Coupling Reaction of Enol Derivatives with Silyl Ketene Acetals Catalyzed by Gallium Trihalides

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• 作者：Dr. Yoshihiro Nishimoto ; Yuji Kita ; Hiroki Ueda ; Hiroto Imaoka ; Kouji Chiba ; Prof. Dr. Makoto Yasuda and Prof. Dr. Akio Baba
• 刊名：Chemistry - A European Journal
• 出版年：2016
• 出版时间：August 8, 2016
• 年：2016
• 卷：22
• 期：33
• 页码：11837-11845
• 全文大小：764K
• ISSN：1521-3765

文摘

A cross-coupling reaction between enol derivatives and silyl ketene acetals catalyzed by GaBr₃ took place to give the corresponding α-alkenyl esters. GaBr₃ showed the most effective catalytic ability, whereas other metal salts such as BF₃⋅OEt₂, AlCl₃, PdCl₂, and lanthanide triflates were not effective. Various types of enol ethers and vinyl carboxylates as enol derivatives are amenable to this coupling. The scope of the reaction with silyl ketene acetals was also broad. We successfully observed an alkylgallium intermediate by using NMR spectroscopy, suggesting a mechanism involving anti-carbogallation among GaBr₃, an enol derivative, and a silyl ketene acetal, followed by syn-β-alkoxy elimination from the alkylgallium. Based on kinetic studies, the turnover-limiting step of the reaction using a vinyl ether and a vinyl carboxylate involved syn-β-alkoxy elimination and anti-carbogallation, respectively. Therefore, the leaving group had a significant effect on the progress of the reaction. Theoretical calculations analysis suggest that the moderate Lewis acidity of gallium would contribute to a flexible conformational change of the alkylgallium intermediate and to the cleavage of the carbon−oxygen bond in the β-alkoxy elimination process, which is the turnover-limiting step in the reaction between a vinyl ether and a silyl ketene acetal.