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).