Ruthenium Carbene Research Articles

Grubbs Metathesis Enabled by a Light‐Driven gem‐Hydrogenation of Internal Alkynes

Abstract[(NHC)(cymene)RuCl2] (NHC=N‐heterocyclic carbene) complexes instigate a light‐driven gem‐hydrogenation of internal alkynes with concomitant formation of discrete Grubbs‐type ruthenium carbene species. This unorthodox reactivity mode is harnessed in the form of a “hydrogenative metathesis” reaction, which converts an enyne substrate into a cyclic alkene. The intervention of ruthenium carbenes formed in the actual gem‐hydrogenation step was proven by the isolation and crystallographic characterization of a rather unusual representative of this series carrying an unconfined alkyl group on a disubstituted carbene center.

Grubbs Metathesis Enabled by a Light-Driven gem-Hydrogenation of Internal Alkynes.

[(NHC)(cymene)RuCl2] (NHC=N‐heterocyclic carbene) complexes instigate a light‐driven gem‐hydrogenation of internal alkynes with concomitant formation of discrete Grubbs‐type ruthenium carbene species. This unorthodox reactivity mode is harnessed in the form of a “hydrogenative metathesis” reaction, which converts an enyne substrate into a cyclic alkene. The intervention of ruthenium carbenes formed in the actual gem‐hydrogenation step was proven by the isolation and crystallographic characterization of a rather unusual representative of this series carrying an unconfined alkyl group on a disubstituted carbene center.

Highly Efficient Ethenolysis and Propenolysis of Methyl Oleate Catalyzed by Abnormal N-Heterocyclic Carbene Ruthenium Complexes in Combination with a Phosphine–Copper Cocatalyst

Fluorinated imidazo[1,5-a]pyridine abnormal carbene ruthenium complexes (F-aImPy–Ru) with a tricyclohexylphosphine copper chloride (Cy3P–CuCl) cocatalyst were developed for the ethenolysis/propenolysis of methyl oleate. These catalysts showed remarkable catalytic efficiencies (turnover numbers of 100,000 for ethenolysis and 200,000 for propenolysis) and excellent α-olefin selectivity (up to 99%). A computational study indicated that the energy barrier of the β-hydride elimination pathway in the presence of a phosphine–copper cocatalyst is higher than that in the absence of the cocatalyst, which might contribute to the improved longevity of the ruthenium catalyst in the reaction.

Ruthenium-Catalyzed Diastereoselective Synthesis of Fully Substituted Pyrrolidines from Anilines and Diazo Pyruvates.

An unprecedented synthesis of polysubstituted pyrrolidines from anilines and diazo pyruvates by ruthenium catalysis under mild reaction conditions is reported. An enol intermediate derived from the N-H insertion of aniline toward the ruthenium carbene species as well as an imine ester intermediate generated by SET-mediated oxidation of enol were proposed as the key intermediates. This strategy provides various pyrrolidines containing four contiguous stereocenters in good efficiency with high diastereoselectivities.

Towards Sustainable Catalysis – Highly Efficient Olefin Metathesis in Protic Media Using Phase Labelled Cyclic Alkyl Amino Carbene (CAAC) Ruthenium Catalysts

AbstractNew generations of Hoveyda and bis‐carbene type of ruthenium‐based olefin metathesis catalysts (10 and 12), containing cationic cyclic alkyl amino carbene (CAAC) ligands, have been synthetized. The catalysts show exceptional stability and activity in environmentally benign, protic media. Various olefin metatheses reactions of OH functionalized feedstock (e. g. RCM, ROMP CM) can be carried out at as low as 0.05 mol % catalyst loading in methanol, isopropanol, water or methanol/water solvent mixture, accomplishing the lowest applied catalyst loading reported so far in these media. The facile olefin metathesis of renewable feedstocks including phospholipids (23) and vegetable oils (20) in protic media has also been demonstrated.

Synthesis and Activity of Six-Membered Cyclic Alkyl Amino Carbene–Ruthenium Olefin Metathesis Catalysts

Ru–cyclic alkyl amino carbene (Ru–CAAC) olefin metathesis catalysts perform extraordinarily in metathesis macrocyclization and ethenolysis, but previous studies have been limited to the use of five-membered CAAC (CAAC-5) ligands. In this work, we synthesized a different group of ruthenium catalysts with more σ-donating and π-accepting six-membered CAAC (CAAC-6) ligands, and their metathesis activity was probed through initiation studies, ring-closing metathesis (RCM), cross-metathesis, and ethenolysis. These catalysts display higher initiation rates than analogous Ru–CAAC-5 complexes but demonstrate lower activity in RCM and ethenolysis.

A Macrocyclic Ruthenium Carbene for Size-Selective Alkene Metathesis.

The synthesis of amacrocyclic Ru carbene catalyst for selective cross alkene metathesis is reported. The new catalyst showed different reactivity for various type 1 alkenes in homodimerization which correlated with the aggregrate size of the allylic substituent. The altered reactivity profile allowed for selective product formation in competition cross alkene metathesis between two different type 1 alkenes and tert-butyl acrylate. Selectivity in these reactions is attributed to the ability of themacrocyclic catalyst to differentiate alkenes based on their size. Two preparative examples of cross metathesis with the macrocyclic catalyst are also provided.

Ruthenium Carbene-Mediated Construction of Strained Allenes via the Enyne Cross-Metathesis/Cyclopropanation of 1,6-Enynes.

Herein, we report on the unprecedented dimerization of 1,6-enynes using a commercially available ruthenium complex RuCl2(PPh3)3, which results in a series of bicyclo[3.1.0]hexyl allene derivatives in moderate to excellent yields. Mechanistic investigation indicates that the in-situ-generated ruthenium vinylidene undergoes a site-selective metathesis process to provide allenyl ruthenium carbene, which can be intramolecularly trapped by the pendent C=C bond of enyne through a [2 + 2] cycloaddition/metal elimination process.

A DFT Study on the Redox Active Behavior of Carbene and Pyridine Ligands in the Oxidative and Reductive Quenching Cycles of Ruthenium Photoredox Catalysts

In this study, a detailed look at the electronic structure changes induced by photon absorption and of the succeeding redox events of the oxidative and reductive quenching cycles of ruthenium–carbene and ruthenium–pyridine photoredox catalysts is provided through an arsenal of density functional theory-based techniques including electron density difference Δρ(r) maps, spin-density distributions, and the non-covalent interaction analysis. We introduced an efficient computational protocol to obtain accurate equilibrium structures and ground-state reduction potentials for these types of complexes, substantiated via a direct comparison to empirical X-ray structures and cyclic voltammetry measurements, respectively. Moreover, we demonstrated the utility of a hitherto unexplored approach to compute excited-state redox potentials based on the Gibbs free energy of the triplet metal-to-ligand charge transfer state (3MLCT). The analyzed Δρ(r) maps revealed the characteristic features of, for example, metal- and ligand-centered reductions and oxidations in both ground and excited states and MLCT processes, disclosing the active participation of carbene ligands in the redox events of homoleptic systems. Beyond analyzing ligand–ligand non-covalent interactions and redox-active behaviors of carbene and pyridine ligands side by side, the effect of such groups on the kinetics of 3MLCT to 3MC transition was scrutinized.

Polymer supported N-heterocyclic carbene ruthenium complex catalyzed transfer hydrogenation of ketones

Polymer supported N-heterocyclic carbene ruthenium complex (Poly-Ru-1) was synthesized by free radical polymerization of Ru complex monomer and divinylbenzene. The structure of Poly-Ru-1 was characterized by 13C solid-state NMR spectroscopy, N2 adsorption-desorption, X-ray photoelectron spectroscopy (XPS). 13C solid-state NMR spectra indicated the complete polymerization of Ru complex monomer. Poly-Ru-1 had the high specific surface area of 286.7 m2 g−1 and ruthenium was in +2 oxidation state. Poly-Ru-1 showed higher catalytic activity than the corresponding homogenous catalyst in transfer hydrogenation of ketones, which was attributed to the high dispersion of the active sites.

Mechanistic Divergence in the Hydrogenative Synthesis of Furans and Butenolides: Ruthenium Carbenes Formed by gem‐Hydrogenation or through Carbophilic Activation of Alkynes

AbstractEnynes with a tethered carbonyl substituent are converted into substituted furan derivatives upon hydrogenation using [Cp*RuCl]4 as the catalyst. Paradoxically, this transformation can occur along two distinct pathways, each of which proceeds via discrete pianostool ruthenium carbenes. In the first case, hydrogenation and carbene formation are synchronized (“gem‐hydrogenation”), whereas the second pathway comprises carbene formation by carbophilic activation of the triple bond, followed by hydrogenative catalyst recycling. Representative carbene intermediates of either route were characterized by X‐ray crystallography; the structural data prove that the attack of the carbonyl group on the electrophilic carbene center follows a Bürgi–Dunitz trajectory.

Mechanistic Divergence in the Hydrogenative Synthesis of Furans and Butenolides: Ruthenium Carbenes Formed by gem-Hydrogenation or through Carbophilic Activation of Alkynes.

Enynes with a tethered carbonyl substituent are converted into substituted furan derivatives upon hydrogenation using [Cp*RuCl]4 as the catalyst. Paradoxically, this transformation can occur along two distinct pathways, each of which proceeds via discrete pianostool ruthenium carbenes. In the first case, hydrogenation and carbene formation are synchronized (“gem‐hydrogenation”), whereas the second pathway comprises carbene formation by carbophilic activation of the triple bond, followed by hydrogenative catalyst recycling. Representative carbene intermediates of either route were characterized by X‐ray crystallography; the structural data prove that the attack of the carbonyl group on the electrophilic carbene center follows a Bürgi–Dunitz trajectory.

Ruthenium Catalyzed Ethenolysis of Internal Olefin Generated from Fischer-Tropsch Process to Produce α-Olefin

Ruthenium olefin metathesis catalysts were evaluated for the ethenolysis of trans-5-decene, a model compound of internal olefins, to produce α-olefin. N-heterocyclic carbene (NHC) ruthenium metathe...

Non‐Glovebox Ethenolysis of Ethyl Oleate and FAME at Larger Scale Utilizing a Cyclic (Alkyl)(Amino)Carbene Ruthenium Catalyst

AbstractA ruthenium metathesis catalyst Ru1 bearing a cyclic (alkyl)(amino)carbene (CAAC) ligand is used in the ethenolysis reaction of biosourced 90% pure undistilled ethyl oleate or technical fatty acid methyl esters mixture with grade 3 ethylene, to yield 1‐decene and valuable terminal unsaturated 9‐decenoic acid ester. Under optimized conditions (25 ppm of catalyst Ru1, 20 bar, 50 °C, 2 h, use of metal scavenger/quenching agent, SnatchCat), the title reaction proceeds with high productivity (up to 70% conversion) and selectivity, and is tested in scale up to 1 L, with the exclusion of a glovebox or Schlenk techniques usage, and the ethenolysis products are isolated by fractional distillation. In addition, a large‐scale synthesis of Ru1 is presented.Practical Applications: Decreasing availability of fossil organic feedstock and current policies of the European Union drives scientists to look for alternative sustainable sources of fine chemicals. In this context, olefin metathesis based ethenolysis of transesterified seed oils studied herein shows a significant potential as a sustainable way to obtain valuable products, such as 1‐decene and valuable terminal unsaturated 9‐decenoic acid ester. Unlike other published studies on ethenolysis, in the present work, the focus is on optimizing this reaction under industrially practical conditions, thus utilizing an air‐stable catalyst, technical grade oleic substrates, grade 3 ethylene, and excluding the use of a glovebox or Schlenk techniques.

Synthesis and ROMP of Benzothiadiazole Paracyclophane-1,9-Dienes to Donor–Acceptor Alternating Arylenevinylene Copolymers

Dialkoxy- and dialkyl-substituted phenylene benzothiadiazole paracyclophane-1,9-diene monomers undergo living ring-opening metathesis polymerization using ruthenium carbene initiators (G3) to give ...

Ruthenium–Carbene Complexes in the Synthesis of Polybutadiene and Its Cross-Metathesis with Polynorbornene

The activity of known and newly synthesized Ru-carbene catalysts in the metathesis polymerization of 1,5-cyclooctadiene and interchain cross-metathesis of synthesized polybutadiene with polynorbornenes is studied. First- and second-generation Grubbs Ru-complexes, second-generation Hoveyda–Grubbs complexes, and their modified analogs with asymmetric fluoro-containing ligands and unsaturated imidazolyl moieties are used as catalysts. It is shown that in the presence of the second-generation modified and nonmodified catalysts a partially crystalline polybutadiene is formed, in which blocks with trans-С=С bonds are about four times longer than blocks containing cis-С=С bonds. In the case of catalysts with bulkier ligands, the length of trans blocks is smaller and the melting temperature of the polymer is lower. The activities of the catalysts are compared by in situ 1Н NMR monitoring. New norbornene-butadiene multiblock copolymers are prepared for the first time by cross-metathesis between polybutadiene and polynorbornene. The highest degree of blockiness of the copolymers is attained when using second-generation Grubbs and Hoveyda–Grubbs catalysts. The activity sequences for the studied catalysts are constructed. It is shown that macromolecular cross-metathesis occurs more intensively in the presence of the catalysts with less bulky ligands.

Well‐Defined End‐Functionalized Conjugated Polymers/Oligomers Exhibiting Unique Emission Properties through the End Groups: The Exclusive Synthesis by Combined Olefin Metathesis with Wittig‐type Coupling

AbstractAcyclic diene metathesis polymerization using ruthenium–carbene catalysts affords defect‐free, high molecular weight poly(arylene vinylene)s containing all trans olefinic double bonds. The exclusive end‐functionalization in the resultant poly(fluorene vinylene)s or poly(phenylene vinylene)s can be attained by treating the vinyl end groups using a molybdenum–alkylidene catalyst/reagent (through olefin metathesis) followed by addition of various aldehydes (Wittig‐type coupling). Some of these end‐modified conjugated materials display unique emission properties, which are different from the original ones, through an interaction (energy transfer or structural change in the excited state) between the conjugated main chain and the end groups [oligo(thiophene)s, F‐BODIPY, etc.]. Exclusive synthesis of well‐defined, all‐trans end‐functionalized oligo(2,5‐dialkoxy‐1,4‐phenylene vinylene)s [(oligo(phenylene vinylene), alkoxy = O(CH2)2OSii Pr3, up to 31 repeat units] is demonstrated by adopting a stepwise synthetic approach (olefin metathesis and the subsequent Wittig‐type cleavage). It is clearly demonstrated that their optical properties (especially the fluorescence spectra including photoluminescence quantum yields) are strongly affected by the end groups as well as the conjugation repeat units.

Alkyne gem-Hydrogenation: Formation of Pianostool Ruthenium Carbene Complexes and Analysis of Their Chemical Character.

Parahydrogen (p-H2 ) induced polarization (PHIP) NMR spectroscopy showed that [CpX Ru] complexes with greatly different electronic properties invariably engage propargyl alcohol derivatives into gem-hydrogenation with formation of pianostool ruthenium carbenes; in so doing, less electron rich CpX rings lower the barriers, stabilize the resulting complexes and hence provide opportunities for harnessing genuine carbene reactivity. The chemical character of the resulting ruthenium complexes was studied by DFT-assisted analysis of the chemical shift tensors determined by solid-state 13 C NMR spectroscopy. The combined experimental and computational data draw the portrait of a family of ruthenium carbenes that amalgamate purely electrophilic behavior with characteristics more befitting metathesis-active Grubbs-type catalysts.

Alkyne gem‐Hydrogenation: Formation of Pianostool Ruthenium Carbene Complexes and Analysis of Their Chemical Character

AbstractParahydrogen (p‐H2) induced polarization (PHIP) NMR spectroscopy showed that [CpXRu] complexes with greatly different electronic properties invariably engage propargyl alcohol derivatives into gem‐hydrogenation with formation of pianostool ruthenium carbenes; in so doing, less electron rich CpX rings lower the barriers, stabilize the resulting complexes and hence provide opportunities for harnessing genuine carbene reactivity. The chemical character of the resulting ruthenium complexes was studied by DFT‐assisted analysis of the chemical shift tensors determined by solid‐state 13C NMR spectroscopy. The combined experimental and computational data draw the portrait of a family of ruthenium carbenes that amalgamate purely electrophilic behavior with characteristics more befitting metathesis‐active Grubbs‐type catalysts.

Hydrogenative Cyclopropanation and Hydrogenative Metathesis.

The unusual geminal hydrogenation of a propargyl alcohol derivative with [CpX RuCl] as the catalyst entails formation of pianostool ruthenium carbenes in the first place; these reactive intermediates can be intercepted with tethered alkenes to give either cyclopropanes or cyclic olefins as the result of a formal metathesis event. The course of the reaction is critically dependent on the substitution pattern of the alkene trap.