Boron Chelates Derived from N-Acylhydrazones as Radical Acceptors: Photocatalyzed Coupling of Hydrazones with Carboxylic Acids
Igor A. Dmitriev, Vitalij V. Levin and Alexander D. Dilman*
*N. D. Zelinsky Institute of Organic Chemistry, 119991 Moscow, Leninsky prosp. 47, Russian Federation, Email: adil25mail.ru
I. A. Dmitriev, V. V. Levin, A. D. Dilman, Org. Lett., 2021, 23, 8973-8977.
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Difluoroboryl complexes obtained from N-acyl hydrazones upon brief treatment with boron trifluoride and allylic silane serve as efficient acceptors of alkyl radicals. These boryl chelates react with carboxylic acids in the presence of an acridine-type photocatalyst to provide N-acyl hydrazides.
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A test tube containing hydrazone (0.50 mmol) was evacuated and filled with argon. 1,2-Dichloroethane (1.5 mL), allyltrimethylsilane (129 µL, 0.75 mmol, 1.5 equiv), boron trifluoride etherate (93 µL, 0.75 mmol, 1.5 equiv) were added successively. The tube was placed in a preheated bath at 65°C for 5 min, then the solvent was evaporated under vacuum, and the residue was dried under vacuum for additional 10 min at 65 °C. The heating bath was removed, the tube was cooled to room temperature, and filled with argon. Then, dichloromethane (2 mL), carboxylic acid (1.25 mmol, 2.5 equiv), 2,4,6-collidine (61 mg, 0.50 mmol, 1.0 equiv), and 9-(2-chlorophenyl)acridine (7.3 mg, 5 mol %, 0.025 mmol; or 10 mol %, 0.050 mmol) were added. The tube was closed with a screw cap, and the mixture was irradiated with 400 nm 60W LED. During irradiation the reaction temperature was maintained at 20°C. For the work-up, saturated aqueous NaHCO3 solution (7 mL) was added, and the mixture was extracted with ethyl acetate (8 mL and 2×4 mL). The combined organic phases were washed with brine (10 mL), dried over Na2SO4, concentrated under vacuum, and the residue was purified by column chromatography on silica gel.
hydrazine derivatives, photochemistry