Enantioselectivity Induced by Stereoselective Interlocking: A Novel Core Motif for Tropos Ligands

Well-defined supramolecular interactions are a powerful tool to control the stereochemistry of a catalytic reaction. In this paper, we report a novel core motif for fluxional 2,2'-biphenyl ligands carrying (S)-amino acid-derived interaction sites in 5,5'-position that cause spontaneous enrichment of the R_ax rotamer. The process is based on strong non-covalent interlocking between interaction sites, which causes diastereoselective formation of a supramolecular ligand dimer, in which the axial chirality of the two subunits is dictated by the stereochemical information in the amino acid residues. The detailed structure of the dimer was elucidated by NMR spectroscopy and single-crystal X-ray analysis. Three different phosphorus-based ligand types, namely a bisphosphine, a bisphosphinite and a phosphoramidite were synthesized and characterized. Whereas the first one was found to exist in a strongly weighted equilibrium, the two others each exhibited stereoconvergent behavior transforming into the diastereopure R_ax rotamer. Enriched ligands were used in rhodium-mediated asymmetric hydrogenation reactions of prochiral olefins in which very high enantioselectivities of up to 96:4 were achieved.