Novel chiral bisoxazoline ligands with a biphenyl backbone: Preparation, complexation, and application in asymmetric catalytic reactions

Novel C-2-symmetric chiral bisoxazoline ligands 1 were easily prepared from enantiomerically pure 2-amino alcohols and achiral 2,2'-biphenyldicarboxyl ic acid via the corresponding amide and mesylate as intermediates. Since th ese ligands bear only two ortho-substituents on the biphenyl backbone, the biphenyl axis is not fixed, and the two diastereomers of these ligands exis t in equilibrium in solution. Interestingly, when the ligands 1 were coordi nated with a metal ion, only one of the two possible diastereomer complexes , an (S,aS,S)-complex, can be formed depending on the combination of the li gand and the metal ion. Thus, copper(I) afforded only the (S,aS,S)-complexe s with all ligands 1, while zinc(II), palladium(II), and silver(I) afforded the (S,aS,S)-complexes as the sole product only with Ib, which has a bulky tert-butyl group on the oxazoline ring, and a mixture of the two diastereo mer complexes with 1a,c,d. The copper(I)-catalyzed asymmetric cyclopropanat ion of styrene with diazoacetate proceeded successfully with these ligands and good to excellent enantioselectivities were afforded.