Reaction of Molecular Oxygen with a PdII-Hydride To Produce a PdII-Hydroperoxide: Experimental Evidence for an HX-Reductive-Elimination Pathway

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• 作者：Michael M. Konnick ; Shannon S. Stahl
• 刊名：Journal of the American Chemical Society
• 出版年：2008
• 出版时间：April 30, 2008
• 年：2008
• 卷：130
• 期：17
• 页码：5753 - 5762
• 全文大小：295K
• 年卷期：v.130,no.17(April 30, 2008)
• ISSN：1520-5126

文摘

The reaction of molecular oxygen with a Pd^II-hydride species to form a Pd^II-hydroperoxide represents one of the proposed catalyst reoxidation pathways in Pd-catalyzed aerobic oxidation reactions, but well-defined examples of this reaction were discovered only recently. Here, we present a mechanistic study of the reaction of O₂ with trans-(IMes)₂Pd(H)(OBz), 1 (IMes = 1,3-dimesitylimidazol-2-ylidene), which yields trans-(IMes)₂Pd(OOH)(OBz), 2. The reaction was monitored by ¹H NMR spectroscopy in benzene-d₆, and kinetic studies reveal a two-term rate law, rate = k₁[1] + k₂[1][BzOH], and a small deuterium kinetic isotope effect, k_Pd−H/k_Pd−D = 1.3(1). The rate is independent of the oxygen pressure. The data support a stepwise mechanism for the conversion of 1 into 2 consisting of rate-limiting reductive elimination of BzOH from 1 followed by rapid reaction of molecular oxygen with (IMes)₂Pd⁰ and protonolysis of a Pd−O bond of the η²-peroxo complex (IMes)₂Pd(O₂). Benzoic acid and other protic additives (H₂O, ArOH) catalyze the oxygenation reaction, probably by stabilizing the transition state for reductive elimination of BzOH from 1. This study provides the first experimental validation of the mechanism traditionally proposed for aerobic oxidation of Pd-hydride species.