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Issue 45, 2015
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P-Trifluoromethyl ligands derived from Josiphos in the Ir-catalysed hydrogenation of 3,4-dihydroisoquinoline hydrochlorides

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Abstract

The synthesis of mono P-trifluoromethyl and therefore P-stereogenic Xyliphos-derived ligands 5 and their application in the Ir-catalyzed enantioselective hydrogenation of 1-substituted 3,4-dihydroisoquinolinium species (DHIQ) are reported. The ligands were prepared following previous procedures involving the reaction of a bistrifluoromethylphosphine with lithiated (R)-Ugi amine 1. Chloroiridium(I) cyclooctadiene precatalysts containing these new partially electron-poor ligands 9 were found to be poorly active in the hydrogenation of free 1-phenyl-3,4-dihydroisoquinoline 12a. However, the corresponding hydrochloride 12a·HCl was smoothly reduced at 55–60 °C and 100 bar hydrogen pressure. The (SP)-configured ligand (SP)-5 yielded significantly higher enantioselectivity in hydrogenation experiments than its P-stereoisomeric counterpart (RP)-5. These new ligands were subsequently applied in the hydrogenation of a series of different 1-substituted 3,4-DHIQ chlorides 12a–l·HCl. Good to excellent enantioselectivity was observed for substrates bearing relatively large substituents in position 1, reaching 96% ee for 1-Ph-DHIQ chloride 12a·HCl without the help of any additives. Furthermore, an interesting counter ion effect was found with chloride being the best and hexafluorophosphate being very detrimental to enantioselectivity.

Graphical abstract: P-Trifluoromethyl ligands derived from Josiphos in the Ir-catalysed hydrogenation of 3,4-dihydroisoquinoline hydrochlorides

  • This article is part of the themed collection: Fluorine
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Publication details

The article was received on 28 May 2015, accepted on 29 Jun 2015 and first published on 30 Jun 2015


Article type: Paper
DOI: 10.1039/C5DT02019K
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Citation: Dalton Trans., 2015,44, 19566-19575
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    P-Trifluoromethyl ligands derived from Josiphos in the Ir-catalysed hydrogenation of 3,4-dihydroisoquinoline hydrochlorides

    R. Schwenk and A. Togni, Dalton Trans., 2015, 44, 19566
    DOI: 10.1039/C5DT02019K

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