Ruthenium N-heterocyclic Carbene Complexes Research Articles

Dehydrogenative Coupling of Primary Alcohols To Form Esters Catalyzed by a Ruthenium N-Heterocyclic Carbene Complex

The ruthenium complex [RuCl2(IiPr)(p-cymene)] catalyzes the direct condensation of primary alcohols into esters and lactones with the release of hydrogen gas. The reaction is most effective with linear aliphatic alcohols and 1,4-diols and is believed to proceed with a ruthenium dihydride as the catalytically active species.

Ligand-Controlled Highly Regioselective and Asymmetric Hydrogenation of Quinoxalines

The research group of Glorius developed a ligand-controlled hydrogenation of quinoxalines. Selecting the right N-heterocyclic carbene ligand facilitates the regioselective hydro-genation of either the heterocyclic or the carbocyclic ring of a substituted quinoxaline. The hydrogenated products were generally obtained in quantitative yields and by employing chiral N-heterocyclic carbene ruthenium complexes, the 5,6,7,8-tetrahydroquinoxalines were furnished with good enantioselectivities.

A computational perspective of olefins metathesis catalyzed by N-heterocyclic carbene ruthenium (pre)catalysts

In recent years olefin metathesis catalyzed by N-heterocyclic carbene ruthenium complexes has attracted remarkable attention as a versatile tool to form new CC bonds. The last developed (pre)catalysts show excellent performances, and this achievement has been possible because of continuous experimental and computational efforts to understand the laws controlling the behavior of these systems. This perspective rapidly traces the ideas and discoveries that computational chemistry contributed to the development of these catalysts, with particular emphasis on catalysts presenting a N-heterocyclic carbene ligand.

Transfer hydrogenation of unfunctionalised alkenes using N-heterocyclic carbene ruthenium catalyst precursors

Transfer hydrogenation of unfunctionalised and aliphatic alkenes in iPrOH/KOH is efficiently catalysed by an olefin-tethered N-heterocyclic carbene ruthenium complex, which also catalyses double bond migration as a competitive and considerably faster process.

Experimental and Computational Investigation of C−N Bond Activation in Ruthenium N-Heterocyclic Carbene Complexes

A combination of experimental studies and density functional theory calculations is used to study C-N bond activation in a series of ruthenium N-alkyl-substituted heterocyclic carbene (NHC) complexes. These show that prior C-H activation of the NHC ligand renders the system susceptible to irreversible C-N activation. In the presence of a source of HCl, C-H activated Ru(I(i)Pr(2)Me(2))'(PPh(3))(2)(CO)H (1, I(i)Pr(2)Me(2) = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) reacts to give Ru(I(i)PrHMe(2))(PPh(3))(2)(CO)HCl (2, I(i)PrHMe(2) = 1-isopropyl-4,5-dimethylimidazol-2-ylidene) and propene. The mechanism involves (i) isomerization to a trans-phosphine isomer, 1c, in which hydride is trans to the metalated alkyl arm, (ii) C-N cleavage to give an intermediate propene complex with a C2-metalated imidazole ligand, and (iii) N-protonation and propene/Cl(-) substitution to give 2. The overall computed activation barrier (ΔE(++)(calcd)) corresponds to the isomerization/C-N cleavage process and has a value of +24.4 kcal/mol. C-N activation in 1c is promoted by the relief of electronic strain arising from the trans disposition of the high-trans-influence hydride and alkyl ligands. Experimental studies on analogues of 1 with different C4/C5 carbene backbone substituents (Ru(I(i)Pr(2)Ph(2))'(PPh(3))(2)(CO)H, Ru(I(i)Pr(2))'(PPh(3))(2)(CO)H) or different N-substituents (Ru(IEt(2)Me(2))'(PPh(3))(2)(CO)H) reveal that Ph substituents promote C-N activation. Calculations confirm that Ru(I(i)Pr(2)Ph(2))'(PPh(3))(2)(CO)H undergoes isomerization/C-N bond cleavage with a low barrier of only +21.4 kcal/mol. Larger N-alkyl groups also facilitate C-N bond activation (Ru(I(t)Bu(2)Me(2))'(PPh(3))(2)(CO)H, ΔE(++)(calcd) = +21.3 kcal/mol), and in this case the reaction is promoted by the formation of the more highly substituted 2-methylpropene.

Amide Synthesis from Alcohols and Amines Catalyzed by Ruthenium N‐Heterocyclic Carbene Complexes

The direct synthesis of amides from alcohols and amines is described with the simultaneous liberation of dihydrogen. The reaction does not require any stoichiometric additives or hydrogen acceptors and is catalyzed by ruthenium N-heterocyclic carbene complexes. Three different catalyst systems are presented that all employ 1,3-diisopropylimidazol-2-ylidene (IiPr) as the carbene ligand. In addition, potassium tert-butoxide and a tricycloalkylphosphine are required for the amidation to proceed. In the first system, the active catalyst is generated in situ from [RuCl(2)(cod)] (cod=1,5-cyclooctadiene), 1,3-diisopropylimidazolium chloride, tricyclopentylphosphonium tetrafluoroborate, and base. The second system uses the complex [RuCl(2)(IiPr)(p-cymene)] together with tricyclohexylphosphine and base, whereas the third system employs the Hoveyda-Grubbs 1st-generation metathesis catalyst together with 1,3-diisopropylimidazolium chloride and base. A range of different primary alcohols and amines have been coupled in the presence of the three catalyst systems to afford the corresponding amides in moderate to excellent yields. The best results are obtained with sterically unhindered alcohols and amines. The three catalyst systems do not show any significant differences in reactivity, which indicates that the same catalytically active species is operating. The reaction is believed to proceed by initial dehydrogenation of the primary alcohol to the aldehyde that stays coordinated to ruthenium and is not released into the reaction mixture. Addition of the amine forms the hemiaminal that undergoes dehydrogenation to the amide. A catalytic cycle is proposed with the {(IiPr)Ru(II)} species as the catalytically active components.

Chelating NHC Ruthenium(II) Complexes as Robust Homogeneous Hydrogenation Catalysts

A series of ruthenium(II) complexes have been prepared by using bidentate chelating N-heterocyclic carbene (NHC) ligands that feature different donor groups E (E = olefin, thioether, carboxylate, and NHC). Rigid coordination of all donor sites was concluded from NMR spectroscopy, and the electronic impact of the donor group was evaluated by electrochemical analyses. The chelating donor group had a strong influence on the activity of the metal center in catalyzing direct hydrogenation of styrene. A thioether group or a second NHC donor site essentially deactivates the metal center. Complexes comprising a NHC tethered with an olefin or a carboxylate group showed appreciable activity, though only the carboxylate-functionalized system proved to be a precursor for homogeneous hydrogenation. According to in situ high-pressure NMR analyses, complexes featuring a carboxylate chelating group are remarkably resistant toward reductive elimination even under strongly reducing conditions and may, therefore, be used repeatedly.

Catalytic Hydrodefluorination of Aromatic Fluorocarbons by Ruthenium N-Heterocyclic Carbene Complexes

The catalytic hydrodefluorination (HDF) of hexafluorobenzene, pentafluorobenzene, and pentafluoropyridine with alkylsilanes is catalyzed by the ruthenium N-heterocyclic carbene (NHC) complexes Ru(NHC)(PPh(3))(2)(CO)H(2) (NHC = SIMes (1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene) 13, SIPr (1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) 14, IPr (1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) 15, IMes (1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) 16). Catalytic activity follows the order 15 > 13 > 16 > 14, with 15 able to catalyze the HDF of C(6)F(5)H with Et(3)SiH with a turnover number of up to 200 and a turnover frequency of up to 0.86 h(-1). The catalytic reactions reveal (i) a novel selectivity for substitution at the 2-position in C(6)F(5)H and C(5)F(5)N, (ii) formation of deuterated fluoroarene products when reactions are performed in C(6)D(6) or C(6)D(5)CD(3), and (iii) a first-order dependence on [fluoroarene] and zero-order relationship with respect to [R(3)SiH]. Mechanisms are proposed for HDF of C(6)F(6) and C(6)F(5)H, the principal difference being that the latter occurs by initial C-H rather than C-F activation.

The intramolecular sp2 and sp3 C–H bond activation of (p-cymene)ruthenium(ii) N-heterocyclic carbene complexes

The intramolecular sp2 and sp3 C-H activated products, as well as the monometalated products, based on the "(p-cymene)Ru(NHC)" framework were synthesised by treatment of a series of NHCs (1-R-3-methylimidazol-2-ylidene [R=Ph (1), Bn (2), t-Bu (3), i-Pr (4), Mes (5), Cy (6)] and 1,3-bis(isopropyl)imidazol-2-ylidene (7)) with [(p-cymene)RuCl2]2 under mild conditions. A new NHC precursor (1-tert-butyl-3-phenyl-4,5-dihydro-imidazol-2-ylidene) was also designed to compare the reactivity of sp2 C-H and sp3 C-H bonds upon cyclometalation, and only the sp3 C-H activated product (8) was observed. The factors that possibly determine the selectivity of intramolecular sp2 or sp3 C-H activation are elucidated by a series of experiments. In the cases where activation of both sp2 C-H and sp3 C-H is possible, steric factors overrode the others to dominate the regioselectivity of activation. All complexes were characterised by 1H NMR, 13C NMR and HRMS spectra. The molecular structures of 1, 3, 5, 6, 7, and 8 were confirmed by X-ray diffraction.

Probing the potential of N-heterocyclic carbenes in molecular electronics: redox-active metal centers interlinked by a rigid ditopic carbene ligand

Bimetallic homonuclear iron(II) and ruthenium(II) N-heterocyclic carbene complexes have been synthesized and crystallographically analyzed. As a spacer ligand for interconnecting the two redox-active metal centers, a ditopic carbene ligand has been used that comprises two carbene sites annelated to benzene. Detailed electrochemical and spectroelectrochemical analyses of the bimetallic systems revealed that despite the potentially pi-delocalized nature of the ditopic ligand, the iron centers are only moderately coupled. In the ruthenium complexes, the intermetallic interactions are very weak and the centers are electrochemically nearly independent. A model is proposed for rationalizing these observations which is based on (I) relatively weak charge delocalization in the spacer ligand and (II) on electrostatic factors governing the metal-carbene bond.

CH Activation Reactions of Ruthenium N-Heterocyclic Carbene Complexes: Application in a Catalytic Tandem Reaction Involving CC Bond Formation from Alcohols

A series of ruthenium hydride N-alkyl heterocyclic carbene complexes has been investigated as catalysts for a tandem oxidation/Wittig/reduction reaction to give C-C bonds from alcohols. The C-H-activated carbene complex Ru(IiPr(2)Me(2))'(PPh(3))(2)(CO)H (9) proves to be the most active precursor catalyzing the reaction of PhCH(2)OH and Ph(3)P=CHCN in 3 h at 70 degrees C. These results provide (a) a rare case in which N-alkyl carbenes afford higher catalytic activity than their N-aryl counterparts and (b) a novel example of the importance of NHC C-H activation in a catalytic cycle.

Synthesis and characterization of a planar chiral and chiral-at-metal ruthenium N-heterocyclic carbene complex

This report describes the conversion of the neutral planar chiral arene-tethered complex [Ru(η 6:η 1-Me 2NC 6H 4C 6H 4PCy 2)Cl 2 ( 1), into [Ru(η 6: η 1-Me 2NC 6H 4C 6H 4PCy 2)(1,3-dibutylimidazol-2-ylidene)Cl]BF 4 ( 2) via silver transmetallation. The cationic title complex is also chiral-at-metal, and forms stereoselectively as a result of the directing effect of the arene-tethered ligand. The structure in solution and in the solid state was examined; a puckered distortion of the η 6-arene was noted in the crystal structure, along with a hindered rotation of the NHC in solution. As a comparison to the previously reported phosphine analogues, the dication derived from 2 was used to catalyze the asymmetric Diels-Alder reaction of methacrolein and cyclopentadiene.

Synthesis and Application of New N-Heterocyclic Carbene Ruthenium Complexes in Catalysis: A Case Study

New imidazolium and imidazolinium salts were synthesized and their ability to act as stable N-heterocyclic carbene (NHC) ligand precursors was investigated in various ruthenium-catalyzed processes. Thus, 1,3- diarylimidazol(in)ium chlorides bearing the phenyl, 1-naphthyl, 4-biphenyl, 3,5-dimethylphenyl, 2-tolyl, 2,6- dimethylphenyl, 2,4,6-trimethylphenyl (mesityl), and 2,6-diisopropylphenyl substituents were prepared. They were combined with the [RuCl2(p-cymene)]2 dimer and potassium tert-butoxide or sodium hydride to generate the corresponding ruthenium-arene complexes [RuCl2(p-cymene)(NHC)] in situ. The catalytic activity of all these species was investigated in the photoinduced ring-opening metathesis polymerization (ROMP) of norbornene and cyclooctene. Results from this study showed that the C4-C5 double bond in the imidazole ring of the NHC ligands was not crucial to achieve high catalytic efficiencies. The presence or the absence of alkyl groups on the ortho positions of the phenyl rings had a more pronounced influence. Blocking all the ortho positions was a requisite for obtaining efficient catalysts. Failure to do so probably resulted in the ortho-metallation of the carbene ligand, thereby altering the coordination sphere of the ruthenium active centers. Catalytic screenings were also carried out with the various imidazol(in)ium salts to evaluate their ability at promoting the cyclopropanation of styrene and cyclooctene with ethyl diazoacetate. Under the experimental conditions adopted, the exact nature of the N,N-diaryl groups had very little influence on the outcome of these reactions. The imidazolium salts were further probed as catalyst modifiers for the Atom Transfer Radical Addition (ATRA) of carbon tetrachloride to styrene. Some species displayed a dual activity and promoted both olefin metathesis and ATRA.

Regioselective allylic alkylation and etherification catalyzed by in situ generated N-heterocyclic carbene ruthenium complexes

Benzimidazolium halides are used for the first time as ligand precursors in ruthenium-catalyzed substitution of allylic carbonates and chlorides by carbon nucleophiles and phenols, respectively. After generation of diaminocarbene species upon deprotonation by tBuOK, their association with [Cp*Ru(MeCN) 3]PF 6 induces a very high regioselectivity in favor of the branched isomers when cinnamyl derivatives are used as starting substrates. They also provide good regioselectivities for the allylation of phenols by unsymmetrical aliphatic allylic substrates such as 3-chloro-4-phenylbut-1-ene, and thus provide a straightforward access to new allylic phenyl ethers.

Ruthenium(II) N-heterocyclic Carbene Complexes in the Transfer Hydrogenation of Ketones

Six ruthenium-N-heterocyclic carbene complexes (2–7) have been prepared and the new compounds characterized by C, H, N analyses, 1H-n.m.r. and 13C-n.m.r. The reduction of ketones to alcohols via transfer hydrogenation was achieved with catalytic amounts of complexes (2–7) in the presence of t-BuOK.

Direct and Transfer Hydrogenation of Ketones and Imines with a Ruthenium N-Heterocyclic Carbene Complex

The dihydride ruthenium N-heterocyclic carbene complex Ru(IMes)(PPh3)2CO(H)2 (1) (IMes=1,3-dimesityl-1,3-dihydro-2H-imidazol-2-ylidene) is an efficient catalyst for both direct hydrogenation and transfer hydrogenation of ketones and imines, in the absence of base.

Di- and Trivalent Ruthenium Complexes of Chelating, Anionic N-Heterocyclic Carbenes

The reactions of a range of Ru(II) complexes with the anionic alkoxy di-N-heterocyclic carbenes M[OCPh(CH2{1-C[NCHCHNBut]})2] (ML; M = Ag, Tl) yield a mixed carbene−imidazolium complex of ruthenium(II), [Ru(p-cymene)[OCPh(CH2{1-C[NCHCHNBut]})(CH2{1-CH[NCHCHNBut]}Cl)]Cl, and the first trivalent ruthenium N-heterocyclic carbene complex, [RuCl2(PPh3)(L)], in this case stabilized by the two bound carbene groups.

Olefin isomerization promoted by olefin metathesis catalysts

A model study was conducted to determine the extent to which olefin isomerization occurs during olefin metathesis of simple olefins with Grubbs ruthenium catalysts and Schrock's molybdenum catalyst under conditions similar to those employed in ADMET polymerization. It was found that the N-heterocyclic carbene (NHC)-ligated ruthenium complex promotes extensive isomerization of both internal and terminal olefins at temperatures of 50–60 °C, whereas the bisphosphine ruthenium complex and Schrock's molybdenum complex do not. Isomerization occurs concurrently with metathesis for the NHC ruthenium complex to produce a mixture of linear olefins of consecutive carbon numbers.