Lewis base activation of Lewis acids: Catalytic enantioselective allylation and propargylation of aldehydes
Lewis base activation of Lewis acids: Catalytic enantioselective allylation and propargylation of aldehydes
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DOI：
10.1021/ja016017e
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发表时间：
2001-06-27
影响因子：
15
通讯作者：
Wynn, T
中科院分区：
文献类型：
--
作者：
Denmark, SE;Wynn, T
自古贺等人发表催化的对映选择性Diels-Alder反应的里程碑式报告以来的22年中，手性刘易斯酸的不对称催化已成为研究最多的领域之一。[2]由于碳-碳键形成反应的核心重要性，已经开发了无数的手性刘易斯酸催化剂体系用于许多转化。典型地，这些催化剂通过强刘易斯酸与手性配体原位或在单独的制备中组合而产生。在主族、前过渡金属和镧系元素基刘易斯酸的几乎所有实例中，由于配体的给体原子的碱性，用手性配体的不对称调节导致催化剂失活。这种行为的一个重要结果是需要手性刘易斯酸的独立合成或过量的配体以确保抑制来自新生刘易斯酸的竞争性非手性背景反应。事实上，这种通过配体使母体刘易斯酸失活的作用已被用于减弱刘易斯酸催化剂的活性以提高选择性。3特别是由于配体取代，如果要实现高选择性，则需要仔细设计手性刘易斯酸催化剂。然而，在某些情况下，刘易斯碱性供体配体可以增强刘易斯酸性受体的活性。根据古特曼提出的一套经验性的键长和电荷密度变化规则，这种违反直觉的情况显然是可以预料到的。[4]具体地说，古特曼第四规则指出，当多原子供体与多原子受体配位时，供体原子上的电子密度将净增加，受体原子上的电子密度将净减少。因此，在刘易斯碱配位时，刘易斯酸的中心原子变得更亲电，过量的电荷驻留在外围配体上！从逻辑上讲，这种电子密度的转移将导致刘易斯酸中的一个配体电离。一旦配体被电离，一个完整的正电荷可以正式分配给中心原子。6阳离子物质的产生导致中心原子的刘易斯酸性显著增加;因此，刘易斯碱活化了刘易斯酸。[7]刘易斯碱活化的概念为配体加速催化提供了有趣的可能性，因为当与刘易斯碱配位时，刘易斯酸的活性最高。因此，通过使用手性刘易斯碱，产生了高活性和手性改性的刘易斯酸。在这种情况下，弱的非手性刘易斯酸可以大量使用，而不需要
In the 22 years since the landmark report by Koga et al. of a catalytic enantioselective Diels-Alder reaction, 1 asymmetric catalysis by chiral Lewis acids has become one of the most heavily investigated fields of research. 2 Because of the central importance of carbon-carbon bond-forming reactions, a myriad of chiral Lewis acid catalyst systems have been developed for many transformations. Typically, these catalysts are generated by the combination of a strong Lewis acid with a chiral ligand either in situ or in a separate preparation. In nearly all examples of main group, early transition metal, and lanthanide-based Lewis acids, asymmetric modulation with chiral ligands leads to deactivation of the catalyst due to the basicity of the donor atoms of the ligand. An important consequence of this behavior is the need for either independent synthesis of the chiral Lewis acid or an excess of the ligand to ensure suppression of competitive, achiral background reaction from the nascent Lewis acid. Indeed, this deactivation of the parent Lewis acid by the ligand has been used to attenuate the activity of Lewis acid catalysts to increase selectivity. 3 Especially because of ligand substitutions, careful design of a chiral Lewis acid catalyst is needed if high selectivities are to be realized. There are however, certain circumstances in which a Lewis basic donor ligand can enhance the activity of a Lewis acidic acceptor. This counter-intuitive situation is clearly anticipated, according to a set of empirical bond-length and charge-density variation rules formulated by Gutmann. 4 Specifically, Gutmann’s fourth rule states that upon coordination of a polyatomic donor to a polyatomic acceptor there will be a net increase in electron density on the donor atom and a net decrease of electron density on the acceptor atom. 5 Thus, upon coordination of a Lewis base, the central atom of a Lewis acid becomes more electrophilic with the excess charge residing on the peripheral ligands! Taken to its logical limit, this transfer of electron density would result in an ionization of one of the ligands from the Lewis acid. Once the ligand is ionized, a full positive charge can be formally assigned to the central atom. 6The generation of a cationic species results in a significant increase in the Lewis acidity of the central atom; thus, the Lewis base has actiVated the Lewis acid. 7 The concept of Lewis base activation leads to intriguing possibilities for ligand-accelerated catalysis because the Lewis acid is most active when coordinated to the Lewis base. 8 Thus, by use of a chiral Lewis base, a highly active and chirally modified Lewis acid is generated. In this scenario a weak, achiral Lewis acid can be used in bulk without