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    Home > Angelw: nickel / photocatalysis for asymmetric acyl coupling

    Angelw: nickel / photocatalysis for asymmetric acyl coupling

    • Last Update: 2019-10-22
    • Source: Internet
    • Author: User
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    In the past few decades, Ni catalyst has been greatly developed in coupling reaction, and many valuable natural products, polymers and drugs have been produced by Ni catalyst In recent years, more and more Ni catalysts have been used in the synthesis of chiral molecules For example, chemists have successfully realized stereoscopic C-C bonding (Figure 1a) by using racemic alkyl electrophilic reagents However, these methods need organometallic reagents or equivalent reducing agents, which reduces the practicability of these methods Recently, the combination of Ni catalyst and photocatalyst provides a solution to this problem (Figure 1b) In addition, the photocatalyst can adjust the valence state of Ni catalyst through the single electron reduction process, which is more conducive to the successful completion of the catalytic cycle Despite these advances, so far there have been only a few examples of asymmetric versions of such reactions (source: angelw Chem Int ed.) in 2019, the P melchiorre group of the Catalan Institute of chemistry in Spain used 4-alkyl-1,4-dihydropyridine (DHPs) to produce alkyl radicals to realize the Ni catalyzed coupling reaction The excited DHPs can not only be used as a source of free radicals, but also as a single electron reductant to regulate the valence of Ni catalyst, Recently, the team has realized the asymmetric coupling reaction catalyzed by Ni with the double action of DHPs, which does not need additional photocatalysts The related research results were published in angelw Chem Int ed (DOI: 10.1002 / anie 201910168) The process of photo induced asymmetric coupling reaction is as follows (Figure 2): firstly, alkyl DHPs 1 reaches excited state 1 * under the action of light, excited state 1 * reduces Ni (II) to Ni (0) through two single electron transfer processes, and produces second-order free radical B Ni (0) oxidizes the anhydrides to produce Ni (Ⅱ) - acyl complex C, and then the free radical B interacts with C to produce Ni (Ⅲ) complex D Finally, D was reduced to produce the final chiral coupling product 3 Ni (I) was regenerated into Ni (0) catalyst by single electron transfer (source: angelw Chem Int ed.) in order to verify this assumption, the author uses 1a and 2A as template substrate for condition optimization (Table 1) Through the optimization of reaction parameters such as Ni catalyst, chiral ligand and solvent, the optimum conditions of the reaction are as follows: NiCl2 is a metal catalyst, L1 is a chiral ligand, THF is a solvent, the reaction is carried out at 10 ℃ for 48 hours under 405 nm light, the substrate is 65% yield and 75% ee is worth to the target product After the optimal reaction conditions are established, the universality of the reaction is studied (Table 2) Firstly, the reaction substrate of the free radical precursor was expanded, and the target product could be obtained from the halogen substituted indole and carbazole skeleton Subsequently, different anhydrides were investigated Some functional groups, such as aryl, ketone and alkyl Chloro Substituted anhydrides, can participate in the reaction (source: angelw Chem Int ed.) later, the author extended this kind of reaction to the preparation of acyclic α, α - aryl and alkylketone, which are useful intermediates in the synthesis of natural products and drugs After the reaction conditions were optimized, the target product could be obtained with high enantioselectivity Among them, different length of alkyl chain and olefin functional group can be compatible with the reaction conditions, and the electronic donor or electron deficient aryl will not affect the reaction, but the ortho substituted aryl will inhibit the reaction (source: angelw Chem Int ed.) Summary: P melchiorre group realized asymmetric free radical coupling reaction by using excited DHPs and chiral Ni catalyst, and obtained α with high enantioselectivity The strategy of visible light induction without external photosensitizers is a good inspiration for other asymmetric catalytic coupling reactions.
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