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

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• 作者：Lisa M. Berreau ; Tomasz Borowski ; Katarzyna Grubel ; Caleb J. Allpress ; Jeffrey P. Wikstrom ; Meaghan E. Germain ; Elena V. Rybak-Akimova ; David L. Tierney
• 刊名：Inorganic Chemistry
• 出版年：2011
• 出版时间：February 7, 2011
• 年：2011
• 卷：50
• 期：3
• 页码：1047-1057
• 全文大小：1024K
• 年卷期：v.50,no.3(February 7, 2011)
• ISSN：1520-510X

文摘

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鈭抍arbon 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 掳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]^{鈭?/sup> 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鈭?/sup>. Independent experiments indicate that the reaction between 1,3-diphenylpropanetrione and HOO鈭?/sup> results in oxidative aliphatic carbon鈭抍arbon 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鈭?/sup> pathway for aliphatic carbon鈭抍arbon 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]鈭?/sup> anion results in a reaction pathway for O₂-dependent aliphatic carbon鈭抍arbon bond cleavage (via a trione intermediate) that differs from that accessible to C(1)鈭扝 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.}