1,1’-Binaphthylazepine-based Ligands for Asymmetric Catalysis. Part II: New Aminoalcohols as Chiral Ligands in the Enantioselective Addition of ZnEt2 to Aromatic Aldehydes

The new enantiopure 1,2-aminoalcohols 1b-1h having 1,1′-binaphthylazepine skeleton have been tested as catalytic precursors in the enantioselective addition of ZnEt2 to benzaldehyde. The best results were seen with ligand 1d, which owes its chirality only to the atropisomerism of the binaphthyl nucleus and does not have any stereogenic carbon atom. In the presence of 1d benzaldehyde was quickly and cleanly transformed to (S)-1-phenylpropanol in 99% yield and 87% e.e. The same ligand was also used in the asymmetric ZnEt2 addition to other aryl aldehydes giving rise to (S)-1-arylpropanols in almost quantitative yields and e.e.s up to 90%.