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. Issue 5 (28th November 2014)

• Record Type: Journal Article
• Authors: Kita, Yusuke; Yamaji, Kenta; Higashida, Kosuke; Sathaiah, Kandula; Iimuro, Atuhiro; Mashima, Kazushi
• Is Part Of: Chemistry. Volume 21:Issue 5(2015)
• Page Start: 1915
• Page End: 1927
• Publication Date: 2014-11-28
• Subjects: Chemistry -- Periodicals; 540
• DOI: 10.1002/chem.201405408
• Languages: English
• ISSNs: 0947-6539

Abstract

<abstract abstract-type="main" xml:lang="en"> <title>Abstract</title> <p>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 ¹H and ³¹P{¹H} 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.</p> </abstract>