Ring-closing olefin metathesis on ruthenium carbene complexes: model DFT study of stereochemistry

Ring-closing metathesis (RCM) is the key step in a recently reported synthesis of salicylihalamide and related model compounds. Experimentally, the stereochemistry of the resulting cycloolefin (cis/trans) depends strongly on the substituents that are present in the diene substrate. To gain insight into the factors that govern the observed stereochemistry, density functional theory (DFT) calculations have been carried out for a simplified dichloro(2-propylidene)(imidazole-2-ylidene)ruthenium catalyst I, as well as for the real catalyst II with two mesityl substituents on the imidazole ring. Four model substrates are considered, which are closely related to the systems studied experimentally, and in each case, two pathways A and B are possible since the RCM reaction can be initiated by coordination of either of the two diene double bonds to the metal center. The first metathesis yields a carbene intermediate, which can then undergo a second metathesis by ring closure, metallacycle formation, and metallacycle cleavage to give the final cycloolefin complex. According to the DFT calculations, the stereochemistry is always determined in the second metathesis reaction, but the rate-determining step may be different for different catalysts, substrates, and pathways. The ancillary N-heterocyclic carbene ligand lies in the Ru-Cl-Cl plane in the simplified catalyst I, but is perpendicular to it in the real catalyst II, and this affects the relative energies of the relevant intermediates and transition states. Likewise, the introduction of methyl substituents in the diene substrates influences these relative energies appreciably. Good agreement with the experimentally observed stereochemistry is only found when using the real catalyst II and the largest model substrates in the DFT calculations.     

Mechanism of olefin metathesis with catalysis by ruthenium carbene complexes: density functional studies on model systems

Gradient-corrected (BP86) density functional calculations were used to study alternative mechanisms of the metathesis reactions between ethene and model catalysts [(PH(3))(L)Cl(2)Ru[double bond]CH(2)] with L=PH3 (I) and L=C(3)N(2)H(4)=imidazol-2-ylidene (II). On the associative pathway, the initial addition of ethene is calculated to be rate-determining for both catalysts (Delta G(22-25)*[double bond] kcal mol(-1)). The dissociative pathway starts with the dissociation of phosphane, which is rather facile (Delta G(298)* is approximately equal to 5-10 kcal mol(-1)). The resulting active species (L)Cl(2)Ru[double bond]CH(2) can coordinate ethene cis or trans to L. The cis addition is unfavorable and mechanistically irrelevant (Delta G(298)* is approximately equal to 21-25 kcal mol(-1)). The trans coordination is barrierless, and the rate-determining step in the subsequent catalytic cycle is either ring closure of the complex to yield the ruthenacyclobutane (catalyst I, Delta G(298)*=12 kcal mol(-1)), or the reverse reaction (catalyst II, ring opening, Delta G(298)*=10 kcal mol(-1)), that is, II is slightly more active than I. For both catalysts, the dissociative mechanism with trans olefin coordination is favored. The relative energies of the species on this pathway can be tuned by ligand variation, as seen in (PMe(3))(2)Cl(2)Ru[double bond]CH(2) (III), in which phosphane dissociation is impeded and olefin insertion is facilitated relative to I. The differences in calculated relative energies for the model catalysts I-III can be rationalized in terms of electronic effects. Comparisons with experiment indicate that steric effects must also be considered for real catalysts containing bulky substituents.     

Synthesis and metathesis activity of ruthenium dimethylvinyl carbene complexes

Two new dimethylvinyl carbene complexes, RuCl₂(SIMes)(PPh₃)CHCHC(CH₃)₂ and RuCl₂(SIMes)(3BP)₂CHCHC(CH₃)₂, were synthesized from RuCl₂(PCp₃)₂CHCHC(CH₃)₂. Complex RuCl₂(SIMes)(3BP)₂CHCHC(CH₃)₂ does not suffer from the problem of incomplete initiation that has been observed for the other dimethylvinyl carbene complexes, as witnessed by complete and rapid reaction with ethyl vinyl ether. Acyclic diene metathesis (ADMET) polymerization of 1,9‐decadiene with these complexes was found to give polymers with chemical and thermal properties similar to those obtained with Schrock's molybdenum catalyst. These complexes are also catalysts for ring‐opening metathesis polymerization. The parent complex RuCl₂(SIMes)(PCp₃)CHCHC(CH₃)₂ was found to give polyoctenamer with high initial heats of fusion, suggesting a dependence of the “as formed” crystallinity of the polymer on the rate of the ROMP reaction. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 6134–6145, 2005     

N-heterocyclic carbene and phosphine ruthenium indenylidene precatalysts: a comparative study in olefin metathesis

Kinetic studies on ring-closing metathesis of unhindered and hindered substrates using phosphine and N-heterocyclic carbene (NHC)-containing ruthenium-indenylidene complexes (first and second generation precatalysts, respectively) have been carried out. These studies reveal an appealing difference, between the phosphine and NHC-containing catalysts, associated with a distinctive rate-determining step in the reaction mechanism. These catalysts have been compared with the benzylidene generation catalysts and their respective representative substrates. Finally, the reaction scope of the two most interesting precatalysts, complexes that contain tricyclohexylphosphine and 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (SIMes), has been investigated for the ring-closing and enyne metathesis for a large range of olefins. Owing to their high thermal stability, the SIMes-based indenylidene complexes were more efficient than their benzylidene analogues in the ring-closing metathesis of tetrasubstituted dienes. Importantly, none of the indenylidene precatalysts were found to be the most efficient for all of the substrates, indeed, a complementary complex-to-substrate activity relationship was observed.     

Molecular modeling of the olefin metathesis by tungsten(0) carbene complexes

Density functional and second-order Moller–Plesset theory were used to model W(0) carbene mediated homogeneous metathesis reaction of propylene. The calculations show that the rate determining step of the metathesis is the initiation. After the initiation has been completed the rate determining step becomes dissociation of olefin–metallocarbene complex. The low stereoselectivity of the olefin metathesis reaction is due to the close matching of activation energies for cis and trans isomer formation and the fast cis–trans isomerization caused by the catalysts. The non-productive olefin metathesis reaction always dominates the reaction mixture owing to its very low activation energy. The electronic structure of metal carbene olefin complexes can be described as a combination of donor–acceptor interactions between HOMO of the olefin and LUMO of metal carbene located at carbene carbon on the one hand, and the Dewar, Chatt and Duncanson back donation scheme on the other.     

Tandem catalytic carbene addition/bicyclization of enynes. One-step synthesis of fluorinated bicyclic amino esters by ruthenium catalysis

The reaction of diazo compounds with enynes, containing a fluorinated amino acid moiety, in the presence of the precatalyst Cp(Cl)Ru(COD) leads to fluorinated alkenyl bicyclo[3.1.0]hexane and [4.1.0]heptane amino acid derivatives. It is remarkable that the catalyst, in situ generated from ruthenium complex and diazo compound, completely inhibits the ring closing metathesis of enyne to the profit of tandem alkenylation/cyclopropanation with high stereoselectivity. The study shows that the Cp(Cl)Ru moiety in ruthenacyclobutane favors reductive elimination versus expected alkene metathesis. [reaction: see text]     

<Emphasis Type="Italic">N</Emphasis>-chelate ruthenium carbene complexes in olefin metathesis and isomerization

The catalytic activity of N-chelate ruthenium carbene complexes in the metathesis of hex-1-ene has been studied in comparison to the second generation Grubbs catalyst.     

Generation and spectroscopic characterization of ruthenacyclobutane and ruthenium olefin carbene intermediates relevant to ring closing metathesis catalysis

The reaction of phosphonium alkylidenes [(H2IMes)RuCl2=CHPR3]+[A]- (R = C6H11, A = OTf or B(C6F5)4, 1-Cy; R = i-C3H7, A = ClB(C6F5)3 or OTf, 1-iPr) with 1 equiv of ethylene at -78 degrees C, in the presence of 2-3 equiv of a trapping olefin substrate, yields intermediates relevant to olefin metathesis catalytic cycles. Dimethyl cyclopent-3-ene-1,1-dicarboxylate gives solutions of a substituted ruthenacyclobutane 3 of relevance to ring closing metathesis catalysis. 1H and 13C NMR data are fully consistent with its assignment as a ruthenacyclobutane, but 1JCC values of 23 Hz for the CalphaH2-Cbeta bond and 8.5 Hz for the CalphaH-Cbeta bond point to an unsymmetrical structure in which the latter bond is more activated than the former. In contrast, trapping with acenaphthylene leads to an olefin carbene complex (6) in which the putative ruthenacyclobutane has opened; this species was also fully characterized by NMR spectroscopy and compared to related species reported previously.     

Poly(fluoroalkyl acrylate)-bound ruthenium carbene complex: a fluorous and recyclable catalyst for ring-closing olefin metathesis

The synthesis of a fluorous olefin metathesis catalyst derived from the Grubbs second-generation ruthenium carbene complex is described. The air stable fluorous polymer-bound ruthenium carbene complex 1 shows high reactivity in effecting the ring-closing metathesis of a broad spectrum of diene and enyne substrates leading to the formation of di-, tri-, and tetrasubstituted cyclic olefins in minimally fluorous solvent systems (PhCF3/CH2Cl2, 1:9-1:49 v/v). The catalyst can be readily separated from the reaction mixture by fluorous extraction with FC-72 and repeatedly reused. The practical advantage offered by the fluorous catalyst is demonstrated by its sequential use in up to five different metathesis reactions.     

An ionic liquid-supported ruthenium carbene complex: a robust and recyclable catalyst for ring-closing olefin metathesis in ionic liquids

The synthesis of an ionic liquid-supported olefin metathesis catalyst derived from Grubb's ruthenium carbene complex is described. This new supported catalyst has been used in BMI.PF6 solvent, and this allowed success in solving the challenging problem of catalyst recycling. The IL catalyst in BMI.PF6 can be recovered and reused up to 10 consecutive cycles in RCM reactions of several dienes with excellent conversions. Moreover, the IL catalyst shows a remarkable stability in BMI.PF6 and can be stored several months without loss of activity. These results clearly demonstrate the importance of anchoring an imidazolium ionic liquid pattern to the catalyst to avoid its leaching from the BMI.PF6 phase.     

Hydrosilylation of alkynes catalyzed by ruthenium carbene complexes

Hydrosilylation of terminal alkynes with a variety of silanes catalyzed by Cl₂(PCy₃)₂RuCHPh (1) affords mainly the Z-isomer via trans addition in excellent yields. The presence of a hydroxyl group in close proximity to the triple bond was observed to exert a strong directing effect, resulting in the highly selective formation of the α-isomer. Intramolecular hydrosilylation of a homopropargylic silyl ether was demonstrated to give the cis addition product.     

Pyridine as trigger for chloride isomerisation in chelated ruthenium benzylidene complexes: implications for olefin metathesis

The cationic pyridine adduct of a ruthenium complex bearing a chelating benzylidene and an N-heterocyclic carbene was identified as an intermediate during the activation of cis dichloro species and a novel triggering concept for olefin metathesis catalysts based on cationic species was disclosed.     

Homo-metathesis of vinylsilanes catalysed by ruthenium carbene complexes

Effective homo-metathesis of a series of dichloro-substituted vinylsilanes H₂C = C(H)SiCl₂R (where R = Me, OSiMe₃, C₆H₅, C₆H₄–Me-4, C₆H₄–CF₃-4) in the presence of second generation Grubbs catalyst [Cl₂(PCy₃)(IMesH₂)Ru(=CHPh)] (I) and Hoveyda–Grubbs catalyst (II) leads to selective formation of E-1,2-bis(silyl)ethenes and ethene. On the basis of the results of experiments with deuterium-labelled reagents, a metallacarbene mechanism has been suggested for these reactions.     

A highly controllable latent ruthenium Schiff base olefin metathesis catalyst: Catalyst activation and mechanistic studies

A ruthenium Schiff base catalyst ( 5 ), bearing an N‐heterocyclic carbene ligand, was found to be a latent catalyst for the ring‐opening metathesis polymerization of cis,cis‐1,5‐cyclooctadiene and dicyclopentadiene and is activated efficiently on addition of hydrochloric acid. A benchmark study was performed using the Grubbs first ( 1 ), second ( 2 ), and third ( 3 ) generation catalyst. Results further illustrate that the catalyst can be stored in dicyclopentadiene at a monomer/catalyst ratio of 15,000/1 without any significant polymerization during at least 12 months. After activation of the catalyst with a Brønsted acid, no undesired loss of performance can be detected. This concept of activation can be easily adapted to a Reaction Injection Molding process, and reaction control is obtained via the in situ generation of the cocatalyst as the outcome of the reaction of alcohols with Lewis acids. Insight into the activation mechanism was gained through an in‐depth nuclear magnetic resonance study, and a plausible mechanism is proposed. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 302–310, 2010     

Mechanistic insights into the cis-trans isomerization of ruthenium complexes relevant to catalysis of olefin metathesis

The mechanism of the trans to cis isomerization in Ru complexes with a chelating alkylidene group has been investigated by using a combined theoretical and experimental approach. Static DFT calculations suggest that a concerted single‐step mechanism is slightly favored over a multistep mechanism, which would require dissociation of one of the ligands from the Ru center. This hypothesis is supported by analysis of the experimental kinetics of isomerization, as followed by ¹H NMR spectroscopy. DFT molecular dynamics simulations revealed that the variation of geometrical parameters around the Ru center in the concerted mechanism is highly uncorrelated; the mechanism actually begins with the transformation of the square‐pyramidal trans isomer, with the Ru?CHR bond in the apical position, into a transition state that resembles a metastable square pyramidal complex with a Cl atom in the apical position. This high‐energy structure collapses into the cis isomer. Then, the influence of the N‐heterocyclic carbene ligand, the halogen, and the chelating alkylidene group on the relative stability of the cis and trans isomers, as well as on the energy barrier separating them, was investigated with static calculations. Finally, we investigated the interconversion between cis and trans isomers of the species involved in the catalytic cycle of olefin metathesis; we characterized an unprecedented square‐pyramidal metallacycle with the N‐heterocyclic carbene ligand in the apical position. Our analysis, which is relevant to the exchange of equatorial ligands in other square pyramidal complexes, presents evidence for a remarkable flexibility well beyond the simple cis–trans isomerization of these Ru complexes.     

C-C and C-H bond activation reactions in N-heterocyclic carbene complexes of ruthenium

Thermolysis of Ru(PPh3)3(CO)H2 with the N-heterocyclic carbene bis(1,3-(2,4,6-trimethylphenyl)imidazol-2-ylidene) (IMes) results in C-C activation of an Ar-CH3 bond in one of the mesityl rings of the carbene ligand. Upon addition of IMes to Ru(PPh3)3(CO)H2 at room temperature in the presence of an alkene, C-H bond activation is observed instead. The thermodynamics of these C-C and C-H cleavage reactions have been probed using density functional theory.     

4-Vinylbenzyl-substituted silver(I) N-heterocyclic carbene complexes and ruthenium(II) N-heterocyclic carbene complexes: synthesis and transfer hydrogenation of ketones

4-Vinylbenzyl-substituted Ag(I) N-heterocyclic carbene (NHC) complexes and Ru(II) NHC complexes have been synthesized. The Ag(I) complexes were synthesized from the imidazolium salts and Ag₂O in dichloromethane at room temperature. The Ru(II) complexes were prepared from Ag(I) NHC complexes by transmetallation. The six 4-Vinylbenzyl-substituted Ag(I) NHC complexes and six 4-Vinylbenzyl-substituted Ru(II) NHC complexes have been characterized by spectroscopic techniques and elemental analyses. The Ru(II) NHC complexes show catalytic activity for the transfer hydrogenation of ketones.     

Transfer hydrogenation of ketones catalyzed by ruthenium complexes

Oxidation of 1,3,5- and 1,2,4-trimethylbenzenes using heteropoly vanadomolybdate as catalyst in the presence of hydrogen peroxide under homogeneous conditions has been investigated. Phenol formation in the case of 1,2,4-trimethylbenzene and only side chain oxidation in the case 1,3,5-trimethylbenzene were observed. This behavior is explained in terms of the charge densities on different centers of both substrate molecules calculated by the MNDO method.