CpRu Research Articles

New Method for Asymmetric Polymerization: Asymmetric Allylic Substitution Catalyzed by a Planar-Chiral Ruthenium Complex

A new type of asymmetric polymerization reaction by asymmetric allylic substitution catalyzed by planar-chiral cyclopentadienyl-ruthenium (Cp′Ru) complexes is described. Achiral AB-type monomers, which feature both the allylic chloride and N-alkoxyamide moieties in a single molecule, are smoothly polymerized by the catalysis of chiral ruthenium complexes with quantitative monomer conversion to afford optically active polyamides in good yields. The resulting polymers are characterized by optical rotation, circular dichroism, and 1H NMR analyses, which clearly show that the polymerization system works in a highly stereoselective manner to afford optically active polyamides with high regio- and enantioselectivities. The results revealed that the planar-chiral Cp′Ru complexes work as a highly efficient catalyst to control the stereochemistry even in the polymerization reaction.

A Chiral Sulfoxide-Ligated Ruthenium Complex for Asymmetric Catalysis: Enantio- and Regioselective Allylic Substitution

The design and synthesis of a novel chiral sulfoxide-ligated cyclopentadienyl ruthenium complex is described. Its utility as an asymmetric variant of [CpRu(MeCN)3]PF6 is demonstrated through its ability to function in the branched-selective asymmetric allylic alkylation of phenols and carboxylic acids. Water has also been shown to act as a competent nucleophile in this reaction to generate branched allyl alcohols with good regio- and enantioselectivities.

Highly Enantiomerically Enriched Planar Chiral Cyclopentadienyl(indenyl)ruthenium Complexes

The functionalization of highly enantiomerically enriched planar chiral cyclopentadienyl(4-bromoindenyl)ruthenium complexes is detailed. Lithium/bromine exchange followed by an electrophilic quench using N,N-dimethylformamide (DMF), trimethylsilyl chloride, benzaldehyde, acetone, or 1,2-diiodoethane afforded the corresponding enantiomerically enriched planar chiral complexes. Suzuki–Miyaura cross-coupling led to cyclopentadienyl(indenyl)ruthenium complexes bearing aryl and alkenyl groups in high yields. Similarly, in situ generation of the boronate intermediate using 9-MeO-9-BBN and a metalated species in the Suzuki–Miyaura reaction gave heteroaryl, alkynyl, and alkyl cyclopentadienyl(indenyl)ruthenium complexes with retention of configuration and enantiomeric excess.

Synthesis, characterization and cytotoxicity of cyclopentadienyl ruthenium(II) complexes containing carbohydrate-derived ligands

We here report the synthesis of new cyclopentadienyl ruthenium(II) complexes of general formula [(η5-C5H5)Ru(PP)(L)]+ (PP = two triphenylphosphine, 1,2-diphenylphosphinoethane), isolated as PF6− salts, with L being galactose and fructose carbohydrate derivative ligands, N-coordinated to the metal centre by nitrile, tetrazole and 1,3,4-oxadiazole moieties. The ten new organometallic compounds were fully characterized by FT-IR, 1H, 13C, and 31P NMR spectroscopies, and by elemental analysis. The cytotoxicity of the ruthenium(II) compounds was tested on HeLa cancer cells (cervical carcinoma), unveiling IC50 values in the low micromolar range.

Oxygenation of Ruthenium Carbene Complexes Containing Naphthothiophene or Naphthofuran: Spectroscopic and DFT Studies

The aryl propargylic alcohol 1-[2-(thiophen-3-yl)phenyl]prop-2-yn-1-ol (1a) is readily prepared from 2-(thiophen-3-yl)benzaldehyde. In the presence of visible light, treatment of 1a with one-half mole equivalent of [Ru]Cl ([Ru]=Cp(dppe)Ru) (dppe=1,2-bis(diphenylphosphino)ethane) and NH4PF6 in O2 affords the naphtha[2,1-b]thiophene-4-carbaldehyde (4a) in high yields. The cyclization reaction of 1a proceeds through the formation of the carbene complex 2a that contains the naphtha[2,1-b]thiophene ring, which is isolated in a 1:1 stoichiometric reaction. The C-C bond formation between the inner carbon of the terminal triple bond and the heterocyclic ring is confirmed by structure determination of 2a using single-crystal X-ray diffraction analysis. Facile oxygenation of 2a by O2 yields the aldehyde product 4a accompanied by the formation of phosphine oxide of dppe. Oxygen is most likely activated by coordination to the ruthenium center when one PPh2 unit of the dppe ligand dissociates. This dissociated PPh2 unit then reacts with the coordinated oxygen nearby to generate half-oxidized dppe ligand and an unobserved oxo-carbene intermediate. Coupling of the oxo/carbene ligands followed by demetalation then yields 4a. Presumably the resulting complex with the half-oxidized dppe ligand continuously promotes cyclization/oxygenation of 1a to yield the second aldehyde molecule. In alcohol such as MeOH or EtOH, the oxygenation reaction affords a mixture of 4a and the corresponding esters 5a or 5a'. Four other aryl propargylic alcohols 1b-e, which contain thiophen-2-yl, isopropenyl, fur-3-yl, and fur-2-yl, respectively, on the aryl ring are also prepared. Analogous aldehydes 4b-e are similarly prepared from 1b-e, respectively. For oxygenations of 1b, 1d, and 1e in alcohol, mixtures of aldehyde 4, ester 5, and acetal 8 are obtained. The carbene complex 2b obtained from 1b was also characterized by single-crystal X-ray diffraction analysis. The UV/Vis spectra of 2a and 2b consist of absorption bands with a high extinction coefficient. From DFT calculations on 2a and 2b, the visible light is found to populate the LUMO antibonding orbital of mainly Ru=C bonds, thereby weakening the Ru=C bond and promoting the oxygenation/demetalation reactions of 2.

Regioselective Cyclometalation of Planar Chiral Pyridine and Oxazoline Derivatives: peri- versus ortho-C–H Functionalization

The cyclopalladation of highly enantioenriched cyclopentadienyl(indenyl)ruthenium complexes featuring pyridine or oxazoline functionalities as directing groups is detailed. Two competitive metalation sites inherent to the fused ring system were selectively activated after appropriate tuning of the reaction conditions, enabling the access to both six (peri)- and five-membered (ortho) membered chelates. A peri → ortho isomerization was identified in the pyridine-based system, unveiling a kinetic preference for the peri-position and thermodynamic control for the ortho-cyclopalladation. In marked contrast, oxazoline derivatives incorporating a stereogenic center at C4 proved reluctant to form ortho-isomers and were exclusively converted into the corresponding peri-palladacyles at low temperatures.

Chemo- and Regioselective Catalytic Reduction of N-Heterocycles by Silane

The ruthenium complex [Cp(iPr3P)Ru(NCCH3)2]+ (1) catalyzes the regioselective hydrosilylation of pyridines to 1,4-dihydropyridines. Substitution in the 3- and 5-positions is tolerated, whereas pyridines with substituents in the 2-, 4-, and 6-positions are not reduced. Reduction of functionalized pyridines having keto and ester substituents results in a mixture of products. N-Silyl-1,4-dihydropyridine reacts with ketones and aldehydes to give products of N–Si addition across the C═O bond. Hydrosilylation of pyridine in acetone results quantitatively in the addition product PhMe2SiO–CMe2–NC5H6, which decomposes in hexane to give the parent dihydropyridine HNC5H6. The phenanthroline complex [Cp(phen)Ru(NCCH3)2]+ (10) catalyzes regioselective 1,4-reduction of phenanthroline by a 3–4-fold excess of silane/water or silane/alcohol mixtures. The Cp* analogue [Cp*(phen)Ru(NCCH3)2]+ (9) catalyzes 1,4-regioselective monohydrosilylation of phenanthroline, quinoline, acridine, and 1,3,5-triazine and the 1,2-reduction of isoquinoline. In contrast, 2-substituted phenanthroline, pyrazine, 2-ethylpyridine, 2,6-lutidine, 2,4-lutidine, and pyrimidine are not reduced under these conditions by either of the catalysts studied.

Straightforward Access to Tetrametallic Complexes with a Square Array by Oxidative Dimerization of Organometallic Wires

The bimetallic ruthenium complex {Cp(dppe)Ru}2(μ-C≡CC≡CC≡C) (3; Cp = cyclopentadienyl, dppe = 1,2-bis(diphenylphosphino)ethane) has been prepared and the molecular structure determined. The cyclic voltammogram of 3 is characterized by three reversible one-electron events with a large potential difference between the two first waves (ΔE° = 0.44 V), indicating the large thermodynamic stability of the MV (mixed-valence) 3(PF6), which can be considered as a class III MV complex. The complex 3(PF6) was quantitatively prepared by treatment of 3 with 1 equiv of [FeCp2](PF6) at −78 °C and characterized by EPR spectroscopy. Above −10 °C, solutions of 3(PF6) provide the asymmetric tetranuclear complex {cyclo-C([Ru])C(CCCC[Ru])C(CC[Ru])C(CC[Ru])}(PF6)2 (4(PF6)2; [Ru] = Cp(dppe)Ru) which was formed regiospecifically and isolated in 92% yield. Thus, despite the thermodynamic stability of the monocation radical 3(PF6) as evidenced by the electrochemical data, and the extensively delocalized electronic structure, a slow...

Mechanistic Aspects on Cyclopentadienylruthenium Complexes in Catalytic Racemization of Alcohols

Cyclopentadienylruthenium complexes commonly serve as efficient transition metal catalysts in the racemization of alcohols. The combination of the racemization reaction with enzymatic resolution leads to dynamic kinetic resolution (DKR). In DKR, a theoretical yield of 100% is possible, making it a powerful tool for enantioselective synthesis. In this Account, we summarize the most important mechanistic aspects of racemization of alcohols reported over the past decade based on both experimental and computational results. Precatalyst activation is often necessary, either by heating the reaction or by adding an alkoxide-type base. The subsequent alcohol-alkoxide exchange is rapid and introduces the substrate into the catalytic cycle. This exchange requires a free coordination site, which may be created via several different mechanisms. Following alkoxide formation, racemization occurs via β-hydride elimination and subsequent readdition. In cyclopentadienyldicarbonylruthenium alkoxide complexes, which are 18-electron complexes, researchers originally considered two mechanisms for the creation of the free coordination site required for β-hydride elimination: a change in hapticity of the cyclopentadienyl ligand from η5 to η3 and dissociation of a CO ligand. Based on computational and experimental results, we have found strong support for the pathway involving CO dissociation. Researchers had also wondered if the substrate remains coordinated to the metal center (the inner-sphere mechanism) during the hydrogen transfer step(s). Using competition and crossover experiments, we found strong evidence for an inner-sphere mechanism. In summary, we have obtained a detailed picture of the racemization of alcohols by cyclopentadienylruthenium catalysts, leading to the development of more efficient catalytic systems for racemization.

Reactions of the cyclopentadienyl ruthenium complexes (C5R5)Ru(cod)Cl and [(C5R5)Ru(MeCN)3]+ (R = H, Me) with phenylacetylene and acetic acid: Unexpected difference in reactivity of CpRu and Cp*Ru complexes

Reaction of CpRu(cod)Cl with phenylacetylene and AcOH gives an unusual binuclear ruthenium complex CpRu(μ-σ,η3:η3,σ-C6H3Ph3)RuCp (5, 58% yield) with a bridging acyclic flyover ligand C6H3Ph3. Under similar conditions [CpRu(MeCN)3]+ undergoes an unexpected cleavage of Cp ligand giving 4,7-diphenyl-3a,7a-dihydroindene (6, 65% yield). In sharp contrast, the pentamethylated congeners Cp*Ru(cod)Cl and [Cp*Ru(MeCN)3]+ react with phenylacetylene and AcOH in catalytic fashion giving 1,4-diphenyl-1-acetoxy-1,3-butadiene. The structures of 5 and 6 were established by X-ray diffraction. The mechanism of Cp ligand cleavage was proposed on the basis of DFT calculations.

Asymmetric Auto‐Tandem Catalysis with a Planar‐Chiral Ruthenium Complex: Sequential Allylic Amidation and Atom‐Transfer Radical Cyclization

Ru does it all: A novel example of an asymmetric auto-tandem reaction catalyzed by a planar-chiral cyclopentadienyl–ruthenium complex is described. The reaction of allylic chloride with α-haloamides provides synthetically useful, diastereomerically and enantiomerically enriched γ-lactams with multiple stereogenic centers through one-pot sequential allylic amidation/atom-transfer radical cyclization. PG=protecting group.

A computational study of the CO dissociation in cyclopentadienyl ruthenium complexes relevant to the racemization of alcohols

The formation of an active 16-electron ruthenium sec-alkoxide complex via loss of the CO ligand is an important step in the mechanism of the racemization of sec-alcohols by (η(5)-Ph(5)C(5))Ru(CO)(2)X ruthenium complexes with X = Cl and O(t)Bu. Here we show with accurate DFT calculations the potential energy profile of the CO dissociation pathway for a series of relevant (η(5)-Ph(5)C(5))Ru(CO)(2)X complexes, where X = Cl, O(t)Bu, H and COO(t)Bu. We have found that the CO dissociation energy increases in the following order: O(t)Bu (lowest), Cl, COO(t)Bu and H (highest). Using the distance between ruthenium and C(CO), r = Ru-C(CO), as a constraint, and by optimizing all other degrees of freedom for a range of Ru-CO distances, we obtained relative energies, ΔE(r) and geometries of a sufficient number of transient structures with the elongated Ru-CO bond up to r = 3.4 Å. Our calculations provide a quantitative understanding of the CO ligand dissociation in (η(5)-Ph(5)C(5))Ru(CO)(2)Cl and (η(5)-Ph(5)C(5))Ru(CO)(2)(O(t)Bu) complexes, which is relevant to the mechanism of their catalytic activity in the racemization of alcohols. We recently reported that exchange of the CO ligand by isotopically labeled (13)CO in the Ru-O(t)Bu complex occurs twenty times faster than that in the Ru-Cl complex. This corresponds to a difference of 1.8 kcal mol(-1) in the CO dissociation energy (at room temperature). This is in very good agreement with the calculated difference between the two potential energy curves for Ru-O(t)Bu and Ru-Cl complexes, which is about 1.8-2 kcal mol(-1) around the corresponding transition states of the CO dissociation. The calculated difference in the total energy for CO dissociation in (η(5)-Ph(5)C(5))Ru(CO)(2)X complexes is related to the stabilization provided by the X group in the final 16-electron complexes, which are formed via product-like transition states. In addition to the calculated transition states of CO dissociation in Ru-O(t)Bu and Ru-Cl complexes, the calculated transient structures with the elongated Ru-CO bond provide insight into how the geometry of the ruthenium complex with a potent heteroatom donor group (X) gradually changes when one of the COs is dissociating.

Synthesis of Tetradeca- and Pentadeca(organo)[60]fullerenes Containing Unique Photo- and Electroluminescent π-Conjugated Systems

Dianions of deca(phenyl)[60]fullerene–bis(cyclopentadienyl ruthenium) complex, (C60Ph10)(RuCp)2 (1a), and dimethylated deca(phenyl)[60]fullerene, C60Ph10Me2 (1b), reacted with benzyl bromide to produce [C60Ph10(CH2Ph2)2](RuCp)2 (3a and 4a) and tetradeca(organo)[60]fullerenes C60Ph10(CH2Ph2)2Me2 (3b and 4b), respectively (Compounds 3 and 4 refer to C2v and C1 symmetric compounds, respectively.). Nucleophilic addition of benzylmagnesium chloride with the C1 symmetric compounds 4a and 4b yielded [C60Ph10(CH2Ph2)3H](RuCp)2 (5a) and pentadeca(organo)[60]fullerenes C60Ph10(CH2Ph2)3Me2H (5b), respectively. Products were structurally characterized by X-ray crystallographic analysis of diruthenium complexes 3a, 4a, and 5a. These X-ray studies revealed that compounds 3, 4, and 5 have unique π-conjugated systems consisting of doubly bridged triphenylene dimer, triphenylene–benzoacenaphthylene, and triphenylene–biphenyl structures, respectively. Quantum mechanical calculations suggested that these π-conjugated systems are aromatic. Containing no metal atom, methylated derivatives 3b, 4b, and 5b exhibited green, pale yellow, and turquoise blue emissions with quantum yields of 0.066, 0.12, and 0.24, respectively. The application of these materials in organic light-emitting diodes was demonstrated.

Facile Catalytic Hydrosilylation of Pyridines

Not only surprisingly facile, a hydrosilylation of pyridines under the catalysis of [Cp(iPr3P)Ru(NCCH3)2]+ has the advantages that it is 1,4-regioselective and reversible. The products can be transformed in a variety of ways (see scheme). The related complex [CpRu(NCCH3)3]+ catalyzes the two-hydrogen-atom reduction of phenanthroline by HSiMe2Ph/water.

Synthesis and characterization of ruthenium heterocyclic-thiocarboxylate complexes

The cyclopentadienyl ruthenium complexes CpRuL2SCO-het (Cp = η5-C5H5; L2 = 2PPh3 (1), dppe (2)) bearing heterocyclic thiocarboxylate ligands have been synthesized from the reaction of CpRuL2SH with heterocyclic acid chlorides (ClCO-2-C4H3S (a); ClCO-2-C4H3O (b); ClCO-1-C4H8N (c)). Bubbling of CO gas through a THF solution of (1) produced the mixed carbonyl–phosphine complexes CpRu(PPh3)(CO)SCO-het (3) with high yields. Complexes (1)-(3) were characterized by spectroscopic methods (i.r., 1H-n.m.r., 31P-n.m.r.) and elemental analysis. The molecular structure of CpRu(PPh3)2SCO-2-C4H3S (1a) verifies that the thiocarboxylate ligands bind via the sulfur atom (Ru–S = 2.406(2) A).

Chemoselective Catalytic Hydrosilylation of Nitriles

Well-behaved: The ruthenium complex [Cp(iPr3P)Ru(NCCH3)2]+ (Cp=cyclopentadienyl) catalyzes monohydrosilylation of nitriles (see scheme) with unprecedented tolerance of most common functional groups. The catalyst is easily synthesized starting from commercially available compounds, is air-stable, recyclable, and works under solvent-free conditions.

Chiral cyclopentadienyl ruthenium(II) complexes with P,P-ligands derived from ( 1R,2R)-1,2-diaminocyclohexane: Synthesis, crystal structures and catalytic activity in Diels–Alder reactions

Chiral cyclopentadienyl ruthenium(II) complexes [CpRu( L1– L3)Cl] ( 5– 7) have been prepared by reaction of [CpRu(PPh 3) 2Cl] with chiral P, P-ligands ( 1R,2R)-1,2-bis(diphenylphosphinamino)cyclohexane ( L1), N,N′-[bis-(3,3′-bis- tert-butyl-5,5′-bis-methoxy-1,1′-biphenyl-2,2′-diyl)phosphite]-( 1R,2R)-1,2-diaminocyclohexane ( L2) and N,N′-[bis-( R)-1,1′-binaphtyl-2,2′-diyl)phosphite]-( 1R,2R)-1,2-diaminocyclohexane ( L3). The molecular structures of 5 and 6 have been determined by single-crystal X-ray analysis. Studies on catalytic activity of the cations derived from ( 5– 7) by treatment with AgSbF 6, are also reported.

Synthesis of cyclopentadienyl ruthenium complexes containing 5-membered N-heterocyclic thiolates

Mononuclear ruthenium–thiolate complexes of structural type CpRu(PPh 3) 2SR ( 1) [R = 2-imidazolyl ( a), 1-methylimidazolyl ( b), 5-methyl-1,3,5-thiadiazolyl ( c) and 5-methyl-4H-1,2,4-triazolyl ( d)] are accessible from the reaction of CpRu(PPh 3) 2Cl with the corresponding thiolate anions. Reaction of CpRu(PPh 3) 2Cl with the heterocyclic-thiolate anions in the presence of the bisphosphine ligands affords CpRu(P–P)SR [P–P = bis(diphenylphosphino)methane; dppm ( 2), bis(diphenylphosphino)ethane; dppe ( 3)]. If CO gas was bubbled through a THF solution of 1b, the complex CpRu(PPh 3)(CO)S(C 4N 2H 5) ( 4b) is produced. These ruthenium–heterocyclic thiolate complexes have been characterized by elemental analysis, spectroscopy (IR, 1H, 31P{ 1H} NMR and MS) and cyclic voltammetry for some samples. The solid-state structures of 3a and 3b are determined by single-crystal X-ray structure analysis.

Synthesis, characterization and DNA binding studies of two cyclopentadienyl ruthenium(II) complexes with amino acid ligands

Two new half-sandwich cyclopentadienyl ruthenium(II) complexes containing α-amino acids, [CpRu(PPh3)2(Ser)] (Ser = l-serine) and [CpRu(PPh3)(Met)] (Met = l-methionine), were synthesized and characterized by physicochemical methods. Interactions of these two complexes with calf thymus DNA were investigated by UV–Vis absorption spectroscopy, emission spectroscopy and competitive binding studies. The results indicate that both complexes can interact with DNA, leading to the damage of the double helix. [CpRu(PPh3)2(Ser)] binds to DNA by intercalation, while the binding mode for [CpRu(PPh3)(Met)] is more complicated due to the formation of an EB-DNA-complex (EB = ethidium bromide). The affinity of the Met complex for DNA is stronger than that of the Ser complex, which could be due to groove–surface combination or electrostatic interaction in addition to intercalative binding.

Systematic Evaluation of Substituted Cyclopentadienyl Ruthenium Complexes, [(η5-C5MenH5−n)RuCl(cod)], for Catalytic Cycloadditions of Diynes

A series of eta(5)-cyclopentadienylruthenium complexes, [(eta(5)-C(5)Me(n)H(5-n))RuCl(cod)] (cod=1,5-cyclooctadiene), are evaluated as catalysts for the cycloaddition of 1,6-diynes with alkynes. As a result, we unexpectedly found that the complex bearing the 1,2,4-Me(3)Cp ligand is the most efficient catalyst in terms of turnover number (TON) for the cycloaddition of a bulky diiododiyne with acetylene, recording the highest TON of 970 with a catalyst loading of 0.1 mol %. To obtain insight into this result, we evaluate the electron richness of all complexes by cyclic voltammetric analyses, which indicate that the electron density of the ruthenium center increases with an increase in methyl substitution on the Cp' ligands. The initial rate (up to 10 % conversion) of the cycloaddition was then measured using (1)H NMR spectroscopy. The initial rate is found to decrease as the number of methyl substituents increases. According to these results, we assumed that the optimum catalytic performance exhibited by the 1,2,4-trimethylcyclopentadienyl complex can be attributed to its robustness under the catalytic cycloaddition conditions. The steric and electronic effects of the Cp' ligands are also investigated in terms of the regioselectivity of the cycloaddition of an unsymmetrical diyne and in terms of the chemoselectivity in the cycloaddition of a 1,6-heptadiyne with norbornene.