Preparation of chiral-at-metal catalysts and their use in asymmetric photoredox chemistry
Preparation of chiral-at-metal catalysts and their use in asymmetric photoredox chemistry
复制标题
DOI：
10.1038/nprot.2017.138
复制
发表时间：
2018-04-01
期刊：
影响因子：
14.8
通讯作者：
Meggers, Eric
中科院分区：
文献类型：
--
作者：
Ma, Jiajia;Zhang, Xiao;Meggers, Eric
不对称催化是合成光学活性化合物的强大方法，可见光构成了实现化学转化的丰富能量来源，而化学转化通常由光诱导电子转移（光氧化还原化学）触发。最近，双环金属化铱（III）和铑（III）配合物作为一类新型催化剂被引入，用于将不对称催化与可见光诱导的光氧化还原化学相结合。这些催化剂之所以有吸引力，是因为它们具有完全源自立体金属中心的不寻常的手性特征，这提供了在底物与金属中心直接配位时特别有效的不对称诱导的前景。由于这些手性催化剂仅包含非手性配体，因此需要特殊的合成策略。在本协议中，我们描述了制备两种类型的手性金属催化剂的策略，即铱络合物 IrS 和铑络合物 RhS 的 Lambda 和 Delta 对映体（分别为左手和右手螺旋桨）。除了两个乙腈配体和一个六氟磷酸盐抗衡离子之外，两者都含有两个环金属化 5-叔丁基-2-苯基苯并噻唑。两个环金属化配体设定了螺旋桨形状的手性几何形状，但乙腈不稳定并且可以被底物分子取代。合成方案由三个阶段组成：首先，配体5-叔丁基-2-苯基苯并噻唑的制备；二是水杨基噻唑啉（用于铱）和水杨基恶唑啉（用于铑）手性助剂的制备；第三，辅助介导的单个对映体纯 Lambda 和 Delta 构型催化剂的合成。这类仅针对金属的立体异构络合物在不对称催化领域具有重要价值，可提供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。提供了可见光诱导的不对称催化的代表性例子。
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 Lambda- and Delta-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 Lambda- and Delta-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.