Research group of Cao Ke, associate researcher of Southwest University of science and technology: Pd / NHC precisely unlocks the selective activation of o-carborane B (3,6) - H

Lead carborane is a kind of boron carbon molecular cluster with three-dimensional aromaticity It has an important application prospect in the fields of high temperature resistant materials, luminescent materials, supramolecular chemistry, coordination chemistry and pharmaceutical chemistry due to its unique photoelectric and biochemical properties Because o-carborane contains 10 incompletely equivalent B-H bonds, and there is little difference in reaction activity between them, how to realize the regioselective functionalization of B-H bonds and the diversification and atom economic synthesis of o-carborane derivatives has become the bottleneck of the development of carborane chemistry and related materials In recent years, great progress has been made in the realization of selective functionalization of B (8,9,10,12) - H and B (4,5) - H bonds by electrophilic boron hydrogen activation and localization group directed boron hydrogen activation However, the selective functionalization of electron deficient B (3,6) - H bonds is rarely reported Recently, the team of Cao Ke, associate researcher of Southwest University of science and technology, has realized the selective activation of o-carborane B (3,6) - h under mild reaction conditions by using the PD NHC catalytic system in situ, which provides a universal catalytic system for the selective functionalization of B (3,6) - H bond The relevant results are published online in org Lett (DOI: 10.1021 / ACS Orglett 9b03790) O-carborane B (3, 6) Selective functionalization of - H bond (source: org Lett.) Cao Ke, associate researcher of the research group, introduced that the research group is mainly based on the synthesis methodology of o-carborane, guided by the application requirements of ICF boron carbide target materials, focusing on the regional selective construction of C-B / b-x The new method of bond, the design and synthesis of boron carbide target materials, from the synthesis method of monomer, performance research to the application of materials, has formed a relatively systematic research system, some research results have been applied to the development of boron carbide target materials At present, there are 3 doctoral students, 1 master's student and 3 undergraduate students in this group In 2011, Cao Ke graduated from the State Key Laboratory of functional organic molecular chemistry, Lanzhou University He studied metal organic chemistry and total synthesis of active natural products under the guidance of academician Tu Yongqiang Since July 2011, he has worked in the school of materials science and engineering, Southwest University of science and technology; since 2013, he has independently carried out the research work of organic boron chemistry and boron carbide target materials in the State Key Laboratory of environmentally friendly energy materials Advanced research results: Pd / NHC precisely unlocks the selective activation of o-carborane B (3,6) - H o-carborane is a three-dimensional aromatic boron carbon cluster, which contains 10 incomplete equivalent B-H bonds Since the appearance of o-carborane in 1960s, the selective functionalization of B-H bond has been an insurmountable problem In recent years, according to the difference of charge density of B-H bond, great progress has been made in the selective functionalization of B (8,9,10,12) - H and B (4,5) - H bonds by using electrophilic B-H activation and localization group directed B-H activation strategies However, the selective functionalization of electron deficient B (3,6) - H bonds is rarely reported In 1988, Sneddon group realized borohydride reaction of propargyne for the first time by using IR to catalyze B (3) - H activation However, this catalytic system is only suitable for propargyne In 2017, Professor Xie Zuowei and Professor Qiu zaozaozao's team realized the selective boration of B (3,6) - H bond by IR catalytic B-H activation, providing a general reaction intermediate for the functionalization of B (3,6) Unfortunately, the efficiency of the synthesis of 3,6-ar 2-o-carborane from 3,6-bpin 2-o-carborane is not ideal Therefore, the development of economic, efficient and universal catalytic system to realize the selective functionalization of B (3,6) - H bond is very important to promote the development of carborane chemistry, but also very challenging Azacarbene (NHC) is a kind of ligands with strong σ electron donating ability, which is widely used in the field of organometallic chemistry and synthetic organic chemistry However, in carborane chemistry, it has been found that NHC can make carborane deproton / debonding reaction, so as to obtain the zwitterionic salt of nested carborane For this reason, transition metal / NHC catalyst system has not been used to explore the activation of B-H bond Because the electron deficient B (3,6) - H is easy to be activated by the electron rich metal center, the team proposed to realize the selective activation of B (3,6) - h by using NHC to regulate the charge density of PD center on the basis of previous research work: (1) PD NHC complex is easy to generate in situ, so it can inhibit the deproton / debonding reaction of carborane; (2) compared with PD (OAC) 2 The strong electron supplying ability of NHC can reduce the electrophilicity of PD NHC, thus inhibiting the electrophilic palladization of B (8,9,10,12) - H and Pd (OAC) 2 Based on this assumption, the team successfully realized the selective activation / arylation of o-carborane B (3,6) - h under mild reaction conditions by using PD NHC generated in situ, and realized the efficient synthesis of asymmetric o-carborane derivatives by step reaction Fig 1 Pd / NHC catalyzes the selective activation / aromatization of B (3,6) - H (source: org Lett.) under the optimal reaction conditions, the author studied the universality of the reaction substrate (Fig 2) It can be seen that the target product (3b-3d, 3f-3i) can be obtained in good yield by halogenated iodobenzene, and the steric effect of ortho substituent has a great influence on the yield (3e) The medium yield product (3j - 3L) can also be obtained from the substrate containing cyano, aldehyde and ester groups However, the reaction efficiency of the substrate with electron donating substituent is very low, which may be due to the slow reaction of oxidation addition of iodobenzene to PD center Fig 2 Study on the scope of application of iodobenzene derivatives (source: org Lett.) then, the author investigated the boron substituted o-carborane (Fig 3) The results show that the B (9) - substituted o-carborane can also successfully obtain the corresponding target product (5a-5d) This reaction provides a simple and efficient method for the synthesis of triaryl substituted o-carborane derivatives Figure 3 Study on the scope of application of o-carborane derivatives (source: org Lett.) in order to further expand the diversity of o-carborane derivatives, the author uses 3-phenyl-o-carborane as the substrate and PD NHC as the catalyst for B (6) - H activation to realize the efficient synthesis of asymmetric o-carborane derivatives (Figure 4), and uses X-ray The precise structure of the product was confirmed by single crystal diffraction Fig 4 Synthesis of asymmetric o-carborane derivatives (source: org Lett.) based on the experimental results, the author proposed a possible reaction mechanism Firstly, PD NHC activated B (3) - h in situ to form b-pd ii-nhc intermediate, then iodobenzene was added to it by oxidation, and 3-ar-o-carborane was obtained by reduction elimination Next, PD NHC reactivates B (6) - H again, and 3,6-ar-o-carborane can be obtained after oxidation addition / reduction elimination In conclusion, the selective activation / aromatization of B (3,6) - h was realized by PD NHC catalyst system, and the efficient synthesis of asymmetric o-carborane derivatives was realized by step reaction This method provides a cost-effective and efficient method for the selective functionalization of B (3,6) - H bonds, and plays an important role in promoting the application of o-carborane derivatives in related fields The experiment was completed by Xu Taotao, a doctoral student The research work was supported by NSFC, special project of national key project, Longshan talent research support plan of Southwest University of science and technology and National Key Laboratory of environmental friendly energy materials 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 website, chembeangoapp, chembeango official micro blog, CBG information wechat subscription number and other platforms jointly launch the column of "people and scientific research", approach the representative research groups in China, pay attention to their research, listen to their stories, record their demeanor, and explore their scientific research spirit