CpRu Research Articles

Transfer Hydrogenation of Unsaturated Substrates by Half-sandwich Ruthenium Catalysts using Ammonium Formate as Reducing Reagent

A series of half-sandwich phosphine- and NHC-supported complexes were screened in catalytic transfer hydrogenation of nitriles, N-heterocycles, olefins, and carbonyls by using ammonium formate as the reductant. Complex [Cp(iPr3P)Ru(NCCH3)2][PF6] (3) was found to be the most efficient catalyst. Mechanistic studies suggested that the formate complexes Cp(iPr3P)Ru(κ2-O2CH) and Cp(iPr3P)Ru(κ1-O2CH)(NCCH3) could be the true catalysts in these reactions. An inner-sphere monohydride mechanism was suggested. Excess free phosphine impedes the catalysis by forming the catalytically inactive bis(phosphine) complex Cp(iPr3P)2RuH.

Chiral Cyclopentadienyl Ruthenium Complexes as Versatile Catalysts for Enantioselective Transformations.

Ruthenium complexes, in particular cyclopentadienyl ruthenium (<small>ii</small>) derivatives, catalyze a vast number of transformations in the field of homogenous catalysis. Herein we describe the first synthesis of efficient chiral cationic cyclopentadienyl ruthenium (<small>ii</small>) catalysts and their application in enantioselective cycloisomerizations yielding 4H-pyrans. A tremendous counterion effect on the selectivity was observed and subsequently explored, giving rise to a complementary set of neutral cyclopentadienyl ruthenium (<small>ii</small>) complexes able to catalyze asymmetric cyclobutene formations.

Cyclopentadienyl-ruthenium(II) complexes as efficient catalysts for the reduction of carbonyl compounds

This work reports the reduction of a large variety of aldehydes and ketones with the system PhSiH3/[CpRu(PPh3)2Cl] in good to excellent yields and high chemoselectivity. The catalyst [CpRu(PPh3)2Cl] can be used in at least 12 catalytic cycles with excellent catalytic activity and several substrates were reduced under solvent free conditions.

ChemInform Abstract: Chiral Cyclopentadienylruthenium Sulfoxide Catalysts for Asymmetric Redox Bicycloisomerization.

AbstractA wide range of cyclopentadienylruthenium complexes containing tethered chiral sulfoxides is synthesized.

Chiral cyclopentadienylruthenium sulfoxide catalysts for asymmetric redox bicycloisomerization.

A full account of our efforts toward an asymmetric redox bicycloisomerization reaction is presented in this article. Cyclopentadienylruthenium (CpRu) complexes containing tethered chiral sulfoxides were synthesized via an oxidative [3 + 2] cycloaddition reaction between an alkyne and an allylruthenium complex. Sulfoxide complex 1 containing a p-anisole moiety on its sulfoxide proved to be the most efficient and selective catalyst for the asymmetric redox bicycloisomerization of 1,6- and 1,7-enynes. This complex was used to synthesize a broad array of [3.1.0] and [4.1.0] bicycles. Sulfonamide- and phosphoramidate-containing products could be deprotected under reducing conditions. Catalysis performed with enantiomerically enriched propargyl alcohols revealed a matched/mismatched effect that was strongly dependent on the nature of the solvent.

A Long-Lived Mononuclear Cyclopentadienyl Ruthenium Complex Grafted onto Anatase TiO2 for Efficient CO2 Photoreduction.

This work shows a novel artificial donor-catalyst-acceptor triad photosystem based on a mononuclear C5 H5 -RuH complex oxo-bridged TiO2 hybrid for efficient CO2 photoreduction. An impressive quantum efficiency of 0.56 % for CH4 under visible-light irradiation was achieved over the triad photocatalyst, in which TiO2 and C5 H5 -RuH serve as the electron collector and CO2 -reduction site and the photon-harvester and water-oxidation site, respectively. The fast electron injection from the excited Ru(2+) cation to TiO2 in ca. 0.5 ps and the slow backward charge recombination in half-life of ca. 9.8 μs result in a long-lived D(+) -C-A(-) charge-separated state responsible for the solar-fuel production.

Half-Sandwich Silane σ-Complexes of Ruthenium Supported by NHC Carbene

Reactions of carbene complex [Cp(IPr)Ru(pyr)2][BF4] (6, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) with excess acetonitrile and LiCl in THF afford complexes [Cp(IPr)Ru(NCCH3)2][PF6] (7) and Cp(IPr)RuCl (8), respectively. Complex 8 was characterized by NMR and X-ray diffraction analysis. Addition of hydrosilanes to 8 results in silane σ-complexes Cp(IPr)Ru(η2-HSiR3)Cl (4), which were characterized by NMR and X-ray studies of Cp(IPr)Ru(η2-HSiMeCl2)Cl (4b) and Cp(IPr)Ru(η2-H3SiPh)Cl (4d). The hydrogen–silicon coupling constants of complexes 4 show an unusual trend in that the J(H–Si) values increase from the less-chlorinated complex Cp(IPr)Ru(η2-HSiMe2Cl)Cl (4c) to the trichloro derivative Cp(IPr)Ru(η2-HSiCl3)Cl (4a). Reaction of Cp(IPr)RuCl (8) with two equivalents of HSiCl3 gave the ruthenate complex [IPrH]+[CpRuCl(H) (SiCl3)2]−, characterized by NMR and X-ray study. Addition of hydrosilanes to the cationic complex [Cp(IPr)Ru(NCCH3)2][BArF4] (9) furnished very unstable cationic silane σ-comple...

Transfer Hydrogenation of Nitriles, Olefins, and N-Heterocycles Catalyzed by an N-Heterocyclic Carbene-Supported Half-Sandwich Complex of Ruthenium

In the presence of KOBut, N-heterocyclic carbene-supported half-sandwich complex [Cp(IPr)Ru(pyr)2][PF6] (3) (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) catalyzes transfer hydrogenation (TH) of nitriles, activated N-heterocycles, olefins, and conjugated olefins in isopropanol at the catalyst loading of 0.5%. The TH of nitriles leads to imines, produced as a result of coupling of the initially formed amines with acetone (produced from isopropanol), and showed good chemoselectivity. Reduction of N-heterocycles occurs for activated polycyclic substrates (e.g., quinoline) and takes place exclusively in the heterocycle. The TH also works well for linear and cyclic olefins but fails for trisubstituted substrates. However, the C═C bond of α,β-unsaturated esters, amides, and acids is easily reduced even for trisubstituted species, such as isovaleriates. Mechanistic studies suggest that the active species in these catalytic reactions is the trihydride Cp(IPr)RuH3 (5), which can catalyze these reactions...

NHC carbene supported half-sandwich hydridosilyl complexes of ruthenium: the impact of supporting ligands on Si∙∙∙H interligand interactions.

Reaction of complex [CpRu(pyr)3][PF6] (3) with the NHC carbene IPr (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) results in the NHC complex [Cp(IPr)Ru(pyr)2][PF6] (4), which was studied by NMR specroscopy and X-ray diffraction analysis. Reaction of [Cp(IPr)Ru(pyr)2][PF6] (4) with LiAlH4 leads to the trihydride Cp(IPr)RuH3 (5) characterised by spectroscopic methods. Heating compound 5 with hydrosilanes gives the dihydrido silyl derivatives Cp(IPr)RuH2(SiR3) (6). Systematic X-ray diffraction studies suggest that complexes 6 have stronger interligand Si∙∙∙H interactions than the isolobal phosphine complexes Cp(Pr3P)RuH2(SiR3).

Enantio- and diastereoselective polymerization: asymmetric allylic alkylation catalyzed by a planar-chiral Cp′Ru complex

In this study, we examined the regio-, diastereo-, and enantioselective polymerization using asymmetric allylic alkylation catalyzed by a planar-chiral cyclopentadienyl-ruthenium (Cp′Ru) complex ((S)-1).

ChemInform Abstract: Enantio‐ and Diastereoselective Asymmetric Allylic Alkylation Catalyzed by a Planar‐Chiral Cyclopentadienyl Ruthenium Complex.

AbstractThe mild reaction conditions result in the formation of chiral diketones having two vicinal stereocenters.

Bioorthogonal Catalysis: A General Method To Evaluate Metal-Catalyzed Reactions in Real Time in Living Systems Using a Cellular Luciferase Reporter System.

The development of abiological catalysts that can function in biological systems is an emerging subject of importance with significant ramifications in synthetic chemistry and the life sciences. Herein we report a biocompatible ruthenium complex [Cp(MQA)Ru(C3H5)]+PF6–2 (Cp = cyclopentadienyl, MQA = 4-methoxyquinoline-2-carboxylate) and a general analytical method for evaluating its performance in real time based on a luciferase reporter system amenable to high throughput screening in cells and by extension to evaluation in luciferase transgenic animals. Precatalyst 2 activates alloc-protected aminoluciferin 4b, a bioluminescence pro-probe, and releases the active luminophore, aminoluciferin (4a), in the presence of luciferase-transfected cells. The formation and enzymatic turnover of 4a, an overall process selected because it emulates pro-drug activation and drug turnover by an intracellular target, is evaluated in real time by photon counting as 4a is converted by intracellular luciferase to oxyaminoluciferin and light. Interestingly, while the catalytic conversion (activation) of 4b to 4a in water produces multiple products, the presence of biological nucleophiles such as thiols prevents byproduct formation and provides almost exclusively luminophore 4a. Our studies show that precatalyst 2 activates 4b extracellularly, exhibits low toxicity at concentrations relevant to catalysis, and is comparably effective in two different cell lines. This proof of concept study shows that precatalyst 2 is a promising lead for bioorthogonal catalytic activation of pro-probes and, by analogy, similarly activatable pro-drugs. More generally, this study provides an analytical method to measure abiological catalytic activation of pro-probes and, by analogy with our earlier studies on pro-Taxol, similarly activatable pro-drugs in real time using a coupled biological catalyst that mediates a bioluminescent readout, providing tools for the study of imaging signal amplification and of targeted therapy.

Chiral Cationic Cp(x)Ru(II) Complexes for Enantioselective Yne-Enone Cyclizations.

The cyclopentadienyl (Cp) group is a ligand of great importance for many transition-metal complexes used in catalysis. Cationic CpRu(II) complexes with three free coordination sites are highly versatile catalysts for many atom-economic transformations. We report the synthesis of a family of Cp(x)Ru(II) complexes with chiral Cp ligands keeping the maximum number of available coordination sites. The cationic members are efficient and selective catalysts for yne-enone cyclizations via formal hetero-Diels-Alder reactions. The transformation proceeds in <1 h at -20 °C and provides pyrans in up to 99:1 er. Unsaturated ester or Weinreb-amide substrates directly yield the iridoid skeleton.

Some reactions of azides with diynyl-bis(phosphine)ruthenium-cyclopentadienyl complexes

Reactions of SiMe3(N3) with Ru(CCH)(PP)Cp’ [PP = (PPh3)2, Cp′ = Cp; PP = dppe, Cp′ = Cp*] afford Ru(N3)(PP)Cp′. Reactions of Ru(CCCCR)(dppe)Cp* with SiMe3(N3) give Ru{N3C(CN)CH2R}(dppe)Cp* (R = H 2, Ph 3). With RuCl(dppe)Cp* in the presence of [NH4]PF6, 2 gives binuclear [Cp*(dppe)Ru{N3C[{(CN)Ru(dppe)Cp*]CMe}]PF64. The reaction between TsN3 and Ru(CCCCH)(dppe)Cp* gives Ru{(NC)CNNTsCHC(NTs)}(dppe)Cp* 5, with a small amount of TsNPPh2CH2CH2PPh2NTs 6 being obtained from some reactions. X-ray determined structures of Ru(N3)(dppe)Cp* and 2–5 are reported, together with plausible routes to 2 and 3.

A heterogeneous mesoporous silica-supported cyclopentadienyl ruthenium(II) complex catalyst for selective hydrosilylation of 1-hexyne at room temperature

A cyclopentadienyl ruthenium(II) complex has been immobilized on MCM-41 modified with aminopropyl group through an amide bond formation reaction. FT-IR and UV–vis spectra show successful immobilization of cyclopentadienyl ruthenium complex onto the mesoporous silica surface by utilizing the amino group as a connector. The coordination state of the ruthenium complex is analyzed in detail by XAFS measurements, which indicate that the immobilization process does not influence its coordination geometry. Moreover, the retaining of long range ordering of the mesoporous structure of MCM-41 after grafting is evident from the results of XRD and N2 adsorption–desorption measurements. The resulting material promotes efficiently the hydrosilylation of 1-hexyne to produce vinylsilane with high α-selectivity under UV-irradiation at room temperature. Furthermore, the catalyst is recyclable for several catalytic runs without significant loss of its catalytic activity.

Racemization of Olefinic Alcohols by a Carbonyl(cyclopentadienyl)ruthenium Complex: Inhibition by the Carbon-Carbon Double Bond

In this article, racemization of various olefinic sec-alcohols by Ru(CO)(2)((5)-C5Ph5)Cl was investigated. The racemization of three aliphatic sec-alcohols with different chain lengths containing terminal double bonds was studied. A dramatic decrease of the racemization rate was found for these sec-alcohols compared to that of the corresponding saturated substrates. The slow racemization rate of the former alcohols is ascribed to coordination of the double bond to the ruthenium centre, which blocks the free site needed for -hydride elimination. This mechanism was supported by a recent study, in which 5-hexen-2-ol was found to form an alkoxycarbonyl complex having the double bond coordinated to the ruthenium atom. Aliphatic sec-alcohol substrates with a di- or trisubstituted double bond were found to give a lower degree of inhibition of the racemization rate than the substrates with a monosubstituted double bond.

Enantio- and diastereoselective asymmetric allylic alkylation catalyzed by a planar-chiral cyclopentadienyl ruthenium complex.

We report asymmetric allylic alkylation of allylic chloride with β-diketones as the prochiral carbon nucleophiles using a planar-chiral Cp'Ru catalyst. The reaction proceeds under mild conditions; the resulting chiral products containing vicinal tertiary stereocenters are obtained with high regio-, diastereo-, and enantioselectivities. These chiral products can then be transformed into a chiral diol by controlling the four stereocentres.

Transfer Hydrogenation of Ketones, Nitriles, and Esters Catalyzed by a Half‐Sandwich Complex of Ruthenium

AbstractHalf‐sandwich complexes [Cp(PiPr3)Ru(CH3CN)2]PF6 (1; Cp=cyclopentadienyl) and [Cp*(phen)Ru(CH3CN)]PF6 (2; Cp*=pentamethylcyclopentadienyl, phen=phenanthroline) catalyse the transfer hydrogenation of ketones to alcohols, aldimines to amines, and nitriles to imines under mild conditions. In the latter process, the imine products come from the coupling of the amines formed initially with acetone derived from the reducing solvent (isopropanol). Among functionally substituted nitriles, the aldo and keto groups are reduced concomitantly with the cyano group, whereas ester and amido groups are tolerated. Amides and alkyl esters are not reduced under these conditions even upon heating to 70 °C. However, phenylbenzoates and trifluoroacetates are reduced to alcohols. Kinetic studies on the reduction of acetophenone in isopropanol established that the reaction is first order in both the substrate and the alcohol. Stoichiometric mechanistic studies showed the formation of a hydride species. A hydride mechanism was proposed to account for these observations.

Metal-Templated Hydrogen Bond Donors as “Organocatalysts” for Carbon–Carbon Bond Forming Reactions: Syntheses, Structures, and Reactivities of 2-Guanidinobenzimidazole Cyclopentadienyl Ruthenium Complexes

The reaction of 2-guanidinobenzimidazole (GBI) and (η5-C5H5)Ru(PPh3)2(Cl) in refluxing toluene gives the chelate [(η5-C5H5)Ru(PPh3)(GBI)]+Cl– (1+Cl–; 96%). Subsequent anion metatheses yield the BF4–, PF6–, and BArf– (B(3,5-C6H3(CF3)2)4–) salts (77–85%). Reactions with CO give the carbonyl complexes [(η5-C5H5)Ru(CO)(GBI)]+X– (2+X–; X– = Cl–, BF4–, PF6–, BArf–; 87–92%). The last three salts can also be obtained by anion metatheses of 2+Cl– (77–87%), as can one with the chiral enantiopure anion P(o-C6Cl4O2)3– ((Δ)-TRISPHAT–; 81%). The reaction of [(η5-C5H5)Ru(CO)(NCCH3)2]+PF6– and GBI also gives 2+PF6– (81%). The pentamethylcyclopentadienyl analogues [(η5-C5Me5)Ru(CO)(GBI)]+X– (3+X–; X– = Cl–, BF4–, PF6–, BArf–; 61–84%) are prepared from (η5-C5Me5)Ru(PPh3)2(Cl), GBI, and CO followed (for the last three) by anion metatheses. An indenyl complex [(η5-C9H7)Ru(PPh3)(GBI)]+Cl– (96%) is prepared from (η5-C9H7)Ru(PPh3)2(Cl) and GBI. All complexes are characterized by NMR (1H, 13C, 31P, 19F, 11B), with 2D spectra aiding assignments. Crystal structures of 1+PF6–·CH2Cl2 and 1+BArf–·CH2Cl2 are determined; the anion is hydrogen bonded to the cation in the former. Complexes 1–3+X– are evaluated as catalysts (10 mol %, RT) for condensations of indoles and trans-β-nitrostyrene. The chloride salts are ineffective (0–5% yields, 48–60 h), but the BArf– salts exhibit excellent reactivities (97–46% yields, 1–48 h), with the BF4– and PF6– salts intermediate. Evidence for hydrogen bonding of the nitro group to the GBI ligand is presented. GBI shows no catalytic activity; a BArf– salt of methylated GBI is active, but much less so than 2–3+BArf–.

ChemInform Abstract: Cyclopentadienyl Ruthenium Complexes with Naphthalene and Other Polycyclic Aromatic Ligands

AbstractReview: synthesis, reactivity and applications; 137 refs.