Enantioselective transition-metal catalysis via an anion-binding approach

Asymmetric transition-metal catalysis represents a powerful strategy for accessing enantiomerically enriched molecules(1-3). The classical strategy for inducing enantioselectivity with transition-metal catalysts relies on direct complexation of chiral ligands to produce a sterically constrained reactive metal site that allows formation of the major product enantiomer while effectively inhibiting the pathway to the minor enantiomer through steric repulsion(4). The chiral-ligand strategy has proven applicable to a wide variety of highly enantioselective transition-metal-catalysed reactions, but important scenarios exist that impose limits to its successful adaptation. Here, we report a new approach for inducing enantioselectivity in transition-metal-catalysed reactions that relies on neutral hydrogen-bond donors (HBDs)(5,6) that bind anions of cationic transition-metal complexes to achieve enantiocontrol and rate enhancement through ion pairing together with other non-covalent interactions(7-9). A cooperative anion-binding effect of a chiral bis-thiourea HBD is demonstrated to lead to high enantioselectivity (up to 99% enantiomeric excess) in intramolecular ruthenium-catalysed propargylic substitution reactions(10). Experimental and computational mechanistic studies show the attractive interactions between electron-deficient arene components of the HBD and the metal complex that underlie enantioinduction and the acceleration effect.