Chiral Oxazolidine‐Fused N‐Heterocyclic Carbene Complexes of Rhodium and Iridium and Their Utility in the Asymmetric Transfer Hydrogenation of Ketones

The catalytic potential of new N‐heterocyclic carbene ligands, derived from a chiral fused bicyclic ring scaffold with restricted rotation along the C–N bond bearing the chiral auxiliary, has been explored in the transition‐metal‐mediated asymmetric transfer hydrogenation reactions of ketones. In particular, the chiral oxazolidine‐fused N‐heterocyclic carbene precursors (3S)‐3‐R‐6‐methyl‐7‐phenyl‐2,3‐dihydroimidazo[5,1‐b]oxazol‐6‐ium iodide [R = sec‐butyl (1f), i‐butyl (2f), isopropyl (3f)] were synthesized from commercially available optically pure amino acids in a multistep sequence that avoids tedious chiral resolution. The reactions of the chiral imidazolium iodide salts 1f–3f with Ag2O yielded the corresponding silver complexes 1g–3g, which were treated with [(COD)MCl]2 (M = Rh, Ir; COD = 1,5‐cyclooctadiene) to afford the rhodium(I) and iridium(I) complexes (1h–3h and 1i–3i, respectively). The rhodium(I) and iridium(I) complexes conveniently catalyze the asymmetric transfer hydrogenation of acetophenones for a wide variety of substrates ranging from electron‐rich ones such as 4‐methylacetophenone, 3,4‐dimethylacetophenone, 4‐tert‐butylacetophenone, and 4‐(methylthio)acetophenone to electron‐deficient ones such as 4‐bromoacetophenone, 4‐chloroacetophenone, 4‐fluoroacetophenone, 4‐nitroacetophenone, and 3‐fluoroacetophenone in moderate‐to‐good yields (ca. 18–95 %) but with low enantioselectivities (ca. 4–41 % ee). The iridium and rhodium complexes of new chiral oxazolidine‐fused N‐heterocyclic carbene ligands effectively catalyze the transfer hydrogenation of ketones with turnover numbers (TONs) of up to 190 and enantiomeric excesses of up to 41 %.