Palladium-catalyzed ligand-directed C-H functionalization reactions.
Palladium-catalyzed ligand-directed C-H functionalization reactions.
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DOI：
10.1021/cr900184e
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发表时间：
2010-02-10
期刊：
影响因子：
62.1
通讯作者：
Sanford MS
中科院分区：
文献类型：
--
作者：
Lyons TW;Sanford MS
发展碳氢键直接转化为碳-氧、碳-卤素、碳-氮、碳-硫和碳-碳键的方法仍然是有机化学中的一个关键挑战。这种温和和选择性的转化无疑将在整个化学领域得到广泛应用，包括在医药、天然产品、农用化学品、聚合物和原料商品化学品的合成中。形成这种官能团的传统方法依赖于反应活性和选择性的预官能化起始材料。然而，在所需的CO、CSX、CN、CS或CC键之前安装官能团的要求增加了整个分子结构的昂贵的化学步骤。因此，规避这一问题不仅可以提高原子经济性，还可以提高多步合成序列的整体效率。直接的CH键功能化反应受到两个基本挑战的限制：(I)大多数碳氢键的惰性性质和(Ii)控制含有不同CH基团的分子中的位置选择性的要求。许多研究都解决了第一个挑战，证明了过渡金属可以与CH键反应生成CSM键，这个过程被称为“CH活化”。1生成的CSM键比它们的CH键活性高得多，在许多情况下，它们可以在温和的条件下转化为新的官能团。第二个主要挑战是实现复杂分子中单个CH键的选择性官能化。虽然已经采用了几种不同的战略来解决这一问题，但最常见的(也是本次审查的主题)涉及使用含有配位配体的底物。这些配体(通常称为“导向基团”)结合到金属中心，并选择性地将催化剂输送到近端的CH键上。许多不同的过渡金属，包括Ru、Rh、Pt和Pd，经历化学计量配位的CH活化反应(也称为环金属化)。2、3此外，在过去的15年里，已经开发了各种催化碳-碳键形成的方法，其中环金属化为关键步骤。1b-d，4目前的综述将特别集中在钯催化的配体导向的CH功能化反应。由于几个原因，钯配合物是这种转化的特别有吸引力的催化剂。首先，Pd中心的配体导向的CH官能化可以用来安装许多不同类型的键，包括碳-氧、碳-卤素、碳氮、碳-硫和碳-碳键。很少有其他催化剂允许如此多样化的键结构，5-7，这种多功能性主要是两个关键特征的结果：(I)许多PdII催化剂与氧化剂的相容性，以及(Ii)选择性地使环钯中间体官能化的能力。第二，钯与多种导向基团一起参与环金属化反应，并且与许多其他过渡金属不同，钯很容易在sp2和sp3CH位促进CH的活化。最后，绝大多数钯催化的定向CH官能化反应可以在环境空气和水分存在的情况下进行，这使得它们在有机合成中的应用格外实用。
The development of methods for the direct conversion of carbonshydrogen bonds into carbon-oxygen, carbon-halogen, carbon-nitrogen, carbon-sulfur, and carbon-carbon bonds remains a critical challenge in organic chemistry. Mild and selective transformations of this type will undoubtedly find widespread application across the chemical field, including in the synthesis of pharmaceuticals, natural products, agrochemicals, polymers, and feedstock commodity chemicals. Traditional approaches for the formation of such functional groups rely on prefunctionalized starting materials for both reactivity and selectivity. However, the requirement for installing a functional group prior to the desired CO, CsX, CN, CS, or CC bond adds costly chemical steps to the overall construction of a molecule. As such, circumventing this issue will not only improve atom economy but also increase the overall efficiency of multistep synthetic sequences. Direct CH bond functionalization reactions are limited by two fundamental challenges:(i) the inert nature of most carbon-hydrogen bonds and (ii) the requirement to control site selectivity in molecules that contain diverse CH groups. A multitude of studies have addressed the first challenge by demonstrating that transition metals can react with CH bonds to produce CsM bonds in a process known as “CH activation”. 1 The resulting CsM bonds are far more reactive than their CH counterparts, and in many cases they can be converted to new functional groups under mild conditions.The second major challenge is achieving selective functionalization of a single CH bond within a complex molecule. While several different strategies have been employed to address this issue, the most common (and the subject of the current review) involves the use of substrates that contain coordinating ligands. These ligands (often termed “directing groups”) bind to the metal center and selectively deliver the catalyst to a proximal CH bond. Many different transition metals, including Ru, Rh, Pt, and Pd, undergo stoichiometric ligand-directed CH activation reactions (also known as cyclometalation). 2, 3 Furthermore, over the past 15 years, a variety of catalytic carbon-carbon bond-forming processes have been developed that involve cyclometalation as a key step. 1b-d, 4 The current review will focus specifically on ligand-directed CH functionalization reactions catalyzed by palladium. Palladium complexes are particularly attractive catalysts for such transformations for several reasons. First, ligand-directed CH functionalization at Pd centers can be used to install many different types of bonds, including carbon-oxygen, carbon-halogen, carbonnitrogen, carbon-sulfur, and carbon-carbon linkages. Few other catalysts allow such diverse bond constructions, 5-7 and this versatility is predominantly the result of two key features:(i) the compatibility of many PdII catalysts with oxidants, and (ii) the ability to selectively functionalize cyclopalladated intermediates. Second, palladium participates in cyclometalation with a wide variety of directing groups and, unlike many other transition metals, readily promotes CH activation at both sp2 and sp3 CH sites. Finally, the vast majority of Pd-catalyzed directed CH functionalization reactions can be performed in the presence of ambient air and moisture, making them exceptionally practical for applications in organic synthesis.
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