO− pathway for aliphatic carbon−carbon bond cleavage. Overall, I is a reasonable structural model for the ES adduct formed in the active site of NiIIARD. However, the presence of phenyl appendages at both C(1) and C(3) in the [PhC(O)C(OH)C(O)Ph]− anion results in a reaction pathway for O2-dependent aliphatic carbon−carbon bond cleavage (via a trione intermediate) that differs from that accessible to C(1)−H acireductone species. This study, as the first detailed investigation of the O2 reactivity of a nickel(II) enolate complex of relevance to NiIIARD, provides insight toward understanding the chemical factors involved in the O2 reactivity of metal acireductone species.