Angelw: Olivier BASL é group realizes visible light induced Rh (I) - catalyzed directed C-H boration

Transition metal (TM) - catalyzed C-H functionalization is an important tool for the synthesis of organic materials, natural products and drug discovery In recent years, the study of visible light driven reactions has promoted the development of new catalytic systems for C-H conversion Most of these new systems involve photooxidation-reduction catalyst (PC) photosensitization, and PC promotes the conversion through hydrogen transfer (HAT) or single electron transfer (set) (Figure 1a) Recently, the selective introduction of functional groups (Figure 1b) has been realized through the dual catalysis strategy of photo redox catalysis and TM catalysis However, there are still some challenges in visible light induced TM catalyzed C-H functionalization, such as the need to develop effective catalysts to solve the selective conversion with inactive C-H bonds Organoboron compounds are common intermediates in pharmaceutical chemistry and materials science, and C-H boration of aromatics is the most direct construction method Although progress has been made in TM catalyzed regioselective C-H boration, only a few photocatalytic systems have been reported and the reactions have poor regioselectivity (Figure 1c) Recently, Olivier BASL é, National Academy of Sciences of France (CNRS Research Institute), realized the visible light induced Rh (I) - catalyzed regioselective C-H boration of aromatic hydrocarbons (Figure 1D) at room temperature, which was recently published in angew Chem Int ed (DOI: 10.1002 / anie 201905924) (image source: angelw Chem Int ed.) currently, the use of N-heterocyclic carbene (NHC) ligands in TM catalyzed C-H boration is rare Recently, a new kind of bidentate carboxylate NHC ligands has been developed and proved to be beneficial to Cp * Rh (III) catalysts in the thermodynamic C-H boration of aromatic and aliphatic compounds In order to realize C-H boration under mild conditions, the Rh (I) complex with strong donor electron dinitrate NHC ligands was compared with the relatively unstable 1, 5-cyclooctadiene (COD) ligands are combined to obtain visible light absorption complexes, and a series of amino acid based NHC ligand precursors 1 are transformed into corresponding Ag intermediates, which are metallized to [Rh (COD) Cl] 2, thus obtaining the expected Rh (I) complex 2 (Figure 2) Among them, the structure of complex 2a was confirmed by X-ray diffraction analysis, showing a slightly distorted square plane shape (Figure 2) In addition, UV-vis spectra show that 2A can be excited by blue LED (picture source: angelw Chem Int ed.) after obtaining the new complex, the author used B 2pin 2 as the boration reagent to study its photocatalytic activity in pyridine oriented C-H boration (Table 1) Under 20W blue LED irradiation and in the presence of complex 2a, 2-phenylpyridine (3a) can be functionalized with 90% conversion and appropriate selectivity to obtain ortho boration products Using 4-methylpyridine as the guiding group can effectively improve the regioselectivity of boration reaction and obtain the product 4B in almost quantitative yield The solvent had a significant effect on the catalytic activity: benzene instead of THF reduced the yield; when dichloromethane and acetonitrile reacted, almost no expected product was formed In addition, hbpin was used instead of B 2pin 2, and the product was obtained in medium yield Subsequently, the effect of NHC ligands on the boration reaction was also studied The results showed that the boration efficiency of complex 2B was basically unchanged, but the boration efficiency of complex 2C based on alanine was significantly reduced In addition, the [Rh (COD) Cl] 2 or the monodentate NHC ligand (imes) Rh (COD) Cl showed very poor catalytic activity in these reactions, which further proved the importance of chelating NHC ligands Finally, the control experiment shows that rhodium catalyst and light are the necessary conditions for the reaction (image source: angelw Chem Int ed.) next, the author studied the boration application range of 2-arylpyridine substrate 3 (Table 2) Under the optimized reaction conditions, the effects of steric resistance and electrical properties on the reaction efficiency were investigated The substrate has good tolerance and reactivity when the substituents are in the para -, meta - and ortho positions of aryl groups Compared with the electron rich substrate 3G, the reactivity of the electron deficient substrate 3H and 3I decreased slightly 2-phenylpyridine substituted by CF 3 and pH on pyridine ring is also suitable as substrate The substrate 3L with steric hindrance can Boronize 1-naphthyl segments efficiently, and isoquinoline group is also suitable for guiding the ortho boronization of electron rich and electron deficient aromatic segments The prochiral ferrocenyl substrate 3R did not react, and the substrate 3S could realize regioselective C (SP 3) - H boration with high efficiency and low enantioselectivity At last, the expected product 5T can be obtained after substrate 3T participates in pyridine oriented desymmetry (image source: angelw Chem Int ed.) in order to study the reaction mechanism of visible light induced C-H boration, the author has carried out a series of experiments Firstly, in the presence of BHT or tempo, the boration activity of substrate 3B did not decrease significantly, indicating that the reaction did not involve free radical process Then, we combine the optical switch experiment with the kinetic isotope effect cross experiment (scheme 1) When the LED is turned off, the reaction does not take place in the dark, indicating that constant light excitation is essential for this transformation In addition, by measuring and comparing the two reaction rate constants, it is determined that the rate controlling step is the fracture of C-H bond Then, the author irradiates the stoichiometric complex 2a and the substrate 3B's D 8-thf (or C6D6) solution with blue LED, and finds that the rh-h characteristic signal (scheme 2) is generated at δ - 21.7, but not in the CD 3CN solution This indicates that the process is accompanied by the dissociation and isomerization of 1,5-cod ligand to 1,3-cod 2D (1H, 13C) - HMBC shows a strong correlation among hydride ligands, C carbenic and ortho C phenyl atoms, which indicates that the expected ring metallized Rh (III) - hydride 6 may be obtained by oxidation addition of the ortho C-H bond of 3B Although the author has not successfully separated 6, the structural characterization of the RH - (μ - OH) intermediate produced by hydrolysis further confirmed the octahedral geometry of the ring metallized Rh (III) intermediate In addition, in the dark environment, when 3B was added to the pre irradiated D 8-thf solution of complex 2a, no hydride species was detected, which confirmed the photoinduced C-H activation process Similar to Gonzalez Herrero's study on the mechanism of C-H activation in the triplet excited state of Pt (II) complex, the author speculates that metal ligand charge transfer promotes the oxidative ortho-c-h addition, which leads to the cyclization of Rh (III) - hydride intermediate 6, and considers that 6 reacts with B 2pin 2 to obtain hbpin and the corresponding Rh (III)- Boride, which is eliminated by reduction to obtain ortho boration product 4 (picture source: angel Chem Int ed.) conclusion: Olivier BASL é group has developed a regioselective C-H boration method of aromatics catalyzed by RH under visible light This mild C-H bond activation method can be used in other functional groups, including asymmetric transformation.