Research group of Professor Xu Pengfei of Lanzhou University: design, synthesis and application of strong reductive organic photocatalyst and progress in activation of acid SP3 hydrocarbon bond without metal participation

With the rapid development of social economy and the acceleration of urbanization, the following environmental and energy problems become more and more prominent These problems have been widely concerned by all sectors of society People began to find new renewable green energy to replace traditional energy As a kind of green and sustainable energy, light energy plays a huge role in all aspects of human life Of course, it is no exception in the field of synthetic chemistry In the past ten years, visible light catalysis has attracted great interest of chemists because of its mild conditions, simplicity and high efficiency, and has been booming, playing a huge role in the discovery of new methods and substances In most visible light catalytic reactions, metal complexes and organic photocatalysts are widely used Due to the high cost and pollution of metal complexes, their development is limited Professor Xu Pengfei's research group of Lanzhou University has long been committed to the development of green organic synthesis methods Recently, in the design, synthesis and application of strong reducing organic photocatalysts (org Lett 2018, 20, 5700-5704; org Lett 2019, DOI: 10.1021/acs.orglett.9b01629) and acid SP 3 hydrocarbon bond activation without metal participation (org Lett 2019, Doi: 10.1021/acs.orglett.9b01329) Prof Xu Pengfei's research group currently has one professor, two associate professors and one young researcher, who are mainly engaged in the research of asymmetric organic small molecules and visible light catalytic reactions Specific directions include: structure oriented organic small molecule catalysis of natural products, design of new organic small molecule catalyst, and put forward the concept of super molecular imine ion catalysis for the first time; design and application of asymmetric photocatalysis and organic photocatalyst; in addition, the research group has long-term cooperation with Jinchuan company to study the flotation mechanism of copper nickel sulfide ore and new flotation agent The development and research achievements of the agent have won many awards for major scientific and technological progress in Gansu Province At present, the research group has published more than 200 papers and edited one academic work in the international chemical journals such as angelw Chem Int ed., acc chem Res., org Lett., chem Commun Prof Xu Pengfei, Professor of Lanzhou University, doctoral supervisor In 1987, he graduated from the Department of chemistry, Lanzhou University, majoring in organic chemistry In 1994 and 1998, he obtained master's degree and doctor's degree respectively He was promoted to associate professor in 1997 and professor at the end of 2002 From 1999 to 2001, he went to ZTE University in Taiwan for postdoctoral research From January 2003 to July 2004, he worked as a visiting researcher and visiting professor with Professor K Tatsumi in the international research center of material science, Nagoya University, Japan In November 2004, he was invited to visit the 7th National Congress of Paris, France Professor Xu Pengfei has successively undertaken and completed the "973" preliminary research project, Gansu Province major science and technology special plan, key projects of the Ministry of education, National Natural Science Foundation and many other topics In 2002, he won the 9th Gansu young teacher talent award, the third prize of Gansu science and technology progress and the second prize of Gansu University Science and technology progress In 2005, he was selected into the "new century excellent talents support plan" of the Ministry of education In the same year, he was selected as the "555" innovative talents of Gansu Province In 2009, he was selected as the leading talents of Gansu Province In 2010 In 2012, he won the first prize of natural science of Gansu Province (the first adult), the first prize of science and technology progress of Jinchuan Group, the special allowance of the State Council in the same year, the "model of teacher's morality" of Lanzhou University in 2013, the "advanced individual of teacher's morality" of Gansu Province, the first prize of science and technology progress of Gansu Province (the first adult), 2014 In 2016, he won the national Baosteel outstanding teacher award, and the Guohua outstanding scholar award of Lanzhou University Won two first prizes for teaching achievements in Gansu Province Cutting edge scientific research achievements: 1 The design, synthesis and application of new highly reductive visible light catalyst; visible light, as a green and clean energy, participates in organic synthesis reaction with mild reaction conditions and good selectivity, so it has been widely concerned by chemists However, most of the small organic molecules are difficult to absorb visible light, so it is necessary to add catalysts with the properties of absorbing visible light in the visible light catalytic reactions The widely used photocatalysts are metal complexes and organic covalent molecules Due to the high cost and pollution of metal complexes, their development is limited Although more and more attention has been paid to the development of organic Photocatalysts in recent years, most organic photocatalysts have a small potential window, especially the organic photocatalysts with strong reducibility In order to solve these problems, a series of organic photocatalysts with strong reducibility (org Lett 2018, 20, 5700-5704) were designed and synthesized by Professor Xu Pengfei of Lanzhou University At the beginning of the study, the author synthesized a series of conjugated molecules containing heteroatoms, and modified cat 5 to get cat 6 - cat 9 In the visible light catalytic reaction, the absorption wavelength and efficiency of the photocatalyst have an important influence on the reaction, so the author measured and characterized the optical absorption properties of the catalyst by UV-Vis absorption The results show that the maximum absorption wavelengths of cat 1 and cat 2 are in the near ultraviolet region and the absorption is weak, but when a nitrogen atom is introduced into the catalyst structure, the maximum absorption wavelengths of the photocatalyst can be red shifted to a large extent Further calculation shows that the Homo of the catalyst is distributed in the whole molecule, while LUMO is mainly distributed in the electron absorption part When nitrogen atom is introduced into the electron absorption part, quinoxaline with strong electron absorption performance has stronger stabilizing effect on LUMO than on Homo, which results in LUMO and Homo The maximum absorption wavelength of the photocatalyst is red shifted (Fig 1) However, when the sulfur atom in cat 3 is replaced by the nitrogen atom, the maximum absorption wavelength of the catalyst does not change significantly, while the absorption efficiency of the photocatalyst has a significant increase (the molar extinction coefficient ε max of cat 5 has a significant increase) The effect of substituents on the maximum absorption wavelength of the catalyst is not significant (cat 6 - cat 9) DFT calculation results also show that when the substituents on the nitrogen atom are changed, the change of e g of cat 6 - cat 9 is small It is worth noting that ε max of these catalysts is greater than 10000 m - 1 cm - 1 Fig 1 UV visible absorption of catalyst and calculation of energy orbit of corresponding catalyst Redox potential is an important parameter of catalyst in visible light catalysis (Fig 2) Through comparison, it is found that cat 5 in excited state shows strong reducibility, while the oxidation in ground state is poor When the substituents are introduced into the catalyst, the reducibility of the excited state is reduced, but the oxidation is strengthened The author thinks that this is because the substituents on the nitrogen atom have certain electron absorption effect It should be noted that the redox potential of the N-substituted catalyst can still be comparable to that of the highly reductive metal complex photocatalyst IR (PPy) 3, even though the reducibility of the excited state of the catalyst decreases After the photo redox potential of the catalyst in Fig 2, the author applied this series of catalysts to some challenging organic reactions, and got good results, such as the activation reaction of carbon halogen bond (Fig 3) In order to further verify the activity of photocatalysts, the authors applied these photocatalysts to the free radical series reaction of 1,6-diyne By comparison, it is found that cat 8 is more effective than ordinary organic photocatalysts, and the substrate is widely used (Fig 4) Fig 4 1,6-diyne's free radical series reaction conditions Through fluorescence quenching, free radical capture experiment and quantum yield calculation, the author thinks that the starting step of the reaction may be through single electron transfer between photocatalyst and oxidant, and there may be free radical chain growth reaction in the reaction process This achievement was published in organic letters (org Lett 2018, 20, 5700-5704) In addition, in order to expand the application prospect of this kind of catalyst, the research group also used this catalyst to realize the construction of difluoroalkyl substituted dibenzo nitrogen compounds The results were recently published in organic letters (DOI: 10.1021 / ACS Orglett 9b01629) The first author of this paper was Liu Dan, Ph.D student, School of chemistry and chemical engineering, Lanzhou University 2、 In traditional chemical reactions, the activation of acid hydrocarbon bond (the hydrocarbon bond adjacent to the electron absorbing functional group) is mainly through the action of alkali to generate nucleophilic carbon anion, and then add some polar double bonds to complete a variety of chemical transformations, such as the classic Michael, aldol Mannich reaction With the development of free radical chemistry, chemists have realized different kinds of acid hydrogen activation through a series of methods They have transformed acid hydrocarbon bond into electrophilic free radical intermediates with high reactivity by various means, which greatly enriched the types of chemical reactions However, these methods have some limitations, such as the use of high-energy ultraviolet light, the reaction at high temperature (80-130 ℃), transition metal involved in the single electron oxidation Hydrogen atom transfer (HAT) has always been one of the most direct and efficient methods to activate hydrocarbon bonds The direct conversion of inert hydrocarbon bonds to highly active free radical intermediates will undoubtedly diversify the types of reactions and improve the step economy of chemical conversion In the past ten years, visible light catalysis has injected new energy into the free radical chemistry and expanded the research of hydrocarbon bond activation In the reported visible light catalyzed hydrocarbon bond activation reactions, there are two main types of hydrocarbon bonds activated by hydrogen atom transfer strategy: 1) heteroatom neighborhood, allylic, benzyl isohydrocarbon bonds, which have relatively weak bond dissociation energy (BDE ≈ 92 kcal / mol); 2) aliphatic hydrocarbon bonds (strong, inactive, aliphatic C C − H Bonds), the bond dissociation energy of these C-H bonds is relatively high, generally around 100 kcal / mol, such as cyclohexane However, the activation of acid C-H bonds at room temperature has not been reported The only mode is to use Ru (bpy) 3 as photocatalyst, and to use aryldiazonium salt as hydrogen extraction reagent at 50 ℃ to realize the free radical activation of acid hydrogen such as acetone and acetonitrile The author thinks how to convert the acid hydrocarbon bond into the electrophilic carbon radical without using metal and under mild conditions, so as to realize the green chemical conversion Firstly, the catalyst and oxidant were screened by using heteroaromatic ring substituted tertiary alcohol as template substrate and acetone as solvent Then, under the optimized reaction conditions, the author expanded the reaction substrate It has been found that no matter the olefin substrate or the type of acid hydrocarbon bond