title: Mechanistic studies of the O2-dependent aliphatic carbon-carbon bond cleavage reaction of a nickel enolate complex.
abstract: The mononuclear nickel(II) enolate complex [(6-Ph(2)TPA)Ni(PhC(O)C(OH)C(O)Ph]ClO(4) (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 O(2)-dependent aliphatic carbon-carbon bond cleavage reactivity was further investigated. Stopped-flow kinetic studies revealed that the reaction of I with O(2) is second-order overall and is ∼80 times slower at 25 °C than the reaction involving the enolate salt [Me(4)N][PhC(O)C(OH)C(O)Ph]. Computational studies of the reaction of the anion [PhC(O)C(OH)C(O)Ph](-) with O(2) 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:CO(2) (∼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:CO(2) ratio is ∼1.5:1. The results for the nickel(II)-containing reaction match those found for the reaction of I with O(2) 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 Ni(II)ARD. 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 O(2)-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 O(2) reactivity of a nickel(II) enolate complex of relevance to Ni(II)ARD, provides insight toward understanding the chemical factors involved in the O(2) reactivity of metal acireductone species.
authors.affiliation: Department of Chemistry and Biochemistry, Utah State University, Logan, Utah 84322-0300, USA. lisa.berreau@usu.edu
authors.forename: Lisa M
authors.initials: LM
authors.lastname: Berreau
authors.forename: Tomasz
authors.initials: T
authors.lastname: Borowski
authors.forename: Katarzyna
authors.initials: K
authors.lastname: Grubel
authors.forename: Caleb J
authors.initials: CJ
authors.lastname: Allpress
authors.forename: Jeffrey P
authors.initials: JP
authors.lastname: Wikstrom
authors.forename: Meaghan E
authors.initials: ME
authors.lastname: Germain
authors.forename: Elena V
authors.initials: EV
authors.lastname: Rybak-Akimova
authors.forename: David L
authors.initials: DL
authors.lastname: Tierney
mesh_terms: D000838:Anions / Q000737:chemistry; D002244:Carbon / Q000737:chemistry*; D049308:Dioxygenases / Q000737:chemistry; D006861:Hydrogen Peroxide / Q000737:chemistry; D007700:Kinetics; D015394:Molecular Structure; D009532:Nickel / Q000737:chemistry*; D010100:Oxygen / Q000737:chemistry*; D013379:Substrate Specificity
chemical_list: D000838:Anions; D002244:Carbon; D009532:Nickel; D006861:Hydrogen Peroxide; D049308:Dioxygenases; D010100:Oxygen