Ruthenium Carbene Complexes Research Articles

Diastereoselective Synthesis of Sulfoxide-Functionalized N-Heterocyclic Carbene Ruthenium Complexes: An Experimental and Computational Study.

The synthesis of sulfoxide-functionalized NHC ligand precursors were carried out by direct and mild oxidation from corresponding thioether precursors with high selectivity. Using these salts, a series of cationic [Ru(II)(η6-p-cymene)(NHC-SO)Cl]+ complexes were obtained in excellent yields by the classical Ag2O transmetallation route. NMR analyses suggested a chelate structure for the metal complexes, and X-ray diffractometry studies of complexes 4 b, 4 c, 4dBArF and 4 e unambiguously confirmed the preference for the bidentate (κ2-C,S) coordination mode of the NHC-SO ligands. Interestingly, only one diastereomer, in the form of an enantiomeric pair, was observed both in 1H NMR and in the solid state for the complexes. DFT calculations showed a possible intrinsic energy difference between the two pairs of diastereomer. The calculated energy barriers suggested that inversion of the sulfoxide is only plausible from the higher energy diastereomer together with bulky substituents. Inverting the configuration at the Ru center instead shows a lower and accessible activation barrier to provide the most stable diastereomer through thermodynamic control, consistent with the observation of a single species by 1H NMR as a pair of enantiomers. All these complexes catalyse the β-alkylation of secondary alcohols. Complex 4dPF6 bearing an NHC-functionalised S-Ad group has been further studied with different primary and secondary alcohols as substrates, showing high reactivity and high to moderate β-ol-selectivities.

Reversible Dihydrogen Activation and Catalytic Transfer Hydrogenation with Iminophosphinoyl‐Tethered Ruthenium Carbene Complexes

AbstractIn the last decades, metal ligand cooperativity has intensively been used as tool in homogeneous catalysis for the development of potent catalysts for a variety of applications, above all hydrogenation and transfer hydrogenation reactions. Herein, we report on the synthesis of iminophosphinoyl‐tethered ruthenium carbene complexes and their use in the activation of dihydrogen and the catalytic transfer hydrogenation of p‐fluoroacetophenone. Variation of the N‐substituent to an electron‐withdrawing p‐nitrophenyl group enabled the reversible activation of elemental hydrogen via a 1,2‐addition across the metal carbon double bond both in solution and in solid state. In comparison to the thiophosphinoyl‐tethered carbene complex the presented iminophosphinoyl systems showed a considerably increased catalytic activity in transfer hydrogenation reactions, thus highlighting the tunability of the cooperative behavior of nucleophilic carbene complexes.

A Ruthenophosphanorcaradiene as a Synthon for an Ambiphilic Metallophosphinidene.

Reaction of the ruthenium carbene complex Cp*(IPr)RuCl (1) (IPr = 1,3-bis(Dipp)imidazol-2-ylidene; Dipp = 2,6-diisopropylphenyl) with sodium phosphaethynolate (NaOCP) led to intramolecular dearomatization of one of the Dipp substituents on the Ru-bound carbene to afford a Ru-bound phosphanorcaradiene, 2. Computations by DFT reveal a transition state characterized by a concerted process whereby CO migrates to the Ru center as the P atom adds to the π system of the aryl group. The phosphanorcaradiene possesses ambiphilic properties and reacts with both nucleophilic and electrophilic substrates, resulting in rearomatization of the ligand aryl group with net P atom transfer to give several unusual metal-bound, P-containing main-group moieties. These new complexes include a metallo-1-phospha-3-azaallene (Ru─P═C═NR), a metalloiminophosphanide (Ru─P═N─R), and a metallophosphaformazan (Ru─P(═N─N═CPh2)2). Reaction of 2 with the carbene 2,3,4,5-tetramethylimidazol-2-ylidene (IMe4) produced the corresponding phosphaalkene DippP═IMe4.

Benzimidazol-2-ylidene ruthenium complexes for C-N bond formation through alcohol dehydrogenation.

A low temperature hydrogen borrowing approach to generate secondary amines using benzimidazole-based N-heterocyclic carbene (BNHC) ruthenium complexes is reported. A series of the piano-stool complexes of the type [(η6-p-cymene)(BNHC)RuCl2] (1a-g) were synthesized via one-pot reaction of the NHC salt precursor, Ag2O, and [RuCl2(p-cymene)]2 and characterized using conventional spectroscopic techniques. The geometry of two precursors, [(η6-p-cymene)(Me4BnMe2BNHCCH2OxMe)RuCl2] (1f) and [(η6-p-cymene)(Me5BnMe2BNHCCH2OxMe)RuCl2] (1g), was studied by single crystal X-ray diffraction. These catalysts were found to dehydrogenate alcohols efficiently at temperatures as low as 50 °C to allow Schiff-base condensation and subsequent imine hydrogenation to afford secondary amines. Notably, this ruthenium-based procedure enables the N-alkylation of aromatic and heteroaromatic primary amines with a wide range of primary alcohols in excellent yields of up to 98%. The present methodology is green and water is liberated as the sole byproduct.

Triazole-modified Ru-carbene complexes: A valid olefin metathesis pre-catalyst for dynamic covalent chemistry via C=C bond formation

The 1,2,3-triazole coordinated ruthenium carbene complexes (TA-Ru) were reported for the first time as a new class of modified Grubbs catalyst to achieve challenging olefin metathesis at higher temperatures without catalyst decomposition. Previously reported N-tethered Ru-carbene complexes all suffered from rapid cis/trans isomerization, causing significantly reduced catalyst reactivity. These new TA-Ru complexes hold the active trans-dichloro conformation even at 80 °C, allowing effective olefin metathesis for challenging substrates. With this new TA-Ru catalyst, cross-metathesis (CM), ring-closing metathesis (RCM) and dynamic covalent chemistry (DCvC) were achieved. Excitingly, the reactivity of TA-Ru prevails all previously reported N-coordinated Ru-carbene precatalysts, Grubbs II, and Hoveyda-Grubbs, making the TA-Ru a transformative catalytic system in olefin catalysis.

β‐Alkylation through Dehydrogenative Coupling of Primary Alcohols and Secondary Alcohols Catalyzed by Thioether‐Functionalized N‐Heterocyclic Carbene Ruthenium Complexes

AbstractA catalytic system for the direct β‐alkylation of secondary alcohol with primary alcohol has been investigated. In this work, a series of cationic Ru(II)(η6‐p‐cymene) complexes with thioether‐functionalized N‐heterocyclic carbene ligands (imidazole‐based 1 a–l and benzimidazole‐based 2 a–e) have been successfully synthesized and evaluated as catalysts. This investigation shows that modifications in the ligand moiety (thioether group and/or NHC core) have a strong effect on both selectivity and reactivity. Imidazole‐based complex 1 c, with only 1 mol % of catalyst loading, displayed the best catalytic activity as well as the highest selectivity for the β‐alcohol up to 98 : 2 for this tandem borrowing hydrogen/aldol methodology. Applied to a wide range of substrates, β‐alkylated secondary alcohols have been obtained in moderate yields, but generally with complete conversion and very high selectivity.

How to Break the Law: trans‐Hydroboration and gem‐Hydroboration of Alkynes

AbstractClassical hydroboration is the textbook example of an organometallic reaction under rigorous frontier orbital control: a synergetic bonding mode between substrate and reagent evolves into a four‐membered transition state that enforces syn/suprafacial delivery of H−BR2 to the π‐system. This strict stereochemical “law” remained valid for decades, even after the advent of metal‐catalyzed hydroboration. During the last decade, however, numerous possibilities emerged that allow this paradigm to be challenged. Thus, direct trans‐hydroboration of terminal as well as internal alkynes was achieved through radical, ionic, organocatalytic, and metal‐catalyzed manifolds, which are summarized in this review. Among them, pathways involving either metal vinylidenes (Rh, Ir, Ru, Fe) derived from terminal alkynes or metallacyclopropenes (η2‐vinylmetal complexes) derived from an internal triple bond and a [CpXRu]‐based catalyst represent the currently most widely applicable solutions. The latter type of intermediates is also accountable for the equally perplexing gem‐hydroboration of alkynes: geminal delivery of a borane to a triple bond has no precedent in the classical canon. The only forerunner was gem‐hydrogenation, which represents an unconventional yet highly useful entry into discrete ruthenium carbene complexes. The close mechanistic ties between these transformations are outlined, and brief reference is also made to remotely related reactions such as trans‐diboration and trans‐carboboration.

Light‐Driven Alkyne gem‐Hydrogenation: An Intramolecular Approach to Hoveyda–Grubbs Catalysts

The light-driven gem-hydrogenation of internal alkynes in the presence of [(NHC)(η6-cymene)RuCl2] generates discrete ruthenium carbene complexes. When applied to appropriately designed enyne substrates, the reactive intermediates thus formed will engage the tethered olefin in metathetic ring closure while splitting off a Hoveyda-Grubbs-type complex as secondary carbene. This unconventional approach to these classical catalysts for olefin metathesis rivals existing methodology in that it is safe, short, phosphine-free, and uses readily available starting materials.

Styrene-alkyne cross ene-yne metathesis: Catalyst screening and optimization for unbranched terminal alkyne substrates

Cross metathesis between styrenes and unbranched terminal alkynes is reported. Styrene cross metathesis with terminal alkynes that lack branching at the propargylic position are unknown. In general, unbranched terminal alkynes have been problematic in cross metathesis due to competing alkyne oligomerization. A catalyst screen was performed for a variety of Grubbs catalysts to optimize the yield of the desired 1,3‑diene product. This catalyst screen identified two ruthenium carbene complexes that have been rarely used for cross metathesis applications. Additionally, a side product arising from a competing alkene metathesis was minimized by choice of catalyst and through reaction optimization.

Benzimidazole-based N[sbnd]heterocyclic carbene ruthenium(II) complexes: Synthesis and C[sbnd]H bond activation properties

The transition-metal-promoted CH activation has become an efficient as well as an atom-economic methodology for the synthesis of a wide array of organic molecules. Herein, the reaction of RuCl2(p-cymene)]2 with 1,3-dialkylbenzimidazolium salts (1a-c) in the presence of a small excess of cesium carbonate yields chelated η6-arene, η1-carbene ruthenium complexes (2a-c). All synthesized compounds were characterized by elemental analysis and NMR spectroscopy. The catalytic activity of [Cl2Ru(η6-arene, η1-carbene)] complexes (2a-c) were evaluated in the direct arylation of 2-phenylpyridine and 1-phenylpyrazole with (hetero)aryl chlorides derivative.

Synthesis and catalytic olefin metathesis activity of amberlyst-15 supported cyclic and bicyclic alkyl amino carbene ruthenium complexes

Amberlyst-15 supported cyclic alkyl amino carbene and bicyclic alkyl amino carbene ruthenium olefin metathesis catalysts for sustainable catalytic applications have been synthesized by the well-known wet impregnation method utilizing ionic complex/support interaction. Surface coverages are as high as 4 and 7 wt% were achieved in the case of the significantly higher pore volume Amberlyst-15, compared to Amberlyst-36. These phase separable catalysts show high activity in cross metathesis, ring closing metathesis and ethenolysis reactions compared to the reported heterogenized olefin metathesis catalysts. Leeching tests revealed no more than 1.5 ppm ruthenium content for the investigated metathesis reactions, which is well below the accepted 10 ppm limit in case of consumer products.

Syntheses, characterizations and structures of ruthenium carbene and allenylidene complexes supported by 1,4,7-trimethyl-1,4,7-triazacyclononane (Me3tacn) ligands

Treatment of ruthenium(II) precursor [(Me3tacn)Ru(dmso)Cl2] (Me3tacn = 1,4,7-trimethyl-1,4,7-triazacyclononane, dmso = dimethylsulfoxide) (1) with concentrated HCl in the presence of air afforded the ruthenium(III) complex [(Me3tacn)RuCl3·H2O] (2), from which a series of bipyridine-ruthenium(II) complexes [(Me3tacn)Ru(R-bpy)(H2O)](PF6)2 (bpy = 2,2′-bipyridine) was obtained according to our previous work. Reactions of [(Me3tacn)Ru(R-bpy)(H2O)](PF6)2 and 1,1-diphenylpropargyl alcohol afforded ruthenium(II) methoxycarbene complexes [(Me3tacn)(R-bpy)Ru=C(OMe)CHCPh2](PF6)2 (R = H,3; 4,4′-Me2, 5; 5,5′-Me2, 7; 4,4′-di-tBu, 9) and ruthenium(II) allenylidene complexes [(Me3tacn)(R-bpy)Ru=C=CCPh2](PF6)2 (R = H,4; 4,4′-Me2, 6; 5,5′-Me2, 8; 4,4′-di-tBu, 10) under different solvent conditions. Complexes 3‒10 were well characterized by infrared, UV/Vis, mass spectrometry, and NMR spectroscopies. The molecular structures of complexes 5, 7, and 8 have been also established by single-crystal X-ray diffraction.

Hydrogenative Cycloisomerization and Sigmatropic Rearrangement Reactions of Cationic Ruthenium Carbenes Formed by Catalytic Alkyne gem‐Hydrogenation

Abstractgem‐Hydrogenation of propargyl alcohol derivatives with [CpXRu(MeCN)3]PF6 (CpX=substituted cyclopentadienyl) as catalysts affords cationic pianostool ruthenium carbene complexes which are so electrophilic that they attack a tethered olefin to furnish cyclopentene products; cyclopropanation or metathesis do not compete with this novel transformation. If the transient carbenes carry appropriate propargylic substituents, however, they engage in ([2,3]‐sigmatropic) rearrangements to give enol esters (carbonates, carbamates, sulfonates) or alkenyl halides. Both pathways are unprecedented in the vast hydrogenation literature. The proposed mechanistic scenarios are in line with labeling experiments and spectroscopic data; most notably, PHIP NMR spectroscopy (PHIP=parahydrogen induced polarization) provides compelling evidence that the reactions are indeed triggered by highly unorthodox gem‐hydrogenation events.

Hydrogenative Cycloisomerization and Sigmatropic Rearrangement Reactions of Cationic Ruthenium Carbenes Formed by Catalytic Alkyne gem-Hydrogenation.

gem‐Hydrogenation of propargyl alcohol derivatives with [CpXRu(MeCN)3]PF6 (CpX=substituted cyclopentadienyl) as catalysts affords cationic pianostool ruthenium carbene complexes which are so electrophilic that they attack a tethered olefin to furnish cyclopentene products; cyclopropanation or metathesis do not compete with this novel transformation. If the transient carbenes carry appropriate propargylic substituents, however, they engage in ([2,3]‐sigmatropic) rearrangements to give enol esters (carbonates, carbamates, sulfonates) or alkenyl halides. Both pathways are unprecedented in the vast hydrogenation literature. The proposed mechanistic scenarios are in line with labeling experiments and spectroscopic data; most notably, PHIP NMR spectroscopy (PHIP=parahydrogen induced polarization) provides compelling evidence that the reactions are indeed triggered by highly unorthodox gem‐hydrogenation events.

Nitrogen–nitrogen-functionalized N-heterocyclic carbene ruthenium(ii) complexes realized efficient CO2 hydrogenation to formate

Three new Ru–CNN complexes are synthesized for the hydrogenation of CO2 to formate. The Ru–CNN complex exhibits a long lifetime of over 400 h at 170 °C with a high TON of 6.5 × 105.

A Fischer-Type Ruthenium Carbene Complex as a Metathesis Catalyst for the Synthesis of Enol Ethers.

The Grubbs G-I or G-II catalyst gives the ruthenium ethoxy carbene complex, which catalyzes ring-opening cross metathesis (ROCM) of a strained cyclic alkene to give a diene where one of the two alkene moieties in the product contains an ethoxy substituent. No polymeric products are detected. Hydrocarbons such as parent norbornene or substituted cyclopropenes can proceed with the reaction smoothly. Tertiary amines, N-alkylimides, esters, and aryl or alkyl bromides remain intact under the reaction conditions. In addition to vinyl ethers, vinylic esters can also be used. The time required to reach a 50% yield of the ROCM product t50 varies from 0.01 to 140 h depending on the strain and nucleophilicity of the double bond. Anchimeric participation of an electron-rich group would result in significant enhancement of the reactivity, and the t50 could be as short as several minutes. A similar substrate without such a neighboring group shows a much slower rate. An exo-norborne derivative reacts much faster than the corresponding endo-isomer. Alkenes with poor nucleophilicity are less favored for the ROCM process, so is less strained cyclooctene.

Cover Feature: Tuning Ruthenium Carbene Complexes for Selective P−H Activation through Metal‐Ligand Cooperation (Chem. Eur. J. 69/2021)

P−H bond activation is an important step in many catalytic applications involving the formation of new P−C bonds. The icebreaker “RV Bond Activation”, which symbolizes our carbene complexes depicted by the constellation in the sky, is able to easily break P−H bonds via the carbene linkage. The splitting of phosphines into protons and phosphide fragments (depicted as ice shards flying away) occurs even at low temperatures. More information can be found in the Full Paper by V. H. Gessner et al. (DOI: 10.1002/chem.202103151).

Tuning Ruthenium Carbene Complexes for Selective P-H Activation through Metal-Ligand Cooperation.

The use of iminophosphoryl‐tethered ruthenium carbene complexes to activate secondary phosphine P−H bonds is reported. Complexes of type [(p‐cymene)‐RuC(SO2Ph)(PPh2NR)] (with R = SiMe3 or 4‐C6H4−NO2) were found to exhibit different reactivities depending on the electronics of the applied phosphine and the substituent at the iminophosphoryl moiety. Hence, the electron‐rich silyl‐substituted complex undergoes cyclometallation or shift of the imine moiety after cooperative activation of the P−H bond across the M=C linkage, depending on the electronics of the applied phosphine. Deuteration experiments and computational studies proved that cyclometallation is initiated by the activation process at the M=C bond and triggered by the high electron density at the metal in the phosphido intermediates. Consistently, replacement of the trimethylsilyl (TMS) group by the electron‐withdrawing 4‐nitrophenyl substituent allowed the selective cooperative P−H activation to form stable activation products.

Light-Driven gem Hydrogenation: An Orthogonal Entry into "Second-Generation" Ruthenium Carbene Catalysts for Olefin Metathesis.

The newly discovered light‐driven gem hydrogenation of alkynes opens an unconventional yet efficient entry into five‐coordinate Grubbs‐type ruthenium carbene complexes with cis‐disposed chloride ligands. Representatives of this class featuring a chelate substructure formed by an iodo‐substituted benzylidene unit react with (substituted) 2‐isopropoxystyrene to give prototypical “second‐generation” Grubbs‐Hoveyda complexes for olefin metathesis. The new approach to this venerable catalyst family is safe and versatile as it uses a triple bond rather than phenyldiazomethane as the ultimate carbene source and does not require any sacrificial phosphines.

Cis-β-Ru(II)heteroatom-stabilized-carbene complexes supported by tetradentate Schiff-base salen ligand

A panel of stable ruthenium carbene complexes bearing a tetradentate salen ligand, with their carbene-C atom adjacent to heteroatom, were synthesized from cis-β-Ru(II)-bis(CO) complex by reacting with the corresponding carbene precursors. These carbene complexes, which were fully characterized by elemental analysis, NMR, IR spectroscopy and X-ray-crystal structure determination, all adopt a cis-β configuration; the heteroatom (N or O) adjacent to the carbene C atom contributes to the high stability of these complexes both in solution and in solid state. DFT calculations were also performed to gain insight into the Ru-Ccarbene bonds.