Anorganisch-Chemisches-Institut, Universität Heidelberg , Im Neuenheimer Feld 270, 69120 Heidelberg, Germany.
Acc Chem Res. 2014 Oct 21;47(10):3162-73. doi: 10.1021/ar5002457. Epub 2014 Aug 28.
Tridentate monoanionic ligands known as "pincers" have gained a prominent place as ligands for transition metals and, more recently, for main-group metals and lanthanides. They have been widely employed as ancillary ligands for metal complexes studied inter alia in bond activation steps relevant to catalytic processes. The central formally anionic aryl or heteroaryl unit acts as an "anchor" in the coordination to the metal, which kinetically stabilizes the resulting complexes. Their stability, activity, and reactivity can be tuned by subtle modifications of substitution patterns on the pincer ligand or by modifying the donor atoms. The challenges in pincer ligand design for enantioselective catalysis have been met by their assembly from rigid heterocycles and chiral ligating units in the "wingtip" positions, which generally contain the stereochemical information. The resulting well-defined geometry and shape of the reactive sector of the molecular catalyst favor orientational control of the substrates. On the other hand, the kinetic stability allows reduced catalyst loadings. Recently, a new generation of tridentate anionic N(∧)N(∧)N pincer ligands has been developed which give rise to highly enantioselective transformations. Their applications in asymmetric catalysis have focused primarily on the asymmetric Nozaki-Hiyama-Kishi coupling of aldehydes with halogenated hydrocarbons as well as Lewis acid catalysis involving enantioselective electrophilic attack onto metal-activated β-keto esters, oxindoles, and related substrates. These include highly selective protocols for Friedel-Crafts alkylations with Michael acceptors, electrophilic fluorinations, trifluoromethylations, azidations, and alkylations and subsequent transformations. Increasingly, these stereodirecting ligands are being employed in other types of transformations, including hydrosilylations, cyclopropanations, and epoxidations. The stability and well-defined nature of the molecular catalysts have made them attractive targets for mechanistic studies into a wide range of these transformations, thus providing the type of insight required for a more rational approach to catalyst development. This Account reviews work performed by us and other groups in the field and places it into perspective in relation to general research efforts in enantioselective catalysis.
作为过渡金属以及最近的主族金属和镧系元素的配体，三齿单阴离子配体（称为“钳子”）已成为重要的配体。它们被广泛用作金属配合物的辅助配体，这些金属配合物在与催化过程相关的键活化步骤中得到了研究。中心的形式阴离子芳基或杂芳基单元充当与金属配位的“锚”，从而动力学稳定所得的配合物。通过对钳子配体的取代模式进行细微修改或通过修饰供体原子，可以调整其稳定性、活性和反应性。通过在“翼尖”位置组装刚性杂环和手性配体单元，可以解决手性催化剂中钳子配体设计的挑战，这些位置通常包含立体化学信息。所得分子催化剂的反应性部分的明确几何形状和形状有利于底物的定向控制。另一方面，动力学稳定性允许降低催化剂的用量。最近，已经开发出了新一代的三齿阴离子 N(∧)N(∧)N 钳子配体，它们可以引发高度对映选择性的转化。它们在不对称催化中的应用主要集中在不对称的 Nozaki-Hiyama-Kishi 偶联反应上，其中包括醛与卤代烃的偶联以及路易斯酸催化反应，涉及对金属活化的β-酮酯、吲哚和相关底物的对映选择性亲电进攻。这些包括与迈克尔受体的 Friedel-Crafts 烷基化反应、亲电氟化反应、三氟甲基化反应、叠氮化反应和烷基化反应以及随后的转化反应的高度选择性方案。越来越多的情况下，这些立体定向配体被用于其他类型的转化反应，包括硅氢化反应、环丙烷化反应和环氧化反应。分子催化剂的稳定性和明确性质使其成为广泛的这些转化反应的机理研究的有吸引力的目标，从而为更合理的催化剂开发方法提供了所需的洞察力。本综述回顾了我们和其他小组在该领域所做的工作，并将其置于与对映选择性催化研究的一般研究努力相关的视角下。
