Synthesis of Alkenylboronates from N-Tosylhydrazones through Palladium-Catalyzed Carbene Migratory Insertion
Yifan Ping, Rui Wang, Qianyue Wang, Taiwei Chang, Jingfeng Huo, Ming Lei* and Jianbo Wang*
*College of Chemistry, Beijing University of Chemical Technology, Beijing 100029; College of Chemistry, Peking University, Beijing 100871, China, Email: leimmail.buct.edu.cn, wangjbpku.edu.cn
Y. Ping, R. Wang, Q. Wang, T. Chang, J. Huo, M. Lei, J. Wang, J. Am. Chem. Soc., 2021, 143, 9769-9780.
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A palladium-catalyzed oxidative borylation reaction of readily available N-tosylhydrazones provides di-, tri-, and tetrasubstituted alkenylboronates. This highly efficient and practical reaction offers mild conditions, broad substrate scope, and good functional group tolerance. One-pot Suzuki coupling and other transformations highlight the synthetic utility of the approach.
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General Procedure for Pd-Catalyzed Oxidative Borylation Reactions of N-tosylhydrazones
For the Synthesis of Tetra-substituted Alkenylboronates: An oven-dried 25 mL Schlenk flask containing a magnetic stir bar was charged with Pd(OAc)2 (1.7 mg, 0.0075 mmol, 2.5 mol%), P(m-tol)3 (4.6 mg, 0.015 mol, 5 mol%), 60% NaH (36 mg, 0.9 mmol, 3 equiv), 2,5-DMBQ (61.2 mg, 0.45 mmol, 1.5 equiv), diboron compound (0.45 mmol, 1.5 equiv), and N-tosylhydrazone (0.3 mmol). After degassing and filling with N2, toluene (6 mL) was added to the flask via syringe. The reaction mixture was stirred at 90°C for 10 h and then cooled to room temperature. The mixture was filtered through a short plug of silica gel and washed with Et2O as the eluent. The filtrate was concentrated under reduced pressure and purified by silica gel column chromatography to afford the desired tetra-substituted alkenylboronate.
For the Synthesis of Tri-substituted Alkenylboronates: Pd(OAc)2 (3.4 mg, 0.015 mmol, 5 mol%), PPh3 (7.9 mg, 0.03 mmol, 10 mol%) was used.
For the Synthesis of Cyclic Alkenylboronates: Pd(OAc)2 (6.7 mg, 0.03 mmol, 10 mol%), PPh3 (15.7 mg, 0.06 mmol, 20 mol%) was used, and the reaction was run at 100°C.
For the Synthesis of Di-substituted Alkenylboronates: Pd(OAc)2 (3.4 mg, 0.015 mmol, 5 mol%), PPh3 (7.9 mg, 0.03 mmol, 10 mol%), TMBQ (67.5 mg, 0.45 mmol, 1.5 equiv) was used, and DCE (6 mL) was employed as the solvent. The reaction was run at 100°C.