image: Activation of carbon-nitrogen bond in aniline. view more
Credit: ©Science China Press
This study is led by Prof. Xuefeng Jiang (Institute Shanghai Key Laboratory of Green Chemistry and Chemical Process, School of Chemistry and Molecular Engineering, East China Normal University).
Carbon-nitrogen bond activation is an intractable challenge among the inert chemical bonds, due to the high bond dissociation energy [BDE (Ph-NH2) = 102.6 ± 1.0 kcal/mol], the intense coordinationg ability [aTM (amines) = 0-1.9 vs aTM (ethers) = -2.5-0.1], and inferior leaving ability [pKa (-NH2) = 36]. Conventionally, prefunctionalization is the essential solution for carbon-nitrogen bond transformation in anilines. The team of Jiang realized the carbon-nitrogen bond activation, which affords phenols from protogenetic anilines via uranyl photoredox catalysis with water in the air at room temperature, enabled the conversion of 40 of protogenetic anilines, 8 of N-substituted anilines, and 9 of anilines-containing natural products/pharmaceuticals. Gram-scale amplification and hundredfold efficiency of flow operation demonstrated the great potential for industrial synthetic application.
The detailed mechanism study was carried out to demonstrate the process of carbon-nitrogen activation: (1) The result of radical trapping experiments, UV-visible experiments, Stern-Volmer analysis and 1H NMR tracking experiments show that active uranyl species, excited by blue light (460 nm) through the process of ligand-to-metal charge transfer (LMCT), was quenched by aniline/TFA complex via single electron transfer. (2) 18O labelling and 15N tracking unambiguously depicted that the oxygen source came from water and the amino group leaved as ammonium salt. (3) Based on the result of other photocatalysts (Control experiments and Stern-Volmer analysis) and previous work (JACS Au 2021, 1, 1141.), uranyl peroxide complexes was obtained from uranyl photolysis of water, which is responsible for the oxygen atom transfer. (4) Both on-off experiments and quantum yield experiments elucidated the existence of a radical chain propagation process during the transformation.
The team of Jiang developed the strategy of carbon-nitrogen bond activation from undecorated anilines to phenols: (1) This is the first time to realized carbon-nitrogen bond activation in protogenetic anilines, which provides a new method for the activation of other inert bonds. (2) Carbon-nitrogen bond activation under uranyl photoredox catalysis through the synergetic mechanism of single electron transfer and oxygen atom transfer, which provides the direction about the development and application of new catalysts.
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See the article:
From Aniline to Phenol: Carbon-Nitrogen Bond Activation via Uranyl Photoredox Catalysis
https://doi.org/10.1093/nsr/nwab156
Journal
National Science Review