478282-20-1Relevant articles and documents
Development of bifunctional salen catalysts: Rapid, chemoselective alkylations of α-ketoesters
DiMauro, Erin F.,Kozlowski, Marisa C.
, p. 12668 - 12669 (2002)
Lewis acid-Lewis base salen complexes have been identified as highly efficient catalysts for the addition of dialkylzincs to α-ketoesters. In contrast to aldehydes or ketones, the reaction between diethylzinc and α-ketoesters is significant in the absence of catalyst. In the presence of catalyst, the reaction rate is increased over 100-fold relative to the background. Furthermore, the reduction product, which is a major coproduct with other catalysts, is not observed with these bifunctional salens. As a result, high yields of the addition products can be obtained (57-99%). Both the Lewis acid and Lewis base portions of the catalyst are critical to the reactivity and selectivity. The two separate portions of the catalyst have been shown to function in a cooperative manner. Copyright
Mechanism and scope of salen bifunctional catalysts in asymmetric aldehyde and α-ketoester alkylation
Fennie, Michael W.,DiMauro, Erin F.,O'Brien, Erin M.,Annamalai, Venkatachalam,Kozlowski, Marisa C.
, p. 6249 - 6265 (2007/10/03)
Metal complexes of C2-symmetric Lewis acid/Lewis base salen ligands provide bifunctional activation resulting in rapid rates in the enantioselective addition of diethylzinc to aldehydes (up to 92% ee). Further experiments probed the reactivity of the individual Lewis acid and Lewis base components of the catalyst and established that both moieties are essential for asymmetric catalysis. These catalysts are also effective in the asymmetric addition of diethylzinc to α-ketoesters. This finding is significant because α-ketoesters alone serve as their own ligands to accelerate racemic 1,2-carbonyl addition of Et2Zn and racemic carbonyl reduction. The latter proceeds via a metalloene pathway, and often accounts for the predominant product. Singular Lewis acid catalysts do not accelerate enantioselective 1,2-addition over these two competing paths. The bifunctional amino salen catalysts, however, rapidly provide enantioenriched 1,2-addition products in excellent yield, complete chemoselectivity, and good enantioselectivity (up to 88% ee). A library of the bifunctional amino salens was synthesized and evaluated in this reaction. The utility of the α-ketoester method has been demonstrated in the synthesis of an opiate antagonist.
