Fachbereich Chemie, Philipps-Universität Marburg, Marburg, Germany.
Nat Protoc. 2018 Apr;13(4):605-632. doi: 10.1038/nprot.2017.138. Epub 2018 Mar 1.
Asymmetric catalysis is a powerful approach for the synthesis of optically active compounds, and visible light constitutes an abundant source of energy to enable chemical transformations, which are often triggered by photoinduced electron transfer (photoredox chemistry). Recently, bis-cyclometalated iridium(III) and rhodium(III) complexes were introduced as a novel class of catalysts for combining asymmetric catalysis with visible-light-induced photoredox chemistry. These catalysts are attractive because of their unusual feature of chirality originating exclusively from a stereogenic metal center, which offers the prospect of an especially effective asymmetric induction upon direct coordination of the substrate to the metal center. As these chiral catalysts contain only achiral ligands, special strategies are required for their synthesis. In this protocol, we describe strategies for preparing two types of chiral-at-metal catalysts, namely the Λ- and Δ-enantiomers (left- and right-handed propellers, respectively) of the iridium complex IrS and the rhodium complex RhS. Both contain two cyclometalating 5-tert-butyl-2-phenylbenzothiazoles in addition to two acetonitrile ligands and a hexafluorophosphate counterion. The two cyclometalated ligands set the propeller-shaped chiral geometry, but the acetonitriles are labile and can be replaced by substrate molecules. The synthesis protocol consists of three stages: first, preparation of the ligand 5-tert-butyl-2-phenylbenzothiazole; second, preparation of salicylthiazoline (used for iridium) and salicyloxazoline (used for rhodium) chiral auxiliaries; and third, the auxiliary-mediated synthesis of the individual enantiopure Λ- and Δ-configured catalysts. This class of stereogenic-only-at-metal complexes is of substantial value in the field of asymmetric catalysis, offering stereocontrolled radical reactions based on visible-light-activated photoredox chemistry. Representative examples of visible-light-induced asymmetric catalysis are provided.
不对称催化是合成手性化合物的一种强大方法，可见光作为丰富的能源，可用于引发化学转化，而这些转化通常是由光诱导电子转移（光氧化还原化学）触发的。最近，双环金属化铱(III)和铑(III)配合物被引入作为一种新型的催化剂，将不对称催化与可见光诱导的光氧化还原化学结合起来。这些催化剂很有吸引力，因为它们的手性特征仅源自立体手性金属中心，这在手性金属中心直接与底物配位时，有望对手性诱导产生特别有效的影响。由于这些手性催化剂仅含有非手性配体，因此需要特殊的策略来合成它们。在本方案中，我们描述了制备两种类型的手性金属催化剂的策略，即铱配合物 IrS 和铑配合物 RhS 的 Λ-和 Δ-对映异构体（分别为左旋和右旋推进器）。两者都含有两个环金属化的 5-叔丁基-2-苯基苯并噻唑，外加两个乙腈配体和一个六氟磷酸盐抗衡离子。两个环金属化配体设定了推进器形状的手性几何形状，但乙腈配体不稳定，可以被底物分子取代。合成方案包括三个阶段：首先，制备配体 5-叔丁基-2-苯基苯并噻唑；其次，制备水杨噻唑啉（用于铱）和水杨恶唑啉（用于铑）手性辅助剂；最后，通过辅助介导的合成，得到单个对映纯的 Λ-和 Δ-构型催化剂。这种仅在金属上具有立体构型的立体异构纯配合物在手性催化领域具有重要价值，提供了基于可见光激活的光氧化还原化学的立体控制自由基反应。提供了可见光诱导不对称催化的代表性实例。
