Functionalized-arene ruthenium half-sandwich compounds as enantioselective hydrogen transfer catalysts. Crystal structures of [RuCl{TsNCH(R)CH(R)NH2}(η6-C6H5OCH2CH2OH)] (R=H or Ph)

Heterocycle-containing Noyori-Ikariya catalysts for asymmetric transfer hydrogenation of ketones

The synthesis of a range of N-(heterocyclesulfonyl)-functionalised Noyori-Ikariya catalysts is described. The complexes were prepared through a short sequence from C2-symmetric 1,2-diphenylethylene-1,2-diamine (DPEN) and were characterised by a range of methods including X-ray crystallography. The complexes were active catalysts for the asymmetric transfer hydrogenation (ATH) of a range of acetophenone derivatives, giving products of high ee in most cases, with notably good results for ortho-substituted acetophenones.

Asymmetric Hydrogenation of Aryl Perfluoroalkyl Ketones Catalyzed by Rhodium(III) Monohydride Complexes Bearing Josiphos Ligands

The asymmetric hydrogenation of 2,2,2-trifluoroacetophenones and aryl perfluoroalkyl ketones was developed using a unique, well-defined chloride-bridged dinuclear rhodium(III) complex bearing Josiphos-type diphosphine ligands. These complexes were prepared from [RhCl(cod)]₂ , Josiphos ligands, and hydrochloric acid. As catalyst precursors, they allow for the efficient and enantioselective synthesis (up to 99 % ee) of chiral secondary alcohols with perfluoroalkyl groups. This system does not require an activating base for the hydrogenation of 2,2,2-trifluoroacetophenones. Additionally, the enantioselective C=O hydrogenations of 2-phenyl-3-(haloacetyl)-indoles, a class of privileged structures in medicinal chemistry, is reported for the first time.

Enantioselective hydrogenation of cyclic imines catalysed by Noyori–Ikariya half-sandwich complexes and their analogues

Trifluoroacetic acid activates cyclic imines in a new non-air sensitive asymmetric hydrogenation method. New transfer hydrogenation catalysts are demonstrated.

Direct formation of tethered Ru(II) catalysts using arene exchange

An ‘arene exchange’ approach has been successfully applied for the first time to the synthesis of Ru(II)-based ‘tethered’ reduction catalysts directly from their ligands in one step. This provides an alternative method for the formation of known complexes, and a route to a series of novel complexes. The novel complexes are highly active in both asymmetric transfer and pressure hydrogenation of ketones.

Phenylalanine--a biogenic ligand with flexible η6- and η6:κ1-coordination at ruthenium(II) centres

The reaction of (S)-2,5-dihydrophenylalanine 1 with ruthenium(III) chloride yields the μ-chloro-bridged dimeric η(6)-phenylalanine ethyl ester complex 3, which can be converted into the monomeric analogue, η(6):κ(1)-phenylalanine ethyl ester complex 12, under basic conditions. Studies were carried out to determine the stability and reactivity of complexes bearing η(6)- and η(6):κ(1)-chelating phenylalanine ligands under various conditions. Reaction of 3 with ethylenediamine derivatives N-p-tosylethylenediamine or 1,4-di-N-p-tosylethylenediamine results in the formation of monomeric η(6):κ(1)-phenylalanine ethyl ester complexes 14 and 15, which could be saponified yielding complexes 16 and 17 without changing the inner coordination sphere of the metal centre. The structure of η(6):κ(1)-phenylalanine complex 17 and an N-κ(1)-phenylalanine complex 13 resulting from the reaction of 3 with an excess of pyridine were confirmed by X-ray crystallography.

Functionalized arene-ruthenium(II) complexes: dangling vs. tethering side chain

The reactivity of compounds [RuCl2(η(6)-C6H5OCH2CH2OH)(L)] (L = phosphine or phosphite) towards the chloride abstractor AgSbF6 has been investigated. Thus, the treatment of the triphenylphosphite complex [RuCl2(η(6)-C6H5OCH2CH2OH){P(OPh)3}] with one equivalent of AgSbF6 gave rise to the formation of the dinuclear dichloro-bridged species [{Ru(μ-Cl)(η(6)-C6H5OCH2CH2OH){P(OPh)3}}2](2+) as the hexafluoroantimonate salt. On the other hand, the triphenylphosphine analog [RuCl2(η(6)-C6H5OCH2CH2OH)(PPh3)] led, under the same experimental conditions, to the di-ruthenium derivative [{RuCl(η(6)-C6H5OCH2CH2OH)(PPh3)}2(μ-Cl)][SbF6] containing only one Cl-bridge. In sharp contrast, treatment of precursors [RuCl2(η(6)-C6H5CH2CH2CH2OH)(L)] (L = P(OPh)3, PPh3, P(OEt)3) with AgSbF6 resulted in the clean formation of the tethered compounds [RuCl{η(6):κ(1)(O)-C6H5CH2CH2CH2OH}(L)][SbF6]. The differences in reactivity observed have been rationalized by theoretical calculations.

Opportunities offered by chiral η⁶-arene/N-arylsulfonyl-diamine-RuII catalysts in the asymmetric transfer hydrogenation of ketones and imines

Methods for the asymmetric transfer hydrogenation (ATH) of ketones and imines are still being intensively studied and developed. Of foremost interest is the use of Noyori's [RuCl(η⁶-arene)(N-TsDPEN)] complexes in the presence of a hydrogen donor (i-PrOH, formic acid). These complexes have found numerous practical applications and have been extensively modified. The resulting derivatives have been heterogenized, used in ATH in water or ionic liquids and even some attempts have been made to approach the properties of biocatalysts. Therefore, an appropriate modification of the catalyst that suits the specific requirements for the reaction conditions is very often readily available. The mechanism of the reaction has also been explored to a great extent. Model substrates, acetophenone (a ketone) and 6,7-dimethoxy-1-methyl-3,4-dihydroisoquinoline (an imine), are both reduced by this Ru catalytic system with almost perfect selectivity. However, in each case the major product is a different enantiomer (S- for an alcohol, R- for an amine when the S,S-catalyst is used), which demanded an in-depth mechanistic investigation. Full-scale molecular modelling of this system enabled us to visualize the plausible 3D structures of the transition states, allowing the proposition of a viable explanation of previous experimental findings.

Mononuclear ruthenium(III) complexes containing chelating thiosemicarbazones: synthesis, characterization and catalytic property

Mononuclear ruthenium(III) complexes of the type [RuX(EPh(3))(2)(L)] (E=P or As; X=Cl or Br; L=dibasic terdentate dehydroacetic acid thiosemicarbazones) have been synthesized from the reaction of thiosemicarbazone ligands with ruthenium(III) precursors, [RuX(3)(EPh(3))(3)] (where E=P, X=Cl; E=As, X=Cl or Br) and [RuBr(3)(PPh(3))(2)(CH(3)OH)] in benzene. The compositions of the complexes have been established by elemental analysis, magnetic susceptibility measurement, FT-IR, UV-vis and EPR spectral data. These complexes are paramagnetic and show intense d-d and charge transfer transitions in dichloromethane. The complexes show rhombic EPR spectra at LNT which are typical of low-spin distorted octahedral ruthenium(III) species. All the complexes are redox active and display an irreversible metal centered redox processes. Complex [RuCl(PPh(3))(2)(DHA-PTSC)] (5) was used as catalyst for transfer hydrogenation of ketones in the presence of isopropanol/KOH and was found to be the active species.

Bifunctional Amine-Tethered Ruthenium(II) Arene Complexes Form Monofunctional Adducts on DNA

The tethered RuII half-sandwich complexes [eta(6):eta(1)-C(6)H(5)(CH(2))(n)NH(2))RuCl(2)] 1 (n = 3) and 2 (n = 2) have been synthesized as potential bifunctional anticancer complexes, and their X-ray crystal structures have been determined. They hydrolyze rapidly in aqueous solution to give predominantly mono-aqua mono-chlorido species. Mono-9EtG adducts, where 9EtG = 9-ethylguanine, form rapidly, but the second 9EtG binds more slowly and more weakly. In the X-ray crystal structure of the di-9EtG adduct [(eta(6):eta(1)-C(6)H(5)(CH(2))(3)NH(2))Ru(9EtG)2](CF(3)SO(3))(2).H(2)O (8.H(2)O), one of the Ru-N7 bonds is significantly longer than the other (2.1588(18) vs 2.101(2) A). The bound guanine bases adopt a head-to-head configuration, stabilized by tether NH2 hydrogen bonding to C6O of 9EtG. The X-ray crystal structure of the dinitrato complex [(eta(6):eta(1)-C(6)H(5)(CH(2))(3)NH(2))Ru(NO(3))(2)] (3) showed both nitrates to be bound to ruthenium. This complex readily rutheniated calf thymus DNA but failed to produce stop sites on pSP73KB plasmid DNA during DNA transcription by an RNA polymerase. This suggested that only monofunctional DNA adducts formed, as did interstrand cross-linking assays. Also, the unwinding angle induced in negatively supercoiled DNA (9 +/- 1 degrees) was less than that induced by cisplatin (13 degrees). These findings may explain why complexes such as 1 and 2 exhibited low cytotoxicities (IC(50) values >100 microM) toward A2780 human ovarian cancer cells.

Highly Selective Hydrogenation of Carbon-Carbon Multiple Bonds Catalyzed by the Cation [(C6Me6)2Ru2(PPh2)H2]+: Molecular Structure of [(C6Me6)2Ru2(PPh2)(CHCHPh)H]+, a Possible Intermediate in the Case of Phenylacetylene Hydrogenation

The dinuclear cation [(C(6)Me(6))(2)Ru(2)(PPh(2))H(2)](+) (1) has been studied as the catalyst for the hydrogenation of carbon-carbon double and triple bonds. In particular, [1][BF(4)] turned out to be a highly selective hydrogenation catalyst for olefin functions in molecules also containing reducible carbonyl functions, such as acrolein, carvone, and methyljasmonate. The hypothesis of molecular catalysis by dinuclear ruthenium complexes is supported by catalyst-poisoning experiments, the absence of an induction period in the kinetics of cyclohexene hydrogenation, and the isolation and single-crystal X-ray structure analysis of the tetrafluoroborate salt of the cation [(C(6)Me(6))(2)Ru(2)(PPh(2))(CHCHPh)H](+) (2), which can be considered as an intermediate in the case of phenylacetylene hydrogenation. On the basis of these findings, a catalytic cycle is proposed which implies that substrate hydrogenation takes place at the intact diruthenium backbone, with the two ruthenium atoms acting cooperatively in the hydrogen-transfer process.

Diastereospecific and Diastereoselective Syntheses of Ruthenium(II) Complexes Using N,N‘ Bidentate Ligands Aryl-pyridin-2-ylmethyl-amine ArNH-CH2-2-C5H4N and Their Oxidation to Imine Ligands

Coordination of N,N' bidentate ligands aryl-pyridin-2-ylmethyl-amine ArNH-CH2-2-C5H4N 1 (Ar = 4-CH3-C6H4, 1a; 4-CH3O-C6H4, 1b; 2,6-(CH3)2-C6H3, 1c; 4-CF3-C6H4, 1d) to the moieties [Ru(bipy)2]2+, [Ru(eta5-C5H5)L]+ (L = CH3CN, CO), or [Ru(eta6-arene)Cl]2+ (arene = benzene, p-cymene) occurs under diastereoselective or diastereospecific conditions. Detailed stereochemical analysis of the new complexes is included. The coordination of these secondary amine ligands activates their oxidation to imines by molecular oxygen in a base-catalyzed reaction and hydrogen peroxide was detected as byproduct. The amine-to-imine oxidation was also observed under the experimental conditions of cyclic voltammetry measurements. Deprotonation of the coordinated amine ligands afforded isolatable amido complexes only for the ligand (1-methyl-1-pyridin-2-yl-ethyl)-p-tolyl-amine, 1e, which doesn't contain hydrogen atoms in a beta position relative to the N-H bond. The structures of [Ru(2,2'-bipyridine)2(1b)](PF6)2, 2b; [Ru(2,2'-bipyridine)(2)(1c)](PF6)2, 2c; trans-[RuCl2(COD)(1a)], 3; and [RuCl2(eta6-C6H6)(1a)]PF6, 4a, have been confirmed by X-ray diffraction studies.

A Versatile Ruthenium Precursor for Biphasic Catalysis and Its Application in Ionic Liquid Biphasic Transfer Hydrogenation: Conventional vs Task-Specific Catalysts

The synthesis of a novel imidazolium-tagged ruthenium complex, which represents a versatile precursor for aqueous and ionic liquid biphasic catalysis, is reported. Its utility is demonstrated in the highly enantioselective ionic liquid biphasic transfer hydrogenation of acetophenone and is compared to conventional (untagged) complexes.

A New Class of “Tethered” Ruthenium(II) Catalyst for Asymmetric Transfer Hydrogenation Reactions

Ruthenium dimer 4 is converted directly to monomeric asymmetric transfer hydrogenation catalyst 2 under the conditions employed for ketone reduction. Using 0.25 mol % of either 4 or 0.5 mol % of 2 in formic acid/triethylamine, it is possible to achieve ketone reduction in quantitative conversion and with ee's as high as 98%. Complex 2 is a robust "single-reagent" catalyst which offers significant scope for modification toward specific substrates. The synthesis and applications of an analogous complex derived from (1R,2S)-norephedrine are also described.