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

AbstractA 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.