Synthesis of chiral C2-symmetric binucleating ligands

The synthesis of a series of chiral enantiomerically pure C-2-symmetric binucleating ligands is reported. Ligands of type 1-4, which consist of a phenolic or heterocyclic unit bridging two chiral dihydrooxazole rings, are readily accessible from chiral amino alcohols. Ligands 5a and 5b are composed of a cyclic urea or thiourea unit, respectively, and two 3,4-dihydro-2H-pyrrole rings containing a stereogenic center next to the iv-atom. Compounds of this type are readily assembled from ethane-1,2-diamine and an imidothioic ester derived from pyroglutamic acid. These new ligands, which can coordinate two metals in close proximity to each other, are of interest regarding possible applications in asymmetric catalysis.