Understanding the Unusual Reduction Mechanism of Pd(II) to Pd(I): Uncovering Hidden Species and Implications in Catalytic Cross-Coupling Reactions.
Understanding the Unusual Reduction Mechanism of Pd(II) to Pd(I): Uncovering Hidden Species and Implications in Catalytic Cross-Coupling Reactions.
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DOI：
10.1021/jacs.7b01110
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发表时间：
2017-04
影响因子：
15
通讯作者：
Carin C. C. Johansson Seechurn-Carin-C.-C.-Johansson-Seechurn-15682258;T. Sperger;Thomas G Scrase;Franziska Schoenebeck;Thomas J. Colacot
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文献类型：
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作者：
Carin C. C. Johansson Seechurn-Carin-C.-C.-Johansson-Seechurn-15682258;T. Sperger;Thomas G Scrase;Franziska Schoenebeck;Thomas J. Colacot
Pd（II）中间体还原为Pd(0)是大量Pd催化过程的关键基本步骤，包括交叉偶联，C-H活化和瓦克化学。对于最强大的新一代膦配体之一，PtBu3，在Pd（II）还原后生成氧化态Pd(I)，而不是Pd(0)。本文通过实验和计算研究了Pd（II）还原成Pd(I)的机理，并对高活性预催化剂{Pd(μ-Br)(PtBu3)}2的形成进行了研究。与Pd(0)络合物（tBu3P）2Pd相反，双核Pd(I)的形成取决于Pd与磷化氢配体的化学计量，试剂的添加顺序，最重要的是，钯前体的性质和所使用的磷化氢配体的选择。此外，通过在钯中克尺度的实验，检测到具有重要机械作用的附加Pd和磷化氢物质。一个离子Pd(II)Br3二聚体副产物被分离、表征，并被确定为Pd(I)溴化二聚体形成机制的关键驱动力。在Buchwald-Hartwig反应、α-芳基化反应和Suzuki-Miyaura反应中，研究了这些副反应的存在对原位形成Pd配合物的潜在影响。使用预成型和分离的溴化钯二聚体作为预催化剂，与原位生成的催化剂相比，在催化活性方面提供了更好的结果。
The reduction of Pd(II) intermediates to Pd(0) is a key elementary step in a vast number of Pd-catalyzed processes, ranging from cross-coupling, C-H activation, to Wacker chemistry. For one of the most powerful new generation phosphine ligands, PtBu3, oxidation state Pd(I), and not Pd(0), is generated upon reduction from Pd(II). The mechanism of the reduction of Pd(II) to Pd(I) has been investigated by means of experimental and computational studies for the formation of the highly active precatalyst {Pd(μ-Br)(PtBu3)}2. The formation of dinuclear Pd(I), as opposed to the Pd(0) complex, (tBu3P)2Pd was shown to depend on the stoichiometry of Pd to phosphine ligand, the order of addition of the reagents, and, most importantly, the nature of the palladium precursor and the choice of the phosphine ligand utilized. In addition, through experiments on gram scale in palladium, mechanistically important additional Pd- and phosphine-containing species were detected. An ionic Pd(II)Br3 dimer side product was isolated, characterized, and identified as the crucial driving force in the mechanism of formation of the Pd(I) bromide dimer. The potential impact of the presence of these side species for in situ formed Pd complexes in catalysis was investigated in Buchwald-Hartwig, α-arylation, and Suzuki-Miyaura reactions. The use of preformed and isolated Pd(I) bromide dimer as a precatalyst provided superior results, in terms of catalytic activity, in comparison to catalysts generated in situ.