Bisoxazolines with one and two sidearms: stereodirecting ligands for copper-catalysed asymmetric allylic oxidations of alkenes.
A series of sidearm functionalized bisoxazoline ligands has been synthesized by reaction of the monolithiated methyl{bis(oxazolinyl)}methane with the appropriate electrophiles, and tested in the copper catalyzed asymmetric allylic oxidation of cyclohexene ("Kharasch-Sosnovski" reaction). The observed enantioselectivities were higher (up to 85% ee) than for the unfunctionalized bisoxazoline ("BOX") derivatives (ca. 60% ee). Regardless of the functional groups incorporated into the sidearm unit, the ee's obtained for the different derivatives were essentially indistinguishable. This implies that the sidearms do not interfere directly in this reaction and only play an indirect role by virtue of their steric demand. Three of the copper complexes have been characterized by X-ray diffraction, establishing a distorted octahedral coordination geometry around the copper atom in all three cases. In the elongated distorted CuN(2)O(4) octahedra, the two nitrogen atoms of the oxazolines and one oxygen atom of each acetate ligand occupy the 'equatorial' positions whereas the sidearms do not interact with the metal centres.