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Molecules 2012, 17(8), 9010-9022; doi:10.3390/molecules17089010

Chiral Zn(II)-Bisamidine Complex as a Lewis-Brønsted Combined Acid Catalyst: Application to Asymmetric Mukaiyama Aldol Reactions of α-Ketoesters

Department of Chemistry and Research Center for Smart Molecules, Rikkyo University, Toshima-Ku, Tokyo 171-8501, Japan
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Received: 11 July 2012 / Revised: 21 July 2012 / Accepted: 21 July 2012 / Published: 30 July 2012
(This article belongs to the Special Issue Stereoselective Synthesis)
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Abstract

Focusing on the steric and electronic properties of the resonance-stabilized amidine framework, a cationic metal-bisamidine complex was designed as a conjugated combined Lewis-Brønsted acid catalyst. The chiral Zn(II)-bisamidine catalyst prepared from the 2,2'-bipyridyl derived bisamidine ligand, ZnCl2, and AgSbF6 promoted asymmetric Mukaiyama aldol reaction of α-ketoester and α,α-disubstituted silyl enol ether to afford the α-hydroxyester having sequential quarternary carbons in good yield, albeit with low enantioselectivity. Addition of 1.0 equivalent of the fluoroalcohol having suitable acidity and bulkiness dramatically increased the enantioselectivity (up to 68% ee). DFT calculations suggested that this additive effect would be caused by self-assembly of the fluoroalcohol on the Zn(II)-bisamidine catalyst.
Keywords: Lewis-Brønsted combined acid; bisamidine; asymmetric Mukaiyama aldol reaction; α-ketoester Lewis-Brønsted combined acid; bisamidine; asymmetric Mukaiyama aldol reaction; α-ketoester
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This is an open access article distributed under the Creative Commons Attribution License (CC BY 3.0).

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MDPI and ACS Style

Gotoh, R.; Yamanaka, M. Chiral Zn(II)-Bisamidine Complex as a Lewis-Brønsted Combined Acid Catalyst: Application to Asymmetric Mukaiyama Aldol Reactions of α-Ketoesters. Molecules 2012, 17, 9010-9022.

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