Cao Ke research group of Southwest University of science and technology: palladium catalyzed B-H / B-H bond oxidative dehydrogenation coupling-o-carborane

Lead carborane is a kind of boron carbon molecular cluster with three-dimensional aromaticity It has an important application prospect in high temperature resistant materials, fluorescent materials, supramolecular chemistry, coordination chemistry, boron neutron capture therapy and other fields 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, the transition metal catalyzed B-H bond activation strategy has made great progress in the realization of selective boron functionalization of o-carborane However, there are few reports on the synthesis of b-b-linked b-o-carborane Recently, the selective synthesis of 4,4 '- bis-o-carborane and 4,5' - bis-o-carborane has been realized for the first time by using palladium catalyzed oxidative dehydrogenation coupling of B-H / B-H by Cao Ke research group of Southwest University of science and technology The related results are published online in org Lett (DOI: 10.1021 / ACS Orglett 9b02129) Brief introduction of Cao Ke's research group the research group is mainly based on the synthesis methodology of o-carborane, guided by the application requirements of ICF target materials, focusing on the new method and new ICF of regioselective construction of C-B / b-X bond The design and synthesis of target materials, from the synthesis methods and properties of monomers to the application of materials, has formed a more systematic research system Some research results have been applied to the development of ICF target materials At present, there are 2 doctoral students, 2 master students and 3 undergraduate students in this group Cao Ke, an associate researcher, graduated from the State Key Laboratory of functional organic molecular chemistry, Lanzhou University in 2011 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 ICF target materials in the State Key Laboratory of environmentally friendly energy materials Frontier research achievements: palladium catalyzed B-H / B-H bond oxidation and dehydrogenation coupling - o-carborane linked by C-C bond - o-carborane is a very important member of carborane family, which has been widely used in the construction of rigid rod polymer, metal carborane, super carborane, etc However, due to the limitation of synthesis methods, the application of b-b-linked b-o-carborane (BBC) is still in the blank In 1967, the brattsev team first reported the method of synthesizing BBC, and then the Kalinin team denied the brattsev team's work Until 1983, godovikov's team accurately synthesized 9,9 '- bis-o-carborane for the first time under the action of stoichiometric TL (ococf 3) 3 However, because a large number of thallium and mercury salts are used in these traditional synthesis methods, they have no practical application value It can be seen that the development of economic, efficient and green synthesis methods to achieve the selective synthesis of BBC plays an important role in promoting the application of BBC and the development of carborane chemistry, but also very challenging Recently, by introducing amide at B (9), the Caoke group has made strong polarization of B (4) - H bond and reversed the reactive order of B-H bond, thus realizing the selective oxidative coupling of B (4) - H and arylboric acid (chem Commun 2018, 54, 13603) On this basis, the selective synthesis of 4,4 '- bis-o-carborane and 4,5' - bis-o-carborane has been realized by using intermolecular B-H / B-H oxidative dehydrogenation coupling (Fig 1) Figure 1 Intermolecular B-H / B-H selective oxidative dehydrogenation coupling (source: org Firstly, the o-carborane modified by arylamide was investigated The results show that the target product can be obtained in a good yield by either the electron withdrawing group or the electron supplying group, and the ratio of the two isomers is about 2.0-2.5:1 However, the steric effect of amide ortho substituents has a great influence on the yield (1J - 1n) It is worth noting that although the conversion of aliphatic amides is not high (1q-1f), a single selective coupling product 4,4 '- bis-o-carborane can be obtained, which provides the possibility for further realization of specific and selective coupling of B-H / B-H bonds Fig 2 Study on the application scope of 9-amide-o-carborane (source: org Lett.) based on the selective B (4) - H arylation of 9-amide-o-carborane and the experimental results of this work, the author proposed a possible reaction mechanism (Fig 3) Firstly, palladium acetate and B (4) - 4-h of 9-amide-o-carborane were electrophilically palladialized to form cyclopd (II) intermediate a, and then a was oxidized to PD (IV) intermediate B under the action of silver salt, while PD (IV) with strong electrophilicity showed weak selectivity when B-H bond of another molecule was activated B (4) - B (4) 'and B (4) - B (5)' were obtained through intermediate C and D respectively Fig 3 Possible reaction mechanism of B-H / B-H bond selective oxidative dehydrogenation coupling (source: org Lett.) In conclusion, the author first realized the atom economic synthesis of B-B bond linked b-o-carborane by using B-H / B-H bond oxidative dehydrogenation coupling This method avoids the disadvantages of using a large number of highly toxic thallium and mercury salts in traditional methods It is the most economical, green and efficient method for the synthesis of BBC derivatives, and plays an important role in promoting the application of lian-o-carborane in related disciplines The experiment was completed by Wu Ji, a doctoral student The research work was supported by National Natural Science Foundation of China (21602182), special project of national key project (* *), research support plan of Longshan talents 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