SupraBox: a New Class of 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 containing five differently substituted oxazoline nuclei, four linkers and three 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 (ee up to 86% and selectivity (s) = 28) and in the desymmetrization of meso-hydrobenzoin (ee up to 88%).