1235263-11-2Relevant academic research and scientific papers
Chiral pincer ruthenium and osmium complexes for the fast and efficient hydrogen transfer reduction of ketones
Baratta, Walter,Benedetti, Fabio,Del Zotto, Alessandro,Fanfoni, Lidia,Felluga, Fulvia,Magnolia, Santo,Putignano, Elisabetta,Rigo, Pierluigi
, p. 3563 - 3570 (2010)
A series of chiral HCNN ligands ((S)-1b-g) (S)-2-(1-aminoethyl)-6-(aryl) pyridine (aryl = 4-MeO-phenyl, 1b; 4-CF3-phenyl, 1c; 3,5-di-Me-phenyl, 1d; 3,5-di-CF3-phenyl, 1e; 1-naphthyl, 1f; 2-naphthyl, 1g) were synthesized starting from commercial 2-acetyl-6- bromopyridine (2), by a chemoenzymatic method involving the dynamic kinetic resolution of the corresponding secondary alcohol (rac-3). The conversion of the resulting (R)-3, obtained in 98% ee, into the homochiral amine ((S)-6), followed by Suzuki coupling with the appropriate arylboronic acids 7b-g, gave access to (S)-1b-g, isolated in 97% ee, with an overall yield up to 50%. The in situ generated pincer complexes [MCl(CNN)(PP)] (M = Ru, Os; PP = Josiphos diphosphine), prepared from [MCl2(PPh3)3], (R,S)-Josiphos diphosphines, and the ligands (S)-1b-g, were found to efficiently catalyze the asymmetric transfer hydrogenation of acetophenone in 2-propanol at 60 °C and in the presence of NaOiPr. On the basis of these data, the 2-naphthyl ruthenium and osmium derivatives [RuCl(CNN)((R,S)-Josiphos*] (8) (HCNN = (S)-1g) and [OsCl(CNN)(PP)] (PP = (R,S)-Josiphos, 9, and (R,S)-Josiphos*, 10) were isolated from [MCl2(PPh 3)3], (R,S)-Josiphos diphosphines, and the ligand (S)-1g. Complexes 8 and 10, displaying the correctly matched chiral PP and CNN - ligands, are highly active and productive catalysts for the transfer hydrogenation of alkyl aryl ketones and methyl pyridyl ketones with TOF = 105-106 h-1, using 0.005 mol % of catalysts and achieving up to 99% ee. The comparison of the catalytic activity of these pincer complexes shows that Ru and Os derivatives display similar rate and enantioselectivity.
