Chiral copper–bipyridine complexes: Synthesis, characterization and mechanistic studies on asymmetric cyclopropanation

Chiral bipyridine ligands of different steric properties when reacted with CuCl2 formed orange, yellow or green solids of new copper(II) complexes, [Cu(L)Cl2] (L = L2–6), in good yield. Together with [Cu(L1)Cl2], these complexes were characterized in solution by UV–Vis spectroscopy and cyclic voltammetry. The complexes give d–d transitions between 860 and 970 nm, and exhibit one quasi-reversible Cu(II)/Cu(I) couple between +0.405 V and +0.516 V versus NHE. Two of the copper(II) complexes, [Cu(L5)Cl2] and [Cu(L6)Cl2], and a copper(I) complex of L1, [Cu(L1)Cl], were characterized by X-ray crystallography. The triflate derivatives of both the Cu(I) and Cu(II) complexes are active catalysts towards the cyclopropanation of ethyl diazoacetate with styrene. The asymmetric induction suffers when the size difference between the alkyl and alkoxyl groups was minimized. The mechanism of the cyclopropanation was studied with kinetic and competition experiments. The rate is first order in catalyst and ethyl diazoacetate, inverse order with styrene and is strongly affected by the counterion.

Graphical abstractChiral bipyridine ligands with different steric properties when reacted with CuCl2 formed orange, yellow or green solids of new copper(II) complexes [Cu(L)Cl2] (L = L2–6) in good yield. They were characterized in solution by UV–Vis spectroscopy and cyclic voltammetry. The triflate derivatives of both the Cu(I) and Cu(II) complexes are active catalysts towards the cyclopropanation of ethyl diazoacetate with styrene. The mechanism of the cyclopropanation was studied with kinetic and competition experiments.Figure optionsDownload full-size imageDownload as PowerPoint slideResearch highlights► Chiral bipyridine ligands with different steric properties were reacted with CuCl2 to form new copper(II) complexes in good yield. ► Two of the copper(II) complexes and a copper(I) complex were characterized by X-ray crystallography. ► The triflate derivatives of all these complexes are active catalysts towards the cyclopropanation of ethyl diazoacetate with styrene. ► The cyclopropanation rate is first order in catalyst and ethyl diazoacetate, inverse order with styrene and is strongly affected by the counterion.