C(sp3)–H Alkenylation Catalyzed by Cationic Alkylhafnium Complexes: Stereoselective Synthesis of Trisubstituted Alkenes from 2,6-Dimethylpyridines and Internal Alkynes

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• 作者：Michael J. Lopez ; Ai Kondo ; Haruki Nagae ; Koji Yamamoto ; Hayato Tsurugi ; Kazushi Mashima
• 刊名：Organometallics
• 出版年：2016
• 出版时间：November 28, 2016
• 年：2016
• 卷：35
• 期：22
• 页码：3816-3827
• 全文大小：772K
• ISSN：1520-6041

文摘

Dibenzylhafnium complexes 3a–d, supported by dianionic bidentate or tridentate ligands, upon activation via abstraction by either [Ph₃C][B(C₆F₅)₄] or B(C₆F₅)₃ served as catalysts for the C(sp³)–H alkenylation of 2,6-dimethylpyridines with dialkylalkynes to give corresponding C(sp³)–H alkenylated products 6. Complex 3c, containing a pyridine arm in the ligand skeleton, exhibited the highest catalytic activity among 3a–d; initial addition of 2,6-dimethylpyridine (4a) to the C₆D₅Br solution of 3c followed by [Ph₃C][B(C₆F₅)₄] and 3-hexyne (5a) produced trisubstituted alkene 6aa in stereoselective manner in up to 50% yield without any byproducts, while the addition of 5a prior to 4a and [Ph₃C][B(C₆F₅)₄] to the C₆D₅Br solution of 3c generated 6aa, together with the formation of byproduct (E)-(2-ethylpent-2-en-1-yl)benzene (7). When an asymmetrical pyridine, 3-bromo-2,6-dimethylpyridine, was used as the coupling partner, the corresponding trisubstituted alkene was obtained selectively. Catalytically active cationic benzylhafnium complexes 8a–d, which were prepared by the reactions of 3a–d and B(C₆F₅)₃, respectively, were characterized by ¹H, ¹³C, and ¹⁹F NMR spectroscopy. Kinetic studies of the catalytic reaction between 4a and 4-octyne (5b) using 3c and [Ph₃C][B(C₆F₅)₄] in C₆D₅Br revealed that the catalytic reaction was zero-order for both 4a and 5b, indicating that the rate-determining step involved the C(sp³)–H bond activation of 4a by vinylhafnium intermediate 11c.