Chiral Pincer Ruthenium and Osmium Complexes for the Fast and Efficient Hydrogen Transfer Reduction of Ketones

Abstract

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(PPh3)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

New N,N,N‐Donors Resulting in Highly Active Ruthenium Catalysts for Transfer Hydrogenation at Room Temperature

“…dipyridine ligand), [6] [RuCl 2 (P) 2 (PN)] (PN = amino-or iminophosphane ligands, oxazolinylferrocenylphosphane), [7] [RuCl 2 (PN) 2 ], [7a,8] [RuCl(η 6 -p-cymene)(PN)][BF 4 ] (PN = phosphole-pyridine), [9] [RuCl(η 6 -p-cymene)(NN)][BF 4 ], [10] [RuCl 2 (NPN)(P)], [11] {RuCl 2 (NPN)}, [12] [RuCl 2 (NNN)(P)] (NPN and NNN = oxazoline-based ligands), [13] [RuCl-(CNN)(PP)] [PP = (R,S)-1-{2-[bis(4-methoxy-3,5-dimethylphenyl)phosphanyl]ferrocenyl}ethyldicyclohexylphos-phane (Josiphos-type diphosphane) and CNN = amine/ aryl/pyridine-based ligands] [14] and [RuCl 2 (PNNP)] systems (PNNP = diphosphane/diamine ligands). [15] Herein, we describe the synthesis and characterization of ruthenium complexes of the type [RuCl 2 (L)(NNN)] (L = PPh 3 or CO), with tridentate dipyrazolpyridines as N,N,Nligands.…”

Catalytic transfer hydrogenation of ketones by ruthenium(II) cyclometallated complex containing para-chloroacetophenone thiosemicarbazone

“…Although several catalytic systems have been reported to support transfer hydrogenation reactions of ketones, a catalyst of this type is novel for its CNS donor, PPh 3 and CO ligand environment. The catalytic activity of the present complex is low when compared to ruthenium complexes containing Schiff base or pincer ligands [11,19]. However, the catalytic efficiency of the complex is better than the other ruthenium complexes in the transfer hydrogenation of acetophenones with respect to catalyst/substrate ratio (1:500) and turn over numbers [20,21].…”

Recent Advances in Osmium-Catalyzed Hydrogenation and Dehydrogenation Reactions

“…17 The best-performing ligand was (S)-44f, which afforded (R)-1-phenylethanol with 87% ee (96% conversion) and TOF = 1.5 × 10 5 h −1 . On this basis, complexes 45 and 46 were prepared from (R,S)-Josiphos …”