title: Enhancing effects of salt formation on catalytic activity and enantioselectivity for asymmetric hydrogenation of isoquinolinium salts by dinuclear halide-bridged iridium complexes bearing chiral diphosphine ligands.
abstract: Asymmetric hydrogenation of 1- and 3-substituted and 1,3-disubstituted isoquinolinium chlorides using triply halide-bridged dinuclear iridium complexes [{Ir(H)(diphosphine)}2 (μ-Cl)3 ]Cl has been achieved by the strategy of HCl salt formation of isoquinolines to afford the corresponding chiral 1,2,3,4-tetrahydroisoquinolines (THIQs) in high yields and with excellent enantioselectivities after simple basic work-up. The effects of salt formation have been investigated by time-course experiments, which revealed that the generation of isoquinolinium chlorides clearly prevented formation of the catalytically inactive dinuclear trihydride complex, which was readily generated in the catalytic reduction of salt-free isoquinoline substrates. Based on mechanistic investigations, including by (1) H and (31) P{(1) H} NMR studies and the isolation and characterization of several intermediates, the function of the chloride anion of the isoquinolinium chlorides has been elucidated, allowing us to propose a new outer-sphere mechanism involving coordination of the chloride anion of the substrates to an iridium dihydride species along with a hydrogen bond between the chloride ligand and the N-H proton of the substrate salt.
authors.affiliation: Department of Chemistry, Graduate School of Engineering Science, Osaka University, CREST, Toyonaka, Osaka 560-8531 (Japan).
authors.forename: Yusuke
authors.initials: Y
authors.lastname: Kita
authors.forename: Kenta
authors.initials: K
authors.lastname: Yamaji
authors.forename: Kosuke
authors.initials: K
authors.lastname: Higashida
authors.forename: Kandula
authors.initials: K
authors.lastname: Sathaiah
authors.forename: Atuhiro
authors.initials: A
authors.lastname: Iimuro
authors.forename: Kazushi
authors.initials: K
authors.lastname: Mashima
keywords: asymmetric catalysis; hydrogenation; iridium; nitrogen heterocycles; reaction mechanisms