Design and synthesis of novel chiral dirhodium(II) carboxylate complexes for asymmetric cyclopropanation reactions

A novel approach to the design of dirhodium(II) tetracarboxylates derived from (<em>S</em>)-amino acid ligands is reported. The approach is founded on tailoring the steric influences of the overall catalyst structure by reducing the local symmetry of the ligand's N-heterocyclic tether. The application of the new approach has led to the uncovering of [Rh₂(<em>S</em>-^{<em>tert</em>}PTTL)₄] as a new member of the dirhodium(II) family with extraordinary selectivity in cyclopropanation reactions. The stereoselectivity of [Rh₂(<em>S</em>-^{<em>tert</em>}PTTL)₄] was found to be comparable to that of [Rh₂(<em>S</em>-PTAD)₄] (up to >99 % <em>ee</em>), with the extra benefit of being more synthetically accessible. Correlations based on X-ray structures to justify the observed enantioinduction are also discussed.