JACS: visible light induced carbene insertion reaction

For a long time, the construction of C-X bond is the basis of organic synthesis, which has aroused extensive interest of researchers In particular, the construction of C-B bond can be used to synthesize many valuable chemical molecules Therefore, in recent years, researchers have developed many simple, mild and efficient ways to construct organoboron compounds So far, transition metal catalyzed carbene insertion reactions have been widely used in the selective construction of C-X bonds Among these methods, carbene insertion into N-H, O-H, S-H and Si-H bonds has developed well, but the method of carbene insertion into B-H bonds has been rarely studied Among them, the insertion of Fischer type metal carbene into the B-H bond is still a huge challenge, which is determined by the nature of its own lack of electricity To solve this problem, researchers activated borane with Lewis base, making carbene easier to insert In addition, the team of Curran and Zhou reported the carbene insertion reaction catalyzed by transition metals Subsequently, researchers developed many different systems to achieve this process, but these methods all need electrical substituents to stabilize diazo compounds, and these diazo compounds also have the potential to explode Although these progresses have been made in the selective construction of B-H bond by carbene insertion reaction, the development of new carbene precursors to achieve B-H bond insertion reaction still has certain research value Recently, the frank glorius group at the University of Minster in Germany has realized the visible light induced B-H bond insertion reaction with the nucleophilic α - siloxane carbene, and synthesized a series of α - alkoxyborate compounds It has been reported that acyl silicon compounds can undergo 1,2-migration to produce α - siloxane carbene by light irradiation, which is a multifunctional synthesis intermediate, but its further application is limited by severe reaction conditions It is assumed that this charged carbene may be inserted into the B-H bond of the absence of electricity, thus realizing the carbene insertion reaction under mild conditions Relevant research results were published in J am Chem SOC (DOI: 10.1021 / JACS 9b08960) (source: J am Chem SOC.) firstly, the author used 3-phenylpropyltrimethylsilane (1a) and hbpin as template substrate to optimize the conditions under light (Table 1) Through the optimization of solvent, reaction temperature and reaction concentration, the following conclusions are obtained: 1) the reaction is sensitive to water and oxygen; 2) the light intensity has no great influence on the reaction, which is conducive to the amplification of the reaction; 3) the target product can be obtained by simple concentration after the reaction Finally, the reaction conditions were determined as follows: n-hexane as solvent, 3 W blue light irradiation for 12 hours at room temperature, and finally the target product was obtained with quantitative yield (source: J am Chem SOC.) after determining the optimal reaction conditions, the author investigated the universality of the reaction (Table 2) Firstly, the substrate containing silicon group was expanded Besides TMS group, TES, TBS, tips and DMPS group can get the target product in quantitative yield In addition, both primary and secondary alkylacyl silicon can participate in the reaction, but the tertiary alkylacyl silicon with larger steric hindrance does not get the target product Then, the functional group tolerance of the reaction was investigated Olefin, halogen, azide, OTS group, phthalimide, sulfonamide, BOC protected amine, sulfonyl group, ester group and other groups were all compatible with the reaction conditions Heterocyclic compounds, including furan and thiophene, have no effect on the reaction Aromatic acyl silicon compounds can also participate in the reaction and get the target product in high yield (source: J am Chem SOC.) finally, the mechanism of the reaction was studied At first, the substrate was tested by UV / Vis Spectrum, and it was found that only acyl silicon had absorbability Then, the control experiment showed that α - siloxane carbene may be formed through the T 1 state path of acyl silicon In addition, the whole reaction process was verified by DFT calculation Summary: Frank glorius group has developed a visible light induced radical insertion reaction to B-H bond and synthesized a series of α - alkoxyborates The reaction does not need metal catalysis, the conditions are mild, and the functional groups are well tolerated, easy to be amplified, and the atom utilization rate is high.