The Mg-oppenauer oxidation as a mild method for the synthesis of aryl and metallocenyl ketones

Abstract

Magnesium alkoxides undergo a hydride-transfer oxidation with benzaldehyde as the oxidant. This magnesium variant of the Oppenauer oxidation was used for the synthesis of polyfunctional biaryl ketones. LiCl was found to promote this reaction by enhancing the solubility of magnesium alkoxides. This mild oxidation method was especially useful for preparing ketones bearing a metallocenyl unit as well as various new ferrocenyl ketones and tricarbonylchromium complexes. This last class of ketones was reduced with the CBS catalyst (CBS = Corey-Bakshi-Shibata, diphenyl oxazaborolidine) to chiral benzhydrol complexes with high enantioselectivity enabling an asymmetric synthesis of electronrich or -poor benzhydryl alcohols (up to 94% ee). © 2007 Wiley-VCH Verlag GmbH & Co. KGaA.