Asymmetric Transfer Hydrogenation of α,β-Unsaturated Carbonyl Compounds to Saturated Alcohols as Catalyzed by Iridium Complexes of Tricyclic Bioxazoline-Fused Imidazole-Derived N-Heterocyclic Carbene Ligands

Abstract

A series of chiral iridium(I) (1-3)e complexes of bioxazoline fused imidazole derived N-heterocyclic carbene (NHC) ligands successfully carried out asymmetric transfer hydrogenation of α, β-unsaturated ketones in good to excellent yields (ca. 36-91%) and in low enantioselectivites (ca. 5-31% ee) at 1 mol % of the catalyst (1-3)e loading in the presence of NaOH as a base in i-PrOH at 75° C in 3 hours of reaction time. The iridium(I) (1-3)e complexes were synthesized directly from the bioxazoline fused imidazole derived NHC precursors namely {(3R,7R)-3,7-di-R-2,3,7,8-tetrahydrodioxazolo[3,2-c:3′,2′-e]imidazol-4-ium trifluoromethanesulfonate, R=s-butyl (1 d), i-butyl (2 d), i-propyl (3 d)} by the treatment with {(COD)IrCl}2 (COD=1,5-cyclooctadiene) in presence of t-BuOK as a base at room temperature. The chiral bioxazoline fused imidazole NHC ligand precursors (1-3)d were synthesized from the commercially available optically pure amino acids by a sequence of reactions without requiring any chiral resolution.