Strategic ligand design for modulating axial sites in dirhodium catalysts: towards enhanced asymmetric...

Dirhodium paddlewheel complexes remain central to asymmetric carbene-transfer catalysis. Recent progress in ligand design has highlighted the influence of sterically adaptable bridging frameworks that create chiral pockets around the metal-metal core, enabling high enantiocontrol in C-H functionalization, cyclopropanation, and other diazo-based transformations. While there are many advances in the design of bridging ligands on dirhodium paddlewheel catalysts for asymmetric transformations, axial coordination remains underexplored but may offer an alternative approach for modulating stereoselectivity. Here, we investigate the incorporation of tethered, axially coordinated ligands (TACLs) on chiral dirhodium(II,II) scaffolds as an approach for positioning chiral information closer to the catalytic center through combined steric and non-covalent interactions. Structurally tailored dirhodium complexes bearing varied bridging and chiral pendant axial ligands were synthesized and evaluated in the asymmetric cyclopropanation of styrene using methyl phenyl diazoacetate as the carbene precursor. The results show that fine-tuning the axial coordination sphere impacts both catalytic yield and enantio-differentiation within the systems examined. These findings highlight the potential of tethered axial-site ligands as complementary elements to traditional equatorial modification and provide a foundation for further exploration of axial coordination in dirhodium-catalyzed asymmetric transformations.