Novel Enantiomerically Pure Heteroleptic Magnesium Complexes for Use in Enantioselective Deprotonation Reactions.

Abstract

Enantioselective synthesesEnantioselective syntheses O 0031 Novel Enantiomerically Pure Heteroleptic Magnesium Complexes for Use in Enantioselective Deprotonation Reactions. -Chiral magnesium amides such as (III) are easily prepared and used as efficient bases in the enantioselective deprotonation of 4-substituted cyclohexanones. -(CARSWELL, E. L.; HAYES, D.; HENDERSON*, K. W.; KERR*, W. J.; RUSSELL, C. J.; Synlett 2003, 7, 1017-1021; Dep. Pure Appl. Chem., Univ. Strathclyde, Glasgow G1 1XL, UK; Eng.) -Mais 41-038

Magnesium Catalysis in Asymmetric Synthesis

“…A typical reaction that can be used to compare different in-situ-generated magnesium complexes prepared by bi-H-bond or mono-H-bond methods is the enantioselective deprotonation of ketones to generate chiral enolate silyl ethers developed by Kerr, Henderson, and coworkers. [85][86][87][88][89] As summarized in Scheme 41, the enantioselective deprotonation of cyclohexanone can be achieved by different magnesium catalysts with either bi-H-bond or mono-H-bond ligands or a combination of the two ligand methods. The addition of the second ligand in the combinational strategies using magnesium catalysts resulted in very different catalytic activities.…”

“…Although the utilization of a phenol as the second ligand did not improve the enantioselective deprotonation reaction, this proposed strategy implies that the addition of another Brønsted acid to form a combinational magnesium catalyst would dramatically change the properties of the Mg(II) complex, and this could be an alternative strategy for tuning the catalyst's activity. [85][86][87][88][89] A classic example of employing a second covalent ligand in a magnesium catalyst to achieve high enantioselectivity was reported by Trost and coworkers. 90,91 Using their ProPhenol ligand, a magnesium-catalyzed direct asymmetric Aldol addition of ethyl diazoacetate to aldehydes was achieved.…”