Efficient stereoselective synthesis of chiral 3,3′-dimethyl-(2,2′-bipyridine)-diol ligand and applications in FeII-catalysis

Abstract

A seven step synthesis of a chiral 2,2′-bipyridinediol ligand with 3,3′-dimethyl substituents was achieved starting from commercially available materials. The O₂-mediated oxidative homocoupling reaction of a chiral pyridine N-oxide was demonstrated to be the key step to prepare the S,S enantiomer of the title ligand with excellent stereoselectivities, i.e., 99% de and >99.5% ee. An unusual heptacoordination of Fe^II when complexed with the chiral 2,2′-bipyridinediol ligand was highlighted from single crystal diffraction analysis. Steric strain due to the 3,3′-dimethyl groups was revealed from the structural analysis of the obtained Fe^II complex. Asymmetric induction using this chiral 3,3′-dimethyl-(2,2′-bipyridine)-diol ligand was studied in the Mukaiyama aldol and thia-Michael reactions. An increase of chiral induction in the latter one was achieved using the Fe^II catalyst made from newly synthesized ligand vs. Bolm's ligand.