Popp Brian V, Stahl Shannon S
Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison, Wisconsin 53706, USA.
J Am Chem Soc. 2007 Apr 11;129(14):4410-22. doi: 10.1021/ja069037v. Epub 2007 Mar 20.
The reaction of a palladiumII-hydride species with molecular oxygen to form palladiumII-hydroperoxide has been proposed as a key step in Pd-catalyzed aerobic oxidation reactions. We recently reported one of the first experimental precedents for such a step (Angew. Chem., Int. Ed. 2006, 45, 2904-2907). DFT calculations have been used to probe the mechanism for this reaction, which consists of formal insertion of O2 into the palladium-hydride bond of trans-(NHC)2Pd(H)OAc (NHC = N-heterocyclic carbene). Four different pathways were considered: (1) hydrogen atom abstraction (HAA) of the Pd-H bond by molecular oxygen, (2) reductive elimination of HX followed by oxygenation of Pd0 and protonolysis of the (eta2-peroxo)-PdII species, (3) oxygenation of palladiumII-hydride with subsequent reductive elimination of the O-H bond from an eta2-peroxo-PdIV center, and (4) formation of a cis-superoxide adduct of the palladium-hydride species followed by O-H bond formation via hydrogen atom migration. The calculations reveal that pathways 1 and 2 are preferred energetically, and both pathways exhibit very similar kinetic barriers. This result suggests that more than one pathway is possible for catalyst reoxidation in Pd-catalyzed aerobic oxidation reactions.
钯（II）-氢化物物种与分子氧反应形成钯（II）-氢过氧化物，这一反应被认为是钯催化的需氧氧化反应中的关键步骤。我们最近报道了这一步骤的首批实验先例之一（《德国应用化学》，国际版，2006年，第45卷，2904 - 2907页）。密度泛函理论（DFT）计算已被用于探究该反应的机理，该反应包括将O₂正式插入反式-（N-杂环卡宾）₂Pd（H）OAc（NHC = N-杂环卡宾）的钯-氢键中。考虑了四种不同的途径：（1）分子氧对Pd-H键的氢原子提取（HAA），（2）HX的还原消除，随后是Pd⁰的氧化和（η²-过氧）-Pd（II）物种的质子解，（3）钯（II）-氢化物的氧化，随后从η²-过氧-Pd（IV）中心还原消除O-H键，以及（4）钯-氢化物物种形成顺式超氧加合物，随后通过氢原子迁移形成O-H键。计算结果表明，途径1和途径2在能量上更占优势，并且这两种途径都表现出非常相似的动力学势垒。这一结果表明，在钯催化的需氧氧化反应中，催化剂再氧化可能存在不止一种途径。
