Bifunctional Ruthenium Catalysts Research Articles

Bifunctional Ruthenium Catalysts for endo-Selective Cycloisomerization of Nucleophile-Functionalized Terminal Alkynes

Bifunctional Ruthenium Catalysts for <i>endo</i>-Selective Cycloisomerization of Nucleophile-Functionalized Terminal Alkynes

A Bifunctional Ruthenium Catalyst for Effective Renewable Hydrogen Production from Biomass-Derived Sorbitol

A Bifunctional Ruthenium Catalyst for Effective Renewable Hydrogen Production from Biomass-Derived Sorbitol

A switchable route for selective transformation of ethylene glycol to hydrogen and glycolic acid using a bifunctional ruthenium catalyst.

The developed bifunctional NNN-Ru complex features a high catalytic efficiency for the selective production of hydrogen and glycolic acid from ethylene glycol under mild reaction conditions, where a TON of 6395 was achieved. Tuning the reaction conditions afforded further dehydrogenation of the organic substrate with higher hydrogen production, and a higher TON of 25 225 was attained. The scale-up reaction yielded 1230 mL of pure hydrogen gas under the optimized reaction conditions. The role of the bifunctional catalyst was studied and mechanistic investigations were performed.

Enantiocontrolled Preparation of ϒ-Substituted Cyclohexenones: Synthesis and Kinase Activity Assays of Cyclopropyl-Fused Cyclohexane Nucleosides.

The enantioselective preparation of the two isomers of 4-hydroxy-2-cyclohexanone derivatives 1a,b was achieved, starting from a common cyclohexenone, through asymmetric transfer hydrogenation (ATH) reactions using bifunctional ruthenium catalysts. From these versatile intermediates, a stereoselective route to a cytosine analogue built on a bicyclo [4.1.0]heptane scaffold is described. Nucleoside kinase activity assays with this cyclopropyl-fused cyclohexane nucleoside, together with other related nucleosides (2a–e), were performed, showing that thymine- and guanine- containing compounds have affinity for herpes simplex virus Type 1 (HSV-1) thymidine kinase (TK) but not for human cytosolic TK-1, thus pointing to their selectivity for herpetic TKs but not cellular TKs.

Exploring the conversion mechanism of formaldehyde to CO2 and H2 catalyzed by bifunctional ruthenium catalysts: A DFT study

Bifunctional catalysts have a wide range of applications in chemical hydrogen storage. However, the versatile structure and complex influencing factors of catalysts still limit the current mechanistic understanding. Herein, a theoretical study based on density functional theory calculation is performed to illuminate the mechanistic preference of the conversion from formaldehyde to CO2 and H2 catalyzed by bifunctional ruthenium catalysts. The computational results indicate: (1) In contrast to the previously proposed mechanism, the catalyst is involved in the formaldehyde hydrolysis reaction and effectively reduces the activation free energy barrier. (2) Metal ligand cooperation mechanism is preferred instead of the metal-centred mechanism due to the stronger interaction between substrate and dual active sites on the bifunctional catalyst. (3) The catalyst with the OH group exhibits better catalytic activity compared with the NH group due to appropriate catalytic driving force, which is governed by the intrinsic electronic effects of the Lewis basic functional ligand. The pKa value can be used as a reliable descriptor to evaluate the catalytic activity of the functional ligand. Our study highlights the advantages of bifunctional catalysts in dehydrogenation-related reactions and proposes feasible strategies for the regulation of bifunctional catalyst activity, which is expected to provide new inspiration for future bifunctional catalyst design.

Fractionation of Birch Wood by Integrating Alkaline-Acid Treatments and Hydrogenation in Ethanol over a Bifunctional Ruthenium Catalyst

For the first time, the fractionation of birch wood into microcrystalline cellulose, xylose and methoxyphenols is suggested based on the integration of alkali-acid pretreatments and hydrogenation in ethanol over a bifunctional Ru/C catalyst. It is established that removal of hemicelluloses during pretreatments of birch wood influences the yields of the liquid, gaseous and solid products of the non-catalytic and catalytic hydrogenation of pretreated samples in ethanol at 225 °C. The bifunctional Ru/carbon catalyst affects in different ways the conversion and yields of products of hydrogenation of the initial and acid- and alkali-pretreated birch wood. The most noticeable influence is characteristic of the hydrogenation of the acid-pretreated wood, where in contrast to the non-catalytic hydrogenation, the wood conversion and the yields of liquid products increase but the yields of the solid and gaseous products decrease. GC-MS, gel permeation chromatography and elemental analysis were used for characterization of the liquid product composition. The molecular mass distribution of the liquid products of hydrogenation of the initial and pretreated wood shifts towards the low-molecular range in the presence of the catalyst. From the GC-MS data, the contents of monomer compounds, predominantly 4-propylsyringol and 4-propanolsyringol, increase in the presence of the ruthenium catalyst. The solid products of catalytic hydrogenation of the pretreated wood contain up to 95 wt% of cellulose with the structure, similar to that of microcrystalline cellulose.

N-Methylation of Amines with Methanol in the Presence of Carbonate Salt Catalyzed by a Metal-Ligand Bifunctional Ruthenium Catalyst [(p-cymene)Ru(2,2'-bpyO)(H2O)

A ruthenium complex [(p-cymene)Ru(2,2'-bpyO)(H2O)] was found to be a general and efficient catalyst for the N-methylation of amines with methanol in the presence of carbonate salt. Moreover, a series of sensitive substituents, such as nitro, ester, cyano, and vinyl groups, were tolerated under present conditions. It was confirmed that OH units in the ligand are crucial for the catalytic activity. Notably, this research exhibited the potential of metal-ligand bifunctional ruthenium catalysts for the hydrogen autotransfer process.

Hydrogen Generation from Catalytic Reforming of Paraformaldehyde and Water by Polymeric Bifunctional Catalysts Comprising Ruthenium and Sulfonic Acid Units.

As a clean and sustainable source of energy, hydrogen shows great potential to be the ultimate energy source in future. In this research, paraformaldehyde is used as hydrogen carrier. Several bifunctional catalysts are prepared for the hydrogen generation from paraformaldehyde. The bifunctional catalysts contain two catalytically active sites. One is a sulfonic acid group for paraformaldehyde hydrolysis, and the other is an organometallic group that catalyzes the hydrogen release from formaldehyde. Bifunctional iridium catalysts and bifunctional rhodium catalysts could only generate traces of hydrogen in the initial phase of paraformaldehyde decomposition. Only the bifunctional ruthenium catalyst shows high activity due to its bifunctional catalytically active sites, thus more than 98.0 % of the initially produced gas contains hydrogen. The initial TOF is 685 h-1 at 363 K when the paraformaldehyde concentration is 20 wt%. A reaction mechanism is proposed for the hydrogen generation from paraformaldehyde in which formaldehyde and formic acid are intermediates Formic acid decomposition is the rate-determining step in the later phase of paraformaldehyde decomposition.

ChemInform Abstract: Hydrogenation of Carboxylic Acid Derivatives with Bifunctional Ruthenium Catalysts

AbstractReview: 42 refs.

Hydrodeoxygenation of lignin-derived phenolic compounds over bi-functional Ru/H-Beta under mild conditions

Bifunctional ruthenium catalysts supported on H-Beta zeolite were designed and applied in the hydrodeoxygenation of lignin-derived phenolic compounds for bio-oil upgrading. The oxygen-containing groups in lignin-derived phenolic monomers and dimers could be efficiently removed through aqueous phase hydrodeoxygenation catalyzed by Ru/H-Beta under very mild conditions, i.e. at 413K and 4MPa H2. Characterization results revealed that ruthenium species existed in the form of highly dispersed nanoparticles on H-Beta zeolite, while the presence of both Brønsted and Lewis acid sites in zeolite was crucial for the remarkable hydrodeoxygenation activity. The effects of reaction conditions and catalyst constitution on the catalytic performance of Ru/H-Beta in hydrodeoxygenation were systematically investigated. The hydrodeoxygenation pathway of diphenyl ether as a model compound catalyzed by optimized 0.5%Ru/H-Beta was discussed and the structure-activity relationship in hydrodeoxygenation was proposed.

Hydrogenation of carboxylic acid derivatives with bifunctional ruthenium catalysts

AbstractThis paper focuses on recent advances in the chemistry of bifunctional Ru catalysts with chelate protic amine ligands effective for hydrogenation of polar carbonyl compounds. The rational design of the cooperating amine ligand that adjusts the balance of the electronic factors on the M/NH units in the bifunctional catalysts is crucial to exploit the characteristic catalytic performance with a wide scope and high practicability. These bifunctional molecular catalysts offer a great opportunity to develop new fundamental processes for the straightforward hydrogenation of carboxylic acid derivatives as a powerful alternative for classical reduction using stoichiometric amounts of metal hydride reagents.

The Mechanism of Borane–Amine Dehydrocoupling with Bifunctional Ruthenium Catalysts

Borane-amine adducts have received considerable attention, both as vectors for chemical hydrogen storage and as precursors for the synthesis of inorganic materials. Transition metal-catalyzed ammonia-borane (H3N-BH3, AB) dehydrocoupling offers, in principle, the possibility of large gravimetric hydrogen release at high rates and the formation of B-N polymers with well-defined microstructure. Several different homogeneous catalysts were reported in the literature. The current mechanistic picture implies that the release of aminoborane (e.g., Ni carbenes and Shvo's catalyst) results in formation of borazine and 2 equiv of H2, while 1 equiv of H2 and polyaminoborane are obtained with catalysts that also couple the dehydroproducts (e.g., Ir and Rh diphosphine and pincer catalysts). However, in comparison with the rapidly growing number of catalysts, the amount of experimental studies that deal with mechanistic details is still limited. Here, we present a comprehensive experimental and theoretical study about the mechanism of AB dehydrocoupling to polyaminoborane with ruthenium amine/amido catalysts, which exhibit particularly high activity. On the basis of kinetics, trapping experiments, polymer characterization by (11)B MQMAS solid-state NMR, spectroscopic experiments with model substrates, and density functional theory (DFT) calculations, we propose for the amine catalyst [Ru(H)2PMe3{HN(CH2CH2PtBu2)2}] two mechanistically connected catalytic cycles that account for both metal-mediated substrate dehydrogenation to aminoborane and catalyzed polymer enchainment by formal aminoborane insertion into a H-NH2BH3 bond. Kinetic results and polymer characterization also indicate that amido catalyst [Ru(H)PMe3{N(CH2CH2PtBu2)2}] does not undergo the same mechanism as was previously proposed in a theoretical study.

Quantum Chemical Calculations with the Inclusion of Nonspecific and Specific Solvation: Asymmetric Transfer Hydrogenation with Bifunctional Ruthenium Catalysts

Details of the mechanism of asymmetric transfer hydrogenation of ketones catalyzed by two chiral bifunctional ruthenium complexes, (S)-RuH[(R,R)-OCH(Ph)CH(Ph)NH(2)](η(6)-benzene) (Ru-1) or (S)-RuH[(R,R)-p-TsNCH(Ph)CH(Ph)NH(2)](η(6)-mesitylene) (Ru-2), were studied computationally by density functional theory, accounting for the solvation effects by using continuum, discrete, and mixed continuum/discrete solvation models via "solvated supermolecules" approach. In contrast to gas phase quantum chemical calculations, where the reactions were found to proceed via a concerted three-bond asynchronous process through a six-membered pericyclic transition state, incorporation of the implicit and/or explicit solvation into the calculations suggests that the same reactions proceed via two steps in solution: (i) enantio-determining hydride transfer and (ii) proton transfer through the contact ion-pair intermediate, stabilized primarily by ionic hydrogen bonding between the cation and the anion. The calculations suggest that the proton source for neutralizing the chiral RO(-) anion may be either the amine group of the cationic Ru complex or, more likely, a protic solvent molecule. In the latter case, the reaction may not necessarily proceed via the 16e amido complex Ru[(R,R)-XCH(Ph)CH(Ph)NH](η(6)-arene). The origin of enantioselectivity is discussed in terms of the newly formulated mechanism.

Mechanism of alkene isomerization by bifunctional ruthenium catalyst: A theoretical study

The molecular mechanism of the isomerization of 1-pentene to form ( E)-2-pentene catalyzed by the bifunctional ruthenium catalyst has been investigated using density functional theory calculations. The reaction is likely to proceed through the following steps: 1) the β-H elimination to generate the ruthenium hydride intermediate; 2) the reductive elimination of the hydride intermediate to generate the nitrogen-protonated allyl intermediate; 3) the transportation of the hydrogen by the dihedral rotation with Ru–P bond acting as axis; 4) the oxidative addition to afford another hydride complex; 5) the reductive elimination of the hydride intermediate to form the C 2-C 3 π-coordinated agostic intermediate; 6) the coordination of the nitrogen to the ruthenium center to give the final product. The rate-determining step is the oxidative addition step (the process of the hydrogen moves to ruthenium center from the nitrogen atom) with the free energy of 31.2 kcal/mol in the acetone solvent. And the N-heterocyclic ligand in the catalyst mainly functions in the two aspects: affords an important internal-basic center (nitrogen atom) and works as a transporter of hydrogen. Our results would be helpful for experimentalists to design more effective bifunctional catalysts for isomerization of a variety of heterofunctionalized alkene derivatives.

ChemInform Abstract: Single Bifunctional Ruthenium Catalyst for One‐Pot Cyclization and Hydration Giving Functionalized Indoles and Benzofurans.

AbstractThe cyclization of a range of aniline and phenol derivatives to indoles and benzofurans is presented.

Single Bifunctional Ruthenium Catalyst for One‐Pot Cyclization and Hydration giving Functionalized Indoles and Benzofurans

DOI: 10.1002/chem.201001075 Single Bifunctional Ruthenium Catalyst for One-Pot Cyclization and Hydration giving Functionalized Indoles and Benzofurans Reji N. Nair, [a] Paul J. Lee, [a] Arnold L. Rheingold, [b] and Douglas B. Grotjahn* [a] Indole [1] and benzofuran [1d, 2] heterocycles are key structur- al units in a variety of biologically active natural products and unnatural synthetic materials. Over the last few years, transition-metal catalysts have been extensively used in the cyclization of o-alkynylarylamines [3a–l] and phenols [3m–p] to construct benzoheterocycles through intramolecular [3] and intermolecular reactions. [4] Many of these reactions required the use of a nitrogen protecting group in the case of indo- les [3g,i,k] or stoichiometric amounts of the metal. [3l] McLory and Trost [3d] synthesized both unprotected indoles or N- benzyl analogues by using rhodium–phosphine complexes, but to optimize the benzofuran synthesis, large amounts of phosphine (0.6 equiv) were needed. Saa and co-workers [3q] recently reported the use of 10 mol % catalyst and amine solvents in benzofuran synthesis at 90 8C. Herein, we report that the bifunctional ruthenium catalyst 1 which has been used for anti-Markovnikov alkyne hydration [5] emerges as a versatile choice for both indole and benzofuran formation, with a number of unique advantages, including the use of only 2 mol % catalyst in most cases and the unprecedented ability of a cycloisomerization catalyst to perform hydration or deuteration in the same reaction. We envisioned that the use of alkyne hydration catalyst 1 on substrates, such as 2, (Scheme 1) would generate the al- dehyde 3, on which cyclization and dehydration would then enable facile synthesis of heterocycles. Indeed, 2 a cyclized to hemiaminal 4 a, [5a] with some equilibrium amount of 3 a, [a] R. N. Nair, P. J. Lee, Prof. D. B. Grotjahn Department of Chemistry and Biochemistry San Diego State University 5500 Campanile Drive, San Diego, CA 92182-1030 (USA) Fax: (+ 1) 619-594-4634 E-mail: grotjahn@chemistry.sdsu.edu [b] Prof. A. L. Rheingold Department of Chemistry and Biochemistry University of California–San Diego La Jolla, CA 92093-0358 (USA) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201001075. Scheme 1. The use of bifunctional catalyst 1 for hydration and cycliza- tion; Cp = cyclopentadienyl. but elimination of water from 4 a required heating in second step, forming 5 a in 46 % overall yield. The same two-step process could be used on the homologous substrate 2 b, to form 5 b in 56 % overall yield. Encouraged by these results, and because of the impor- tance of indoles, we shifted our attention to substrates de- rived from o-ethynylaniline. The first four entries of Table 1 demonstrate that 1 cyclizes a wide range of aniline deriva- tives; most significantly, the simplest, unprotected parent compound 6 cyclized to indole 17 in 99 % yield (Table 1, entry 1). Sulfonamide 7 (Table 1, entry 2) was cyclized to give the N-tosyl (Ts) protected indole in just 2 h. Entries 3 and 4 in Table 1 show the ability of the catalyst to form ni- trogen analogues with different alkyl substituents. Especially notable is the selectivity of 1: the N-allyl group (Table 1, entry 3) is fully tolerated without the decomposition by iso- merization seen when using a Rh-based catalyst. [3d] The fact that a wide variety of nucleophilic nitrogen cen- ters could be used in indole formation encouraged us to ex- plore the scope of 1 not only in making more complex indole derivatives, but also in cyclizing phenol derivatives to benzofurans. Gratifyingly, 1 forms both 7-aza- and 6-nitroin- doles (Table 1, entries 5 and 6), showing tolerance of various ring substituents, including those that might coordinate to the catalyst. Furthermore, entry 9 (Table 1) shows the ability of 1 to form the benzofuran nucleus in essentially quantita- 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eur. J. 2010, 16, 7992 – 7995

One-Pot Formation of Functionalized Indole and Benzofuran Derivatives Using a Single Bifunctional Ruthenium Catalyst

We report a single bifunctional ruthenium catalyst for cyclization of terminal alkynylaryl amines and -phenols to corresponding indole and benzofuran derivatives in good yields. Key features the ability to cyclize and hydrate terminal alkynes in one step and to deuterate the heteroaromatic compounds formed.

Asymmetric nitrile-hydration with bifunctional ruthenium catalysts bearing chiral N-sulfonyldiamine ligands

The asymmetric hydration of α-benzyl-α-methylmalononitrile was examined by using chiral bifunctional catalysts. Enantiomer discrimination of the prochiral dinitrile in the presence of water was demonstrated with a well-defined bifunctional amido-Ru complex having N-sulfonyldiamine ligands, and providing the corresponding amidato(amine) complexes diastereoselectively. Under the catalytic conditions, the ( R)-cyanoamide was successfully obtained without further hydrolysis to carboxylic acids.

Direct Amination via Bifunctional Iridium Amido Complex

While methods for the enantio­selective direct amination reaction often rely on Lewis acid catalysis and organocatalysis, a limited number of metal-based approaches have been reported. Based on the previous development of a bifunctional ruthenium catalyst for asymmetric Michael reactions (J. Am. Chem. Soc. 2004, 126, 11148), the authors report a bifunctional iridium amido complex for the enantioselective direct amination of α-cyanoacetates using dialkyl diazocarboxylates. A variety of aminated products could be obtained in consistently quantitative yields and excellent enantioselectivities. Interestingly, a related ruthenium complex exhibited low levels of enantioselection.