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    Home > Nat. Catalyst: site selective C-H direct carboxylation of aromatic rings with CO2 participation

    Nat. Catalyst: site selective C-H direct carboxylation of aromatic rings with CO2 participation

    • Last Update: 2018-06-09
    • Source: Internet
    • Author: User
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    How to use CO2 more efficiently in chemical synthesis has become a hot issue for chemists On the other hand, C-H bond activation is also the frontier of organic chemistry Recently, Li Gang, a research team from Fujian Institute of material structure, Chinese Academy of Sciences, used CO2 as a C1 synthon to realize the transition metal catalyzed, site-selective direct carboxylation of aromatic C-H bonds (NAT Catalyst., 2018, DOI: 10.1038 / s41929-018-0080-y) Brief introduction of Li Gang Research Group Li Gang research group of Fujian Institute of material structure, Chinese Academy of Sciences was founded in December 2013 The main research directions of the research group are: (1) the research of new efficient and economic organic synthesis methodology; (2) the exploration of novel and efficient natural product total synthesis strategy; (3) the total synthesis of natural products with important biological activities, the synthesis of drugs and organic functional materials There are 3 staff members, 1 postdoctoral student, 4 doctoral students and 5 Master students (including joint training students) in the research group The research group recruits postdoctoral students and other researchers all the year round We welcome young people who love organic chemistry to join us For detailed recruitment information, please refer to the website: http://www.fjirsm.cas.cn/research/r1/gang'li'group/lgktzzszp Brief introduction to researcher Li Gang, doctor, researcher, doctoral supervisor In 2005, he graduated from China University of science and technology In 2009, he graduated from the University of Wisconsin Madison with a doctor's degree From 2009 to 2013, he successively engaged in postdoctoral research at Emory University and the Scripps Research Institute The research results were published in nature, nature catalysis, nature communications, Journal of the American Chemical Society, angewandtechie International Edition, organicletters and other magazines In December 2013, he was introduced to Fujian Institute of material structure of Chinese Academy of Sciences with high-level talents and served as a researcher and project leader Selected into the 10th batch of national "thousand talents plan" youth projects in 2013, automatically selected into the "hundred talents plan" of Chinese Academy of Sciences, and the "hundred talents plan" of high-level entrepreneurship and innovation talents in Fujian Province Leading scientific research results: CO2 is generally considered as the main greenhouse gas in the site-selective C-H direct carboxylation reaction of aromatic ring, but it is also a cheap, abundant, safe and renewable C1 synthon in the eyes of chemists The use of CO2 for the synthesis of organic fine chemicals is a frontier research area that is conducive to the sustainable development of society This field has been widely concerned by chemists, and various strategies to utilize CO2 have been developed However, due to the thermodynamic and kinetic stability of CO2 molecules, the previous carboxylation reactions of aromatic compounds usually need to use active species (such as Grignard reagent) or pre activated aromatic compounds (such as organometallic reagents and aromatic halides) and CO2 to react to form C-C bonds In addition, phenol and other aromatic compounds with strong nucleophilic properties can also undergo carboxylation at specific nucleophilic sites under alkaline conditions Due to the particularity of the substrate and reaction conditions, these reactions are limited in application Greenhouse gas CO2 (source: Network) the C-H bond carboxylation of aromatic compounds catalyzed by transition metals, which is participated by CO2, has shown good development potential because of its high efficiency, green and atomic economy In addition, unlike the traditional carboxylation of aromatics under alkaline conditions, transition metal catalyzed carboxylation is easy to be optimized and controlled, so it is possible to get rid of the control of the intrinsic properties of the substrate, such as electronic properties However, due to the inertness of CO 2 and the limited nucleophilicity of C-H bond activated intermediates, the research on the direct carboxylation of C-H bond of inert aromatic ring catalyzed by transition metal is still very limited, which has always been a challenging research in the field of catalysis At present, the relevant reports of this kind of reactions are limited to some special aromatic compounds with acid C-H bond (such as pkadmso < 32.5), and the conditions of carboxylation of aromatic compounds are relatively harsh (such as the use of active and flammable methylaluminum reagent to change catalyst activity) The selective carboxylation of C-H bonds of weakly acidic / non activated aromatic compounds under mild conditions is still a great challenge Li Gang, research group of the State Key Laboratory of structural chemistry and the Key Laboratory of coal to glycol and related technologies of Fujian Institute of physical composition, using the transition metal Rh (II) as catalyst, under the control of ligand, successfully realized the CO 2 oxidation-reduction under neutral condition The site selective carboxylation of inert aromatic C-H bonds in 2-arylphenol compounds involved (Fig 1a) This reaction breaks through the selective control of the classical Kolbe Schmitt type reaction (Fig 1a, b), and obtains the 3,4-benzocoumarins widely existing in the bioactive molecules (Fig 1c) Fig 1 the most remarkable highlight of Rh (II) - catalyzed site selective carboxylation reaction of aromatic C-H bond is that it breaks through the site selectivity of traditional Kolbe Schmitt reaction In Kolbe Schmitt reaction, the active sites are the ortho and para positions of hydroxyl groups on the electron rich phenol ring In this report, by adding Rh (II) and appropriate phosphine ligands into the system, the author regulated the position of carboxylation reaction to the C-H bond which could not be achieved by traditional methods (Fig 2) The reaction conditions such as ligand, alkali and temperature were compared in detail Under the optimal reaction conditions, the separation yield of target product 2A can reach 95% (entry22) At this time, the side reactions via Kolbe Schmitt pathway are completely inhibited Another advantage of this reaction is that the conditions are mild The reaction does not need any oxidant and reductant, the system is simple, and it can be completed under a atmospheric pressure of CO2, and the operation is simple At the same time, the author found that PCY 3 can keep the activity of catalyst better Even if the amount of catalyst was reduced to 1 mol%, the yield of the substrate was good (91%, entry28) Fig 2 Optimization of reaction conditions (source: nature Catalysis) The Rh (II) catalyzed site selective C-H carboxylation with CO 2 participation has good substrate applicability The effects of electron donor or electron withdrawing substituents at different positions on the reaction results were investigated in detail Under the optimal reaction conditions, these substrates can give medium to excellent separation yield At the same time, when the amount of catalyst is reduced to 1mol%, most of the substrates can get ideal reaction results under the action of PCY 3 (Fig 3) Fig 3 application scope of substrate (source: naturecatalysis) In addition, in order to further highlight the universality of the method, the author extended the scope of substrate to aromatic systems containing heterocycles such as furan, thiophene, pyrrole and electron poor pyridine The results show that the catalytic system can also be used in the carboxylation of a series of heterocyclic aromatic C-H bonds Finally, under the condition of 1 mol% catalyst, several heterocyclic substrates obtained good yield (Fig 3) Fig 4 application scope of heterocyclic substrates (source: naturecatalysis) Finally, the mechanism of the reaction was preliminarily explored, and more excellent ligands were found on the basis of the mechanism study, thus reducing the amount of catalyst Under the simulated reaction conditions, the author observed and successfully separated the two complexes formed by rhodium acetate and sphos, and obtained their crystal structure (Fig 5) By studying the catalytic properties of these two complexes, the author found that complex 5 has good catalytic activity, while complex 6 has poor performance Therefore, the author thinks that complex 5 may be the key catalyst in the reaction In the process of the reaction, complex 5 will gradually convert to complex 6, resulting in the decrease of catalyst activity Based on this, the authors predict that the complex formed by PCY 3 and rhodium acetate, similar to complex 5, can avoid deactivation via this pathway, thus having longer life and better catalytic activity Inspired by this, the author tried PCY 3 (see above) The experimental results show that the amount of catalyst can be greatly reduced to 1 mol under the action of PCY 3, which is in good agreement with the prediction Fig 5 structure and catalytic activity of two active catalyst complexes (source: naturecatalysis) Based on the above study of the complex structure in the system, combined with a series of deuterium experiments and control experiments, the author proposed the possible catalytic cycle of the reaction (Fig 6) At the same time, the author points out that although DMF and t-BuOK have the possibility of acting as reducing agents, experiments show that the catalytic cycle can still be carried out smoothly when other non reducing agents are used to replace them Therefore, the reaction was carried out under redox neutral condition Fig 6 possible mechanism diagram of catalytic cycle (source: naturecatalysis) This study has brought new ideas and new catalytic systems to the utilization of CO 2 in the field of C-H bond activation, and laid a foundation for further study on the reaction mechanism of cyclometal complexes with CO 2 The above research results are published in nature Catalysis (NAT Catalyst 2018, DOI: 10.1038/s41929-018-0080-y) The research work was mainly completed by Fu Lei, a postdoctoral of Fujian Institute of physical architecture, and Li Shangda, a deputy researcher The research work has been greatly supported by the national "youth thousand talents plan", the special project of strategic leading science and technology of the Chinese Academy of Sciences (class B), the National Natural Science Fund, the hundred talents plan of Fujian Province and the natural science fund of Fujian Province Nowadays, people and scientific research have been paid more and more attention in the economic life China has ushered in the "node of science and technology explosion" Behind the progress of science and technology is the work of countless scientists In the field of chemistry, in the context of the pursuit of innovation driven, international cooperation has been strengthened, the influence of Returned Scholars in the field of R & D has become increasingly prominent, and many excellent research groups have emerged in China For this reason, CBG information adopts the 1 + X reporting mechanism CBG information, chembeango app, chembeango official microblog, CBG wechat subscription number and other platforms jointly launch the column of "people and scientific research", approach the domestic representative research group, pay attention to their research, listen to their stories, record their demeanor, and explore their scientific research spirit.
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