Chirality-Switchable 2,2'-Bipyridine Ligands Attached to Helical Poly(quinoxaline-2,3-diyl)s for Copper-Catalyzed Asymmetric Cyclopropanation of Alkenes.

Helical poly(quinoxaline-2,3-diyl)s copolymers PQXbpy consisting of a coordinating unit, which bears a varied achiral, substituted 2,2'-bipyridyl pendant, and a chiral unit bearing chiral side chains were synthesized and used as a ligand in copper-catalyzed asymmetric cyclopropanation of olefins with diazoacetates, giving up to 91:9 er with high chemical yield. The enantioselection relied on the helical structure of PQXbpy . A single PQXbpy afforded either of a pair of enantiomers with high enantioselectivity by switching its helical sense by changing the reaction solvent from pure toluene to a 3/1 mixture of toluene and 1,1,2-trichloroethane.