Imidazol(in)ium-2-Carboxylates as N-Heterocyclic Carbene Ligand Precursors for Ruthenium Metathesis Initiators

Summary: Imidazol(in)ium-2-carboxylates were used as N-heterocyclic carbene (NHC) ligand precursors to convert the [RuCl₂(p-cymene)]₂ dimer into three ruthenium-arene complexes of the [RuCl₂(p-cymene)(NHC)] type. The decarboxylation of NHC · CO₂ betaines also provided a convenient synthetic path to prepare five well-known ruthenium-NHC catalysts for olefin metathesis and related reactions, including the second generation Grubbs and Hoveyda–Grubbs catalysts, via ligand exchange with phosphine-containing, first generation ruthenium-benzylidene or indenylidene complexes. Both procedures are particularly attractive from a practical point of view, because NHC · CO₂ adducts are stable zwitterionic compounds that can be stored and handled with no particular precautions.     

Electrochemistry as a correlation tool with the catalytic activities in [RuCl2(p-cymene)(PAr3)]-catalysed Kharasch additions

[RuCl₂(p-cymene)] complexes containing triarylphosphine ligands with various substituents at the para position were used to catalyse the atom transfer radical addition of carbon tetrachloride to various olefins, and their catalytic activities were nicely correlated with their electrochemical parameters.     

Rhodium or ruthenium units peripherally coordinated to carbosilane dendrimers functionalized with P-stereogenic monophosphines

Carbosilane dendrimers containing P-stereogenic monophosphines as terminal groups, Dend-{CH₂PPhR}_n (R = 2-biphenylyl or 9-phenanthryl), were reacted with [RhCl(COD)]₂ or [RuCl₂(p-cymene)]₂ to afford the corresponding chiral metalladendrimers Dend-{CH₂PPhR(RhCl(COD))}_n or Dend-{CH₂PPhR(RuCl₂(p-cymene))}_n, respectively. Attempts to obtain the first generation Ru-dendrimer for R = 2-biphenylyl proved unsuccessful, probably due to the steric hindrance of R. Complete characterization of these species was achieved by multinuclear NMR spectra, including 2D experiments, mass spectrometry, and optical rotation determinations. The catalytic properties of the rhodium dendrimers were tested in the hydrogenation of dimethylitaconate and those of the ruthenium derivatives in the asymmetric hydrogen transfer of acetophenone. The following model chiral compounds, (CH₃)₃Si{CH₂PPhR(RhCl(COD))} and (CH₃)₃Si{CH₂PPhR(RuCl₂(p-cymene))}, were prepared in order to detect potential dendritic effects. All compounds were active in the catalytic conditions tested, but low or null e.e. were found.     

Imidazol(in)ium-2-carboxylates as N-heterocyclic carbene precursors in ruthenium–arene catalysts for olefin metathesis and cyclopropanation

Five imidazol(in)ium-2-carboxylates bearing cyclohexyl, mesityl, or 2,6-diisopropylphenyl substituents on their nitrogen atoms were prepared from the corresponding N-heterocyclic carbenes (NHCs) by reaction with carbon dioxide. They were characterized by IR and NMR spectroscopies, and by TGA. Their ability to act as NHC precursors for in situ catalytic applications was probed in ruthenium-promoted olefin metathesis and cyclopropanation reactions. When visible light induced ring-opening metathesis polymerization of cyclooctene or cyclopropanation of styrene with ethyl diazoacetate were carried out at 60 °C in the presence of [RuCl₂(p-cymene)]₂, the NHC · CO₂ adducts and their NHC · HX counterparts (X = Cl, BF₄) displayed similar activities. When metathesis polymerizations were performed at room temperature, the carboxylates proved far superior to the corresponding imidazol(in)ium acid salts. They displayed the same level of activity as the preformed RuCl₂(p-cymene)(IMes) complex, whereas the combination of NHC · HX and KO-t-Bu were almost totally inactive. Results obtained for cyclopropanation reactions at room temperature did not show such a large discrepancy of behavior between the two types of adducts.     

Dual catalytic performance of arene-ruthenium amine complexes for norbornene ring-opening metathesis and methyl methacrylate atom-transfer radical polymerizations

Arene ruthenium(II) complexes bearing the cyclic amines RuCl₂(η⁶-p-cymene)(pyrrolidine)] ( 1 ), [RuCl₂(η⁶-p-cymene)(piperidine)] ( 2 ), and [RuCl₂(η⁶-p-cymene)(peridroazepine)] ( 3 ) were successfully synthesized. Complexes 1 – 3 were fully characterized by means of Fourier transform infrared, UV–visible, and NMR spectroscopy, elemental analysis, cyclic voltammetry, computational methods, and one of the complexes was further studied by single crystal X-ray crystallography. These compounds were evaluated as catalytic precursors for ring-opening metathesis polymerization (ROMP) of norbornene (NBE) and atom-transfer radical polymerization (ATRP) of methyl methacrylate (MMA). NBE polymerization via ROMP was evaluated using complexes 1 – 3 as precatalysts in the presence of ethyl diazoacetate (EDA) under different [NBE]/[EDA]/[Ru] ratios, temperatures (25 and 50°C), and reaction times (5–60 min). The highest yields of polyNBE were obtained with [NBE]/[EDA]/[Ru] = 5000/28/1 for 60 min at 50°C. MMA polymerization via ATRP was conducted using 1 – 3 as catalysts in the presence of ethyl-α-bromoisobutyrate (EBiB) as initiator. The catalytic tests were evaluated as a function of the reaction time using the initial molar ratio of [MMA]/[EBiB]/[Ru] = 1000/2/1 at 95°C. The increase in molecular weight as function of time indicates that complexes 1–3 were able to mediate the MMA polymerization with an acceptable rate and some level of control. Differences in the rate of polymerization were observed in the order 3 > 2 > 1 for the ROMP and ATRP.     

Transfer Hydrogenation of Ketones Catalyzed by 1-Alkylbenzimidazole Ruthenium(II) Complexes

Summary. Six [RuCl₂(1-alkylbenzimidazole)(p-cymene)] complexes have been prepared and the new compounds characterized by C, H, N analyses, ¹H NMR, and ¹³C NMR. The reduction of ketones to alcohols via transfer hydrogenation was achieved with catalytic amounts of the complexes in the presence of t-BuOK.     

Improved hydrogen generation from formic acid

The ruthenium-catalyzed generation of hydrogen from formic acid was investigated in the presence of amines and halide additives. While amidines and halide additives increase the production of hydrogen with [RuCl₂(p-cymene)]₂, >330 mL hydrogen/h is generated in the presence of [RuCl₂(benzene)]₂/dppe and N,N-dimethyl-n-hexylamine.     

Racemization of secondary alcohols catalyzed by ruthenium: application to chemoenzymatic dynamic resolution

In this paper, we have shown that the [RuCl₂(p-cymene)]₂ complex associated with simple hemisalen ligands is able to racemize (S)-1-phenylethanol. The influence on the racemization process of the ligand’s structure as well as the nature of a co-catalyst have been evaluated and optimized. This [RuCl₂(p-cymene)]₂/Ligand/TEMPO racemization system was then associated with the Candida Antarctica B lipase in order to carry out dynamic kinetic resolution experiments on rac-phenylethanol. This led us to identify the best conditions for effective DKR, which was then applied to various secondary benzylic and aliphatic alcohols. It was thus possible to obtain (R)-1-cyclohexylethyl acetate from rac-1-cyclohexylethanol in quantitative conversion and with high enantioselectivity (98%).     

[(NHC)(NHCewg)RuCl2(CHPh)] Complexes with Modified NHCewg Ligands for Efficient Ring‐Closing Metathesis Leading to Tetrasubstituted Olefins

Imidazolium salts (NHC_ewg ? HCl) with electronically variable substituents in the 4,5‐position (H,H or Cl,Cl or H,NO₂ or CN,CN) and sterically variable substituents in the 1,3‐position (Me,Me or Et,Et or iPr,iPr or Me,iPr) were synthesized and converted into the respective [AgI(NHC)_ewg] complexes. The reactions of [(NHC)RuCl₂(CHPh)(py)₂] with the [AgI(NHC_ewg)] complexes provide the respective [(NHC)(NHC_ewg)RuCl₂(CHPh)] complexes in excellent yields. The catalytic activity of such complexes in ring‐closing metathesis (RCM) reactions leading to tetrasubstituted olefins was studied. To obtain quantitative substrate conversion, catalyst loadings of 0.2–0.5 mol % at 80 °C in toluene are sufficient. The complex with the best catalytic activity in such RCM reactions and the fastest initiation rate has an NHC_ewg group with 1,3‐Me,iPr and 4,5‐Cl,Cl substituents and can be synthesized in 95 % isolated yield from the ruthenium precursor. To learn which one of the two NHC ligands acts as the leaving group in olefin metathesis reactions two complexes, [(FL‐NHC)(NHC_ewg)RuCl₂(CHPh)] and [(FL‐NHC_ewg)(NHC)RuCl₂(CHPh)], with a dansyl fluorophore (FL)‐tagged electron‐rich NHC ligand (FL‐NHC) and an electron‐deficient NHC ligand (FL‐NHC_ewg) were prepared. The fluorescence of the dansyl fluorophore is quenched as long as it is in close vicinity to ruthenium, but increases strongly upon dissociation of the respective fluorophore‐tagged ligand. In this manner, it was shown for ring‐opening metathesis ploymerization (ROMP) reactions at room temperature that the NHC_ewg ligand normally acts as the leaving group, whereas the other NHC ligand remains ligated to ruthenium.     

Di- and tetranulcear Ru complexes of phenylene-1,4-diaminotetra(phosphonite), p-C6H4[N{P(OC6H4OMe-o)2}2]2 and their catalytic investigation towards transfer hydrogenation reactions

The stoichiometric reaction of phenylene-1,4-diaminotetra(phosphonite), p-C₆H₄[N{P(OC₆H₄OMe-o)₂}₂]₂ (P₂NФNP₂) (1) with [RuCl₂(p-cymene)]₂ in acetonitrile produces cis,cis-[{RuCl₂(CH₃CN)₂}₂(P₂NФNP₂)] (2), whereas the similar reaction of 1 with [RuCl₂(p-cymene)]₂ in THF medium affords a tri-chloro-bridged tetrametallic complex, [{(η⁶-p-cymene)Ru₂(μ₂-Cl)₃Cl}₂(P₂NФNP₂)] (3) irrespective of the stoichiometry and reaction conditions. The formation and structure of complexes 2 and 3 are assigned through various spectroscopic and micro analysis data. The molecular structure of 2 is confirmed by single crystal X-ray diffraction study. The catalytic activities of complexes 2 and 3 have been investigated in transfer hydrogenation reactions.     

N-Heterocyclic Carbene (NHC)-Stabilized Ru0 Nanoparticles: In Situ Generation of an Efficient Transfer Hydrogenation Catalyst

Tethered and untethered ruthenium half-sandwich complexes were synthesized and characterized spectroscopically. X-ray crystallographic analysis of three untethered and two tethered Ru N-heterocyclic carbene (NHC) complexes were also carried out. These RuNHC complexes catalyze transfer hydrogenation of aromatic ketones in 2-propanol under reflux, optimally in the presence of (25 mol %) KOH. Under these conditions, the formation of 2–3 nm-sized Ru⁰ nanoparticles was detected by TEM measurements. A solid-state NMR investigation of the nanoparticles suggested that the NHC ligands were bound to the surface of the Ru nanoparticles (NPs). This base-promoted route to NHC-stabilized ruthenium nanoparticles directly from arene-tethered ruthenium–NHC complexes and from untethered ruthenium–NHC complexes is more convenient than previously known routes to NHC-stabilized Ru nanocatalysts. Similar catalytically active RuNPs were also generated from the reaction of a mixture of [RuCl₂(p-cymene)]₂ and the NHC precursor with KOH in isopropanol under reflux. The transfer hydrogenation catalyzed by these NHC-stabilized RuNPs possess a high turnover number. The catalytic efficiency was significantly reduced if nanoparticles were exposed to air or allowed to aggregate and precipitate by cooling the reaction mixtures during the reaction.     

Coordination chemistry of the [Pt2(μ-S)2(PPh3)4] metalloligand with π-hydrocarbon derivatives of d ruthenium(II), osmium(II), rhodium(III) and iridium(III)

The reactivity of [Pt₂(μ-S)₂(PPh₃)₄] towards [RuCl₂(η⁶-arene)]₂ (arene=C₆H₆, C₆Me₆, p-MeC₆H₄Prⁱ=p-cymene), [OsCl₂(η⁶-p-cymene)]₂ and [MCl₂(η⁵-C₅Me₅)]₂ (M=Rh, Ir) have been probed using electrospray ionisation mass spectrometry. In all cases, dicationic products of the type [Pt₂(μ-S)₂(PPh₃)₄ML]²⁺ (L=π-hydrocarbon ligand) are observed, and a number of complexes have been prepared on the synthetic scale, isolated as their BPh₄⁻ or PF₆⁻ salts, and fully characterised. A single-crystal X-ray structure determination on the Ru p-cymene derivative confirms the presence of a pseudo-five-coordinate Ru centre. This resists addition of small donor ligands such as CO and pyridine. The reaction of [Pt₂(μ-S)₂(PPh₃)₄] with RuClCp(PPh₃)₂ (Cp=η⁵-C₅H₅) gives [Pt₂(μ-S)₂(PPh₃)₄RuCp]⁺. In addition, the reaction of [Pt₂(μ-S)₂(PPh₃)₄] with the related carbonyl complex [RuCl₂(CO)₃]₂, monitored by electrospray mass spectrometry, gives [Pt₂(μ-S)₂(PPh₃)₄Ru(CO)₃Cl]⁺.     

Thermal decomposition of new chlorido(p-cymene) ruthenium(II) complexes containing N-alkylphenothiazines

The thermal decomposition of [RuCl₂(η⁶-p-cymene)]₂ (1) and its biologically active N-alkylphenothiazine compounds of composition L[RuCl₃(η⁶-p-cymene)] where L = CPH⁺ (2), TFH⁺·HCl (3), and TRH⁺ (4) (chlorpromazine hydrochloride, CP·HCl; trifluoperazine dihydrochloride, TF·2HCl; and thioridazine hydrochloride, TR·HCl, respectively) has been studied. The crystal and molecular structure of compound 3 was determined earlier by single crystal X-ray diffraction analysis. The thermal data were collected by simultaneous TG/DSC measurements. For evolved gas detection, the qualitative reaction of chlorides with AgNO₃ in an acidic solution was applied. The measurements were carried out in the temperature range to 700 °C in nitrogen atmosphere. Compounds of L[RuCl₃(η⁶-p-cymene)] crystallize with water or water/2-propanole. On the basis of thermal data, the trend in the solvent bonding energies was assessed.     

Selective ruthenium-catalyzed double reductive aminations using hydrosilane to access tertiary amines and piperidine derivatives

A highly selective double reductive aminations of aldehydes with anilines to give tertiary amines, in the presence of [RuCl₂(p-cymene)]₂ catalyst and PhSiH₃, was performed under neat conditions. Piperidine derivatives were successfully synthesized by a double reductive amination followed by cyclisation from glutaric dialdehyde with anilines.     

Ruthenium(II)-catalyzed transfer hydrogenation of aldehydes with new water-soluble monotosylated ethylenediamines as ligands

New water-soluble monotosylated ethylenediamines containing quaternary ammonium groups were conveniently synthesized from ethylenediamine. The ruthenium catalysts prepared in situ from ruthenium complex [RuCl₂(p-cymene)]₂ and water-soluble monotosylated ethylenediamine ligands were used in transfer hydrogenation of aldehydes, and excellent conversions and chemoselectivities were achieved with sodium formate as reductant in neat water.     

Substituted salen–Ru(II) complexes as catalysts in the asymmetric cyclopropanation of styrene by ethyl diazoacetate: the influence of substituents and achiral additives on activity and enantioselectivity

A series of alkyl-, halogen- and nitro-substituted salen ligands, 1, have been employed in the asymmetric cyclopropanation of styrene with ethyl diazoacetate by its ruthenium(II) complex with [RuCl₂(p-cymene)]₂ or RuCl₂(PPh₃)₃ as precursors. The introduction of appropriate electron withdrawing groups in the salen ligands benefited the enantioselectivity of the reaction. Some additives, including O-donor, N-donor and P-donor ligands, were added to the reaction to improve the enantioselectivity and activity, and e.e.s of up to 80% were achieved. In the salen/[RuCl₂(p-cymene)]₂ system, the (1R,2S)-isomer was obtained in 80.2% e.e. by using the salen ligand 1f derived from 3,5-dibrominated salicylaldehyde with Et₃N as additive. E.e.s of up to 81.3% for (1S,2R)-isomers were achieved by using the complex 2 synthesized from the nitro-substituted ligand 1m and RuCl₂(PPh₃)₃. A possible mechanism was also discussed.     

Ru(II)-Catalyzed rearrangement of 2-aryl-2-(phenylthio)penta-3,4-dienyl 2,2,2-trichloroacetimidates

[RuCl₂(p-cymene)]₂ efficiently catalyzes the rearrangement of 2-aryl-2-(phenylthio)penta-3,4-dienyl 2,2,2-trichloroacetimidate to afford (Z)-2,2,2-trichloro-N-(4-aryl-1-(phenylthio)penta-2,4-dien-2-yl)acetamide. Ru carbene is assumed as the reactive intermediate in this rearrangement.     

“One pot” synthesis of tertiary amines: Ru(II) catalyzed direct reductive N-benzylation of imines with benzyl bromide derivatives

The direct reductive N-benzylation of imines by reaction with benzyl bromide derivatives, in the presence of [RuCl₂(p-cymene)]₂ catalyst and PhSiH₃, is performed under mild conditions without additional base. This reaction proceeds by a tandem imine hydrosilylation/nucleophilic substitution with benzyl bromide derivatives to result the tertiary amines.     

Catalytic activity of a half-sandwich Ru(II)-N-heterocyclic carbene complex in the oligomerization of alkynes

The ruthenium(II)-N-heterocyclic carbene complex, [RuCl₂(1-butyl-3-methylimidazol-2-ylidene)(p-cymene)] selectively catalyzes oligomerization of phenylacetylene (PA) and its derivatives to linear oligomers, containing positively charged imidazolium end-group and uncharged ones. The charged oligomer chain consists of maximum 9-11 PA monomer units after 36 h reaction at 80 °C whereas mainly pentamers are formed as other products. The H₂ atmosphere retards oligomerization of PA and hydrogenation to vinylbenzene and ethylbenzene is observed instead.     

Water-soluble (η-arene)ruthenium(II)-phosphine complexes and their catalytic activity in the hydrogenation of bicarbonate in aqueous solution

The reactions of [(η⁶-C₆H₆)RuCl₂]₂ and [(η⁶-p-cymene)RuCl₂]₂ with hydrogen in the presence of the water-soluble phosphines tppts (meta-trisulfonated triphenylphosphine) and pta (1,3,5-triaza-7-phosphaadamantane) afforded as the main species [(η⁶-C₆H₆)RuH(tppts)₂]⁺, [(η⁶-C₆H₆)RuH(pta)₂]⁺, [(η⁶-p-cymene)RuH(tppts)₂]⁺ and [(η⁶-p-cymene)RuH(pta)₂]⁺. This latter complex was also formed in the reaction of [(η⁶-p-cymene)RuCl₂(pta)] and hydrogen with a redistribution of pta. In addition, prolonged hydrogenation at elevated temperatures and in the presence of excess of pta led to the formation of the arene-free [RuH(pta)₄Cl], [RuH(pta)₄(H₂O)]⁺, [RuH₂(pta)₄] and [RuH(pta)₅]⁺ complexes. Ru-hydrides, such as [(η⁶-arene)RuH(L)₂]⁺, catalyzed the hydrogenation of bicarbonate to formate in aqueous solutions at p(H₂)=100 bar, T=50-70 °C.