title: DFT study of the mechanism and origin of enantioselectivity in chiral BINOL-phosphoric acid catalyzed transfer hydrogenation of ketimine and α-imino ester using benzothiazoline.
abstract: Benzothiazoline is an efficient reducing agent for the chiral BINOL-phosphoric acid catalyzed enantioselective transfer hydrogenation of ketimines and α-imino esters to afford the corresponding amines with high enantioselectivities. DFT studies (M05-2X/6-31G*//ONIOM(B3LYP/6-31G*:HF/3-21G)) revealed the reaction mechanism and the origin of the high enantioselectivity in the present BINOL-phosphoric acid catalyzed transfer hydrogenation of ketimines and α-imino esters using benzothiazoline. The reaction mechanism is similar to that reported in the asymmetric transfer hydrogenation of ketimines using Hantzsch ester. Phosphoric acid simultaneously activates ketimine (α-imino ester) and benzothiazoline to form cyclic transition structures. The high enantioselectivity is attributed to the steric interaction between the substituents at the 3,3'-positions of BINOL-phosphoric acid and substrates. In contrast to the C2-symmetrical Hantzsch ester, the readily tunable 2-aryl substituent of unsymmetrical benzothiazoline plays a significant role in the steric interaction, influencing the asymmetric induction. This feature is responsible for the advantage of benzothiazoline over Hantzsch ester.
authors.affiliation: Department of Chemistry and Research Center for Smart Molecules, Faculty of Science, Rikkyo University, 3-34-1 Nishi-Ikebukuro, Toshima-ku, Tokyo 171-8501, Japan.
authors.forename: Yukihiro
authors.initials: Y
authors.lastname: Shibata
authors.forename: Masahiro
authors.initials: M
authors.lastname: Yamanaka
mesh_terms: D052160:Benzothiazoles / Q000737:chemistry*; D001713:Biphenyl Compounds / Q000737:chemistry*; D002384:Catalysis; D006865:Hydrogenation; D007097:Imines / Q000737:chemistry*; D008958:Models, Molecular; D015394:Molecular Structure; D009570:Nitriles / Q000737:chemistry*; D010084:Oxidation-Reduction; D010756:Phosphoric Acids / Q000737:chemistry*; D011789:Quantum Theory; D013237:Stereoisomerism
chemical_list: D052160:Benzothiazoles; D001713:Biphenyl Compounds; D007097:Imines; D009570:Nitriles; D010756:Phosphoric Acids; C542835:ketimine; C058836:benzothiazoline