Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison, Wisconsin 53706, United States.
J Am Chem Soc. 2011 Aug 31;133(34):13268-71. doi: 10.1021/ja204989p. Epub 2011 Aug 4.
(Sp)PdCl(2) [Sp = (-)-sparteine] catalyzes a number of different aerobic oxidation reactions, and reaction of O(2) with a Pd(II)-hydride intermediate, (Sp)Pd(H)Cl (1), is a key step in the proposed catalytic mechanism. Previous computational studies suggest that O(2) inserts into the Pd(II)-H bond, initiated by abstraction of the hydrogen atom by O(2). Experimental and computational results obtained in the present study challenge this conclusion. Oxygenation of in-situ-generated (Sp)Pd(H)Cl exhibits a zero-order dependence on [O(2)]. This result is inconsistent with a bimolecular H-atom-abstraction pathway, and DFT computational studies identify a novel "reductive elimination" mechanism, in which the chelating nitrogen ligand undergoes intramolecular deprotonation of the Pd(II)-hydride. The relevance of this mechanism to other Pd(II) oxidation catalysts with chelating nitrogen ligands is evaluated.
(Sp)PdCl(2)[Sp=(-)-sparteine]催化许多不同的有氧氧化反应,而 O(2)与 Pd(II)-氢化物中间体(Sp)Pd(H)Cl(1)的反应是所提出的催化机理中的关键步骤。先前的计算研究表明,O(2)通过 O(2)夺取氢原子而插入 Pd(II)-H 键。本研究中获得的实验和计算结果对这一结论提出了挑战。原位生成的(Sp)Pd(H)Cl 的氧化对[O(2)]表现出零级依赖性。这一结果与双分子 H 原子攫取途径不一致,并且 DFT 计算研究确定了一种新的“还原消除”机制,其中螯合氮配体经历 Pd(II)-氢化物的分子内去质子化。评估了该机理对具有螯合氮配体的其他 Pd(II)氧化催化剂的相关性。