Journal
CHEMISTRY-A EUROPEAN JOURNAL
Volume 15, Issue 3, Pages 726-732Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/chem.200802112
Keywords
asymmetric catalysis; chiral resolution; hydrogen transfer; hydrogenation; ruthenium
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Chiral pincer ruthenium complexes of formula [RuCl(CNN)(Josiphos)] (2-7: Josiphos= 1-[1-(dicyclohexylphosphano)ethyl]-2-(diarylphos- phano)ferrocene) have been prepared by treating [RuCl2(PPh3)(3)] with (S,R)-Josiphos diphosphanes and 1-substituted-1-(6-arylpyridin-2-yl)methanamines (HCNN; substituent=H (1a), Me (1b), and tBu (1c)) with NEt3. By using 1b and 1c as a racemic mixture, complexes 4-7 were obtained through a diastereoselective synthesis promoted by acetic acid. These pincer complexes, which display correctly matched chiral PP and CNN ligands, are remarkably active catalysts for the asymmetric reduction of alkyl aryl ketones in basic alcohol media by both transfer hydrogenation (TH) and hydrogenation (HY) achieving enantioselectivities of up to 99%. In 2-propanol, the enantioselective TH of ketones was accomplished by using a catalyst loading as low as 0.002 mol% and afforded a turnover frequency (TOF) of 10(6) h(-1) h(-1) (60 and 82 degrees C). In methanol/ethanol mixtures, the CNN pincer complexes catalyzed the asymmetric HY of ketones with H, (5 atm) at 0.01 mol%, relative to the complex with a TOF of approximate to 10(4) h(-1) at 40 degrees C.
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