SupraBox: Chiral Supramolecular Oxazoline Ligands

A new class of oxazoline ligands, named SupraBox, was studied. These ligands possess an additional urea functionality to generate supramolecular bidentate ligands in transition-metal complexes, by the establishment of hydrogen bonds between the urea N-hydrogens of one ligand and the carbonyl oxygen of a second one. A library of 16 SupraBox ligands was prepared using 5 differently substituted oxazoline nuclei, 4 linkers and 3 different urea substituents. The formation of copper(II) and palladium(II) complexes was investigated by MS, UV/Vis and 1H-NMR spectroscopy. The SupraBox library was screened in the copper-catalyzed asymmetric benzoylation of vic-diols. Good selectivities were obtained in the kinetic resolution of racemic hydrobenzoin [up to 86?%?ee and selectivity (s) = 28] and in the desymmetrization of meso-hydrobenzoin (up to 88?%?ee).