On April 17, 2023, the research team of Qianghui Zhou/Honggang Cheng of Wuhan University and the team of Jinquan Yu of Scripps Research Institute of the United States published the research results entitled “Synthesis of planar chiral ferrocenes via enantioselective remote C–H activation” in the journal Nature Chemistry.
In this study, a strategy based on palladium/norbornene relay co-catalysis is reported to achieve long-range enantioselective C−H activation and functional grouping, thereby constructing 1,3-disubstituted and 1,2,4-trisubstituted planar chiral metallocene compounds that are difficult to synthesize by conventional methods.
Zhou Lan and Associate Professor Cheng Honggang, doctoral students of Wuhan University, are the co-first authors of the paper, and Associate Professor Cheng Honggang, Professor Yu Jinquan and Professor Zhou Qianghui are the co-corresponding authors of the paper.
Planar chiral ferrocene has been widely used in catalysis, materials and biomedicine, and is the most important type of planar chiral skeleton so far. Among them, 1,3-disubstituted planar chiral ferrocene has unique properties different from other ferrocene, for example, they can be developed as chiral ligands with remote stereoinduction, can also be used as unique structural units of molecular machines, and have the highest anisotropy and crystallinity in chiral ferrocene liquid crystals. However, the current research on the synthesis of planar chiral ferrocene mainly focuses on 1,2-substituted planar chiral ferrocene, and the synthesis of 1,3-disubstituted planar chiral ferrocene has rarely been reported. At present, the synthesis of 1,3-disubstituted planar chiral ferrocene mainly adopts an indirect method, involving tedious multi-step operations or chiral splitting. The direct construction of 1,3-disubstituted planar chiral ferrocene by long-range C−H bond activation of monosubstituted ferrocene is the most attractive strategy, but it has failed to achieve a breakthrough so far, mainly because the distance between the existing functional group of monosubstituted ferrocene and the C−H bond between the Mao ring is long, and it is difficult to achieve effective directional activation. Therefore, this is recognized as a difficult problem in the field of planar chiral synthesis.
Figure 1: Planar chiral ferrocene compounds and their synthesis strategies.
The Zhou Qianghui/Cheng Honggang team has been working in the field of palladium/norbornene (Pd/NBE) asymmetric synergistic catalysis for many years, and has successively realized the construction of axial chirality and central chirality (Nat. Catal. 2020, 3. 727; Chem 2021, 7, 1917）。 In view of the above synthesis problem of 1,3-substituted planar chiral ferrocene, they envisage the construction of 1,3-disubstituted planar chiral ferrocene by long-range enantioselective C−H activation and functional grouping by palladium/norbornene relay synergistic catalysis, based on the existing research on the asymmetric activation of ortho-C−H bonds of ferrocene.
Figure 2: Catalytic cycle of the reaction of interest.
Through a large number of explorations, they found that the combination of palladium acetate, chiral amino acid ligand (MPAA, chiral source) and bridgehead-substituted large-site blocking cryoliene catalysts can realize the above research ideas, and successfully prepare 1,3-disubstituted and 1,2,4-trisubstituted planar chiral ferrocene compounds that are difficult to synthesize by conventional methods. This strategy can also be applied to the efficient preparation of planar chiral ruthenocene. This is the first international research report on the asymmetric activation of remote C−H bonds to construct planar chirality by relay synergistic catalytic strategy. Based on this innovative synthesis method, the authors prepared a variety of novel planar chiral ferrocene ligands and catalysts, and proved that they have good application potential in asymmetric catalysis.
Figure 3: Product transformation and application.
In this study, a new asymmetric relay collaborative catalytic system was developed, which provided a new method for the efficient synthesis of 1,3-disubstituted planar chiral metallocene, and also provided a new idea for asymmetric remote C−H activation.
The research was supported by the National Natural Science Foundation of China, the Natural Science Foundation of Jiangsu Province, the State Key Laboratory of Elemental Organic Chemistry of Nankai University, the Special Fund for Basic Scientific Research Business Funds of Central Universities, the Talent Start-up Fund of Wuhan University and the Supercomputing Center of Wuhan University. (Source: Science Network)
Related paper information:https://doi.org/10.1038/s41557-023-01176-3
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