New perfluoroalkyl-substituted bisoxazolines as chiral ligands in asymmetric Cu-II-catalyzed reactions

Two new enantiomerically pure perfluoroalkyl-substituted bisoxazolines (F-Box) have been prepared by a reaction sequence that involves formation of the properly functionalized Box followed by introduction of one or two n-C8F17 residues. These ligands were employed for the first time in the Mukaiyama aldol addition of silylketene thioacetals to methylpyruvate promoted by Cu(OTf)(2) (up to 85% ee) and in the ene reaction between a-methylstyrene and ethylglyoxalate in the presence of Cu(OTf)(2) (up to 74% ee). We compared the reactivities of the new ligands with those of two other chiral fluorous bisoxazolines that we have reported previously. In both cases, the F-Box that has the lower fluorine content performed better, in terms of chemical yield and enantioselectivity, than the more-fluorinated one. The recovery and recycling of the ligands is realized either by phase separation of the reaction solvents or by filtration through a short plug of fluorous silica gel.