Chiral 2-pyridyl alcoholate copper complexes: crystal structures of [bis(2-pyridyl alcoholate)copper(II)] and the use of [(2-pyridyl alcoholate)copper(II)]+ and [(2-pyridyl alcoholate)copper(I)] in asymmetric cyclopropanation of styrene and allylic ox

Abstract

Chiral N,O pyridine alcohols HL1–HL6 were used to form complexes with copper(II) ions. Ligands HL1 and HL2 formed complexes with copper(II) ions when Cu(OAc)2 and HL were refluxed in methanol/ethanol mixture. Ligand HL3 formed a complex with copper(II) when deprotonated with NaH and stirred in a Cu(II) acetate THF solution. Ligands HL4–HL6 did not form complexes with copper(II) under similar conditions. Two complexes, [Cu(L1)2] and [Cu(L2)2], were isolated as single crystals and characterized by X-ray crystallography. These complexes showed low catalytic activities in asymmetric reactions. However, they became active when reacted with triflic acid. Copper complexes, [Cu(L)] or [Cu(L)]+, formed in situ by reacting ligands HL with copper(I) or (II) ions, respectively, were also found to be active copper catalysts for asymmetric cyclopropanation of styrene with ethyl diazoacetate and allylic oxidation of cyclohexene with t-butylperoxybenzoate. Enantioselectivities up to 56% and 38% were obtained in asymmetric cyclopropanation of styrene and asymmetric allylic oxidation of cyclohexene, respectively.