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Coupling Reaction of Enol Derivatives with Silyl Ketene Acetals Catalyzed by Gallium Trihalides
Author(s) -
Nishimoto Yoshihiro,
Kita Yuji,
Ueda Hiroki,
Imaoka Hiroto,
Chiba Kouji,
Yasuda Makoto,
Baba Akio
Publication year - 2016
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201602150
Subject(s) - ketene , silylation , enol , chemistry , acetal , alkoxy group , enol ether , silyl enol ether , medicinal chemistry , catalysis , coupling reaction , lewis acids and bases , alkylation , organic chemistry , photochemistry , alkyl
Abstract A cross‐coupling reaction between enol derivatives and silyl ketene acetals catalyzed by GaBr 3 took place to give the corresponding α‐alkenyl esters. GaBr 3 showed the most effective catalytic ability, whereas other metal salts such as BF 3 ⋅OEt 2 , AlCl 3 , PdCl 2 , 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 GaBr 3 , 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.