Mechanistic Studies and Expansion of the Substrate Scope of Direct Enantioselective Alkynylation of α-Ketiminoesters Catalyzed by Adaptable (Phebox)Rhodium(III) Complexes.
Mechanistic studies and expansion of the substrate scope of direct enantioselective alkynylation of α-ketiminoesters catalyzed by adaptable (phebox)rhodium(III) complexes are described. The mechanistic studies revealed that less acidic alkyne rather than more acidic acetic acid acted as a proton source in the catalytic cycle, and the generation of more active (acetato-κ(2)O,O')(alkynyl)(phebox)rhodium(III) complexes from the starting (diacetato)rhodium(III) complexes limited the overall reactivity of the reaction. These findings, as well as facile exchange of the alkynyl ligand on the (alkynyl)rhodium(III) complexes led us to use (acetato-κ(2)O,O')(trimethylsilylethynyl)(phebox)rhodium(III) complexes as a general precatalyst for various (alkynyl)rhodium(III) complexes. Use of the (trimethylsilylethynyl)rhodium(III) complexes as precatalysts enhanced the catalytic performance of the reactions with an α-ketiminoester derived from ethyl trifluoropyruvate at a catalyst loading as low as 0.5 mol % and expanded the substrate scope to unprecedented α-ketiminophosphonate and cyclic N-sulfonyl α-ketiminoesters.
Graduate School of Pharmaceutical Sciences, Kyushu University , Fukuoka 812-8582, Japan.
Kazuhiro
K
Morisaki
Graduate School of Pharmaceutical Sciences, Kyushu University , Fukuoka 812-8582, Japan.
Masanao
M
Sawa
Graduate School of Pharmaceutical Sciences, Kyushu University , Fukuoka 812-8582, Japan.
Ryohei
R
Yonesaki
Graduate School of Pharmaceutical Sciences, Kyushu University , Fukuoka 812-8582, Japan.
Hiroyuki
H
Morimoto
Department of Chemistry, Graduate School of Engineering Science, Osaka University , Toyonaka, Osaka 560-8531, Japan.
Kazushi
K
Mashima
Graduate School of Pharmaceutical Sciences, Kyushu University , Fukuoka 812-8582, Japan.
Takashi
T
Ohshima