Mechanism Of Olefin Metathesis Research Articles

Recent advancement on the mechanism of olefin metathesis by Grubbs catalysts: A computational perspective

The second-generation Grubbs catalyst contains N-heterocyclic carbene (NHC) coordinated to Ru center. Various such complexes have been employed as catalysts for the olefin metathesis reaction. Based on experimental results and theoretical analysis, it has been established that a cyclic four-membered metallacyclobutane is formed as an active intermediate. The use of the right computational recipe is prescribed in the context of the Grubbs catalyst. The carbene electronic structure in the catalyst and the course of the reaction are discussed. A recent computational study examined the possibility of an oxidation state as +2 or +4 in a comparative manner. Further, the role of catalysts in the chemo, regio, and stereoselectivity are also discussed. In particular, in recent years, Z selective catalysts (cyclometalated and dithiolate-containing) were developed through an in-depth understanding of the electronic and steric principles. Besides, the substituents present on the NHC carbene also plays an important role. Furthermore, the presence of a chiral backbone leads to stereoselectivity in the product. Studies also predict a more suitable catalyst for certain types of substrates. In a recent study, the effect of aromaticity while discussing alkene vs. arene substrate has been considered. Catalyst undergoes decomposition in the presence of primary alcohol and Brønsted/Lewis base. Mechanism of such type of deterioration via β hydride elimination of the metallacyclobutane intermediate are discussed. Finally, catalyst decomposition involving chelated complexes are also emphasized. Thus, such theoretical studies may help the experimental chemist reduce their effort in choosing a suitable catalyst. In this review article, recent advancements in the olefin metathesis mechanistic studies by Grubbs catalyst are comprehensively summarized.

Role of Substrate Substituents in Alkene Metathesis Mediated by a Ru Alkylidene Catalyst

Quantum mechanical calculations on the mechanism of olefin metathesis with a variety of substituents mediated by a Ru alkylidene catalyst reveal multistep processes along the general reactants → ad...

Mechanism of Olefin Metathesis with Neutral and Cationic Molybdenum Imido Alkylidene N-Heterocyclic Carbene Complexes.

A series of neutral molybdenum imido alkylidene N-heterocyclic carbene (NHC) bistriflate and monotriflate monoalkoxide complexes as well as cationic molybdenum imido alkylidene triflate complexes have been subjected to NMR spectroscopic, X-ray crystallographic, and reaction kinetic measurements in order to gain a comprehensive understanding about the underlying mechanism in olefin metathesis of this new type of catalysts. On the basis of experimental evidence and on DFT calculations (BP86/def2-TZVP/D3/cosmo) for the entire mechanism, olefinic substrates coordinate trans to the NHC of neutral 16-electron complexes via an associative mechanism, followed by dissociation of an anionic ligand (e.g., triflate) and formation of an intermediary molybdacyclobutane trans to the NHC. Formation of a cationic complex is crucial in order to become olefin metathesis active. Variations in the NHC, the imido, the alkoxide, and the noncoordinating anion revealed their influence on reactivity. The reaction of neutral 16-electron complexes with 2-methoxystyrene is faster for catalysts bearing one triflate and one fluorinated alkoxide than for catalysts bearing two triflate ligands. This is also reflected by the Gibbs free energy values for the transition states, ΔG‡303, which are significantly lower for catalysts bearing only one triflate than for the corresponding bistriflate complexes. Reaction of a solvent-stabilized cationic molybdenum imido alkylidene N-heterocyclic carbene (NHC) monotriflate complex with 2-methoxystyrene proceeded via an associative mechanism too. Reaction rates of both solvent-free and solvent-stabilized cationic Mo imido alkylidene NHC catalysts with 2-methoxystyrene are controlled by the cross-metathesis step but not by adduct formation.

In Situ Catalyst Modification in Atom Transfer Radical Reactions with Ruthenium Benzylidene Complexes.

Ruthenium benzylidene complexes are well-known as olefin metathesis catalysts. Several reports have demonstrated the ability of these catalysts to also facilitate atom transfer radical (ATR) reactions, such as atom transfer radical addition (ATRA) and atom transfer radical polymerization (ATRP). However, while the mechanism of olefin metathesis with ruthenium benzylidenes has been well-studied, the mechanism by which ruthenium benzylidenes promote ATR reactions remains unknown. To probe this question, we have analyzed seven different ruthenium benzylidene complexes for ATR reactivity. Kinetic studies by (1)H NMR revealed that ruthenium benzylidene complexes are rapidly converted into new ATRA-active, metathesis-inactive species under typical ATRA conditions. When ruthenium benzylidene complexes were activated prior to substrate addition, the resulting activated species exhibited enhanced kinetic reactivity in ATRA with no significant difference in overall product yield compared to the original complexes. Even at low temperature, where the original intact complexes did not catalyze the reaction, preactivated catalysts successfully reacted. Only the ruthenium benzylidene complexes that could be rapidly transformed into ATRA-active species could successfully catalyze ATRP, whereas other complexes preferred redox-initiated free radical polymerization. Kinetic measurements along with additional mechanistic and computational studies show that a metathesis-inactive ruthenium species, generated in situ from the ruthenium benzylidene complexes, is the active catalyst in ATR reactions. Based on data from (1) H, (13)C, and (31)P NMR spectroscopy and X-ray crystallography, we suspect that this ATRA-active species is a RuxCly(PCy3)z complex.

Regarding the Mechanism of Olefin Metathesis with Sol−Gel-Supported Ru-Based Complexes Bearing a Bidentate Carbene Ligand. Spectroscopic Evidence for Return of the Propagating Ru Carbene

Two isotopically and structurally labeled Ru-based carbenes (2-d4 and 13) have been prepared and attached to the surface of monolithic sol-gel glass. The resulting glass-supported complexes (18-dn and 19) exhibit significant catalytic activity in promoting olefin metathesis reactions and provide products of high purity. Through analysis of the derivatized glass pellets used in a sequence of catalytic ring-closing metathesis reactions mediated by various supported Ru carbenes, it is demonstrated that free Ru carbene intermediates in solution can be scavenged by support-bound styrene ether ligands prior to the onset of competing transition metal decomposition. The observations detailed herein provide rigorous evidence that the initially proposed release/return mechanism is, at least partially, operative. The present investigations shed light on a critical aspect of the mechanism of an important class of Ru-based metathesis complexes (those bearing a bidentate styrene ether ligand).

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.

Mechanism of olefin metathesis over a supported molybdenum catalyst

The metathesis of mixtures of 2,8-decadiene and 2,8-decadiene-1,1,1,10,10,10-d 6 was carried out over a MoO 2/CoO/Al 2O 3 catalyst. From the ratio of the product 2-butenes-d 0:d 3:d 6, it was determined that the reaction proceeded by a chain mechanism, as described by the Chauvin scheme. During the initiation stages of the reaction propylene was formed and the reaction showed a significant isotope effect. This observation is best explained by the formation of a π-alkyl complex as a key intermediate in the initiation step.

The mechanism of olefin metathesis

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTThe mechanism of olefin metathesisFrank D. MangoCite this: J. Am. Chem. Soc. 1977, 99, 18, 6117–6119Publication Date (Print):August 1, 1977Publication History Published online1 May 2002Published inissue 1 August 1977https://doi.org/10.1021/ja00460a055RIGHTS & PERMISSIONSArticle Views329Altmetric-Citations12LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (447 KB) Get e-Alerts Get e-Alerts