Asymmetric Transfer Hydrogenation of Arylketones Catalyzed by Enantiopure Ruthenium(II)/Pybox Complexes Containing Achiral Phosphonite and Phosphinite Ligands.

A family of complexes of the formula trans-[RuCl2(L)(R-pybox)] (R-pybox = (S,S)-iPr-pybox, (R,R)-Ph-pybox, L = monodentate phosphonite, PPh(OR)2, and phosphinite, L = PPh2(OR), ligands) were screened in the catalytic asymmetric transfer hydrogenation of acetophenone, observing a strong influence of the nature of both the R-pybox substituents and the L ligand in the process. The best results were obtained with complex trans-[RuCl2{PPh2(OEt)}{(R,R)-Ph-pybox}] (2c), which provided high conversion and enantioselectivity (up to 96% enantiomeric excess, e.e.) for the reduction of a variety of aromatic ketones, affording the (S)-benzylalcohols.

Synthesis of Silver(I) and Gold(I) Complexes Containing Enantiopure Pybox Ligands. First Assays on the Silver(I)-Catalyzed Asymmetric Addition of Alkynes to Imines.

Dinuclear complexes [Ag2(CF3SO3){(S,S)-(i)Pr-pybox}2][CF3SO3] (1), [Ag2(R-pybox)2][X]2 [R-pybox = 2,6-bis[4-(S)-isopropyloxazolin-2-yl]pyridine (S,S)-(i)Pr-pybox and X = PF6 (2) and BF4 (3); R-pybox = 2,6-bis[(3aS,8aR)-8,8a-dihydro-3aH-indeno[1,2-d]oxazol-2-yl]pyridine (3aS,3a'S,8aR,8a'R)-indane-pybox and X = CF3SO3 (4)], [Ag2{(S,S)-(i)Pr-pybox}{(3aS,3a'S,8aR,8a'R)-indane-pybox}][CF3SO3]2 (5), and [Ag2(R-pybox)3][X]2 [R-pybox = (3aS,3a'S,8aR,8a'R)-indane-pybox and X = CF3SO3 (10), SF6 (11), and PF6 (12)] as well as mononuclear complexes [Ag(R-pybox)2][X] [R-pybox = (S,S)-(i)Pr-pybox and X = SbF6 (6), PF6 (7), and BF4 (8); R-pybox = (3aS,3a'S,8aR,8a'R)-indane-pybox) and X = BF4 (9)] have been prepared by the reaction of the corresponding silver salts and pybox ligands using the appropriate molar ratio conditions. The first gold(I)/pybox complex [Au6Cl4{(S,S)-(i)Pr-pybox}4][AuCl2]2 (13) has been synthesized by the reaction of [AuCl{S(CH3)2}] and (S,S)-(i)Pr-pybox (1:1 molar ratio) in acetonitrile. The structures of the dinuclear (1, 4, 5, 10, and 11) and mononuclear (6 and 9) silver complexes and the hexanuclear gold complex 13 have been determined by single-crystal X-ray diffraction analysis. These studies have been complemented with a solution-state study by NMR spectroscopy, which included structure elucidation, variable-temperature measurements, and diffusion studies using diffusion-ordered spectroscopy (DOSY; for complexes 1, 4, 10, and 12). Complexes 1, 2, 4, and 10 have been assayed as catalysts in the asymmetric addition of phenylacetylene to N-benzylideneaniline.