Huang Xiaoqiang, Meggers Eric
Fachbereich Chemie , Philipps-Universität Marburg , Hans-Meerwein Straße 4 , 35043 Marburg , Germany.
Acc Chem Res. 2019 Mar 19;52(3):833-847. doi: 10.1021/acs.accounts.9b00028. Epub 2019 Mar 6.
Aspects of sustainability are playing an increasingly important role for the development of new synthetic methods. In this context, the combination of asymmetric catalysis, which is considered one of the most economic strategies to generate nonracemic chiral compounds, and visible light as an abundant source of energy to induce or activate chemical reactions has recently gained much attention. Furthermore, the combination of photochemistry with asymmetric catalysis provides new opportunity for the development of mechanistically unique reaction schemes. However, the development of such asymmetric photocatalysis is very challenging and two main problems can be pinpointed to undesirable photochemical background reactions and to difficulties in controlling the stereochemistry with photochemically generated highly reactive intermediates. In this Account, we present and discuss asymmetric photocatalysis using one of the currently most versatile photoactivatable asymmetric catalysts, namely, reactive bis-cyclometalated rhodium(III) complexes. The catalysts contain two inert cyclometalating 5-( tert-butyl)-2-phenyl benzoxazole or benzothiazole ligands together with two labile acetonitriles, and the overall chirality is due to a stereogenic metal center. The bis-cyclometalated rhodium complexes serve as excellent chiral Lewis acids for substrates such as 2-acyl imidazoles and N-acyl pyrazoles, which, upon replacement of the two labile acetonitrile ligands, coordinate to the rhodium center in a 2-point fashion. These rhodium-substrate intermediates display unique photophysical and photochemical properties and are often the photoactive intermediates in the developed asymmetric photocatalysis reaction schemes. This combination of visible light excitation to generate long-lived photoexcited states and intrinsic Lewis acid reactivity opens the door for a multitude of visible-light-induced asymmetric conversions. In a first mode of reactivity, bis-cyclometalated rhodium complexes function as chiral Lewis acids to control asymmetric radical reactions of rhodium enolates with electron-deficient radicals, rhodium-coordinated enones with electron-rich radicals, or rhodium-bound radicals generated by photoinduced single electron transfer. The rhodium-substrate complexes in their ground states are key intermediates of the asymmetric catalysis, while separate photoredox cycles initiate radical generations via single electron transfer with either the rhodium-substrate complexes or additional photoactive compounds serving as the photoredox catalyst (secondary asymmetric photocatalysis). In a second mode of reactivity, the rhodium-substrate complexes serve as photoexcited intermediates within the asymmetric catalysis cycle (primary asymmetric photocatalysis) and undergo stereocontrolled chemistry either upon single electron transfer or by direct bond forming reactions out of the excited state. These multiple modes of intertwining photochemistry with asymmetric catalysis have been applied to asymmetric α- and β-alkylations, α- and β-aminations, β-C-H functionalization of carbonyl compounds, [3 + 2] photocycloadditions between cyclopropanes and alkenes or alkynes, [2 + 2] photocycloadditions of enones with alkenes, dearomative [2 + 2] photocycloadditions, and [2 + 3] photocycloadditions of enones with vinyl azides. We anticipate that these reaction schemes of chiral bis-cyclometalated rhodium complexes as (photoactive) chiral Lewis acids will spur the development of new photocatalysts for visible-light-induced asymmetric catalysis.
可持续性的各个方面在新合成方法的发展中发挥着越来越重要的作用。在此背景下,不对称催化(被认为是生成非外消旋手性化合物最经济的策略之一)与可见光(作为丰富的能源来诱导或激活化学反应)的结合最近备受关注。此外,光化学与不对称催化的结合为开发具有独特机理的反应方案提供了新机会。然而,这种不对称光催化的发展极具挑战性,主要存在两个问题:一是不良的光化学背景反应,二是难以用光化学产生的高活性中间体控制立体化学。在本综述中,我们展示并讨论使用目前最通用的光可活化不对称催化剂之一,即反应性双环金属化铑(III)配合物进行的不对称光催化。这些催化剂包含两个惰性的环金属化5-(叔丁基)-2-苯基苯并恶唑或苯并噻唑配体以及两个不稳定的乙腈,整体手性归因于一个手性金属中心。双环金属化铑配合物对于诸如2-酰基咪唑和N-酰基吡唑等底物而言是出色的手性路易斯酸,这些底物在取代两个不稳定的乙腈配体后,以两点方式与铑中心配位。这些铑-底物中间体展现出独特的光物理和光化学性质,并且常常是所开发的不对称光催化反应方案中的光活性中间体。可见光激发产生长寿命光激发态与内在路易斯酸反应性的这种结合为众多可见光诱导的不对称转化打开了大门。在第一种反应模式中,双环金属化铑配合物作为手性路易斯酸来控制铑烯醇盐与缺电子自由基、铑配位的烯酮与富电子自由基或光诱导单电子转移产生的铑结合自由基的不对称自由基反应。处于基态的铑-底物配合物是不对称催化的关键中间体,而单独的光氧化还原循环通过与铑-底物配合物或用作光氧化还原催化剂的额外光活性化合物进行单电子转移来引发自由基生成(二级不对称光催化)。在第二种反应模式中,铑-底物配合物在不对称催化循环内作为光激发中间体(一级不对称光催化),并且在单电子转移后或通过激发态的直接成键反应进行立体控制化学过程。这种光化学与不对称催化相互交织的多种模式已应用于不对称α-和β-烷基化、α-和β-胺化、羰基化合物的β-C-H官能化、环丙烷与烯烃或炔烃之间的[3 + 2]光环加成、烯酮与烯烃的[ + 2]光环加成、脱芳构化[2 + 2]光环加成以及烯酮与乙烯基叠氮化物的[2 + 3]光环加成。我们预计,这些作为(光活性)手性路易斯酸的手性双环金属化铑配合物的反应方案将推动用于可见光诱导不对称催化的新型光催化剂的发展。