Showing papers on "Metathesis published in 2008"
TL;DR: The application of the new catalysts are demonstrated in an enantioselective synthesis of the Aspidosperma alkaloid, quebrachamine, through an alkene metathesis reaction that cannot be promoted by any of the previously reported chiral catalysts.
Abstract: Discovery of efficient catalysts is one of the most compelling objectives of modern chemistry. Chiral catalysts are in particularly high demand, as they facilitate synthesis of enantiomerically enriched small molecules that are critical to developments in medicine, biology and materials science. Especially noteworthy are catalysts that promote-with otherwise inaccessible efficiency and selectivity levels-reactions demonstrated to be of great utility in chemical synthesis. Here we report a class of chiral catalysts that initiate alkene metathesis with very high efficiency and enantioselectivity. Such attributes arise from structural fluxionality of the chiral catalysts and the central role that enhanced electronic factors have in the catalytic cycle. The new catalysts have a stereogenic metal centre and carry only monodentate ligands; the molybdenum-based complexes are prepared stereoselectively by a ligand exchange process involving an enantiomerically pure aryloxide, a class of ligands scarcely used in enantioselective catalysis. We demonstrate the application of the new catalysts in an enantioselective synthesis of the Aspidosperma alkaloid, quebrachamine, through an alkene metathesis reaction that cannot be promoted by any of the previously reported chiral catalysts.
249 citations
TL;DR: The data reported in this paper demonstrate that great care must be taken when choosing an appropriate catalyst for a given metathesis reaction, and some empirical rules can be deduced from the model experiments, providing preliminary hints for the selection of the optimal catalysts.
Abstract: The data reported in this paper demonstrate that great care must be taken when choosing an appropriate catalyst for a given metathesis reaction. First-generation catalysts were found to be useful in the metathesis of sterically unhindered substrates. Second-generation catalysts (under optimised conditions) showed good to excellent activities toward sterically hindered and electron-withdrawing group (EWG)-substituted alkenes that do not react using the first-generation complexes. A strong temperature effect was noted on all of the reactions tested. Interestingly, attempts to force a reaction by increasing the catalyst loading were much less effective. Therefore, when possible, it is suggested that metathesis transformations should be carried out with a second-generation catalyst at 70 degrees C in toluene. However, different second-generation catalysts proved to be optimal for different applications and no single catalyst outperformed all others in all cases. Nevertheless, some empirical rules can be deduced from the model experiments, providing preliminary hints for the selection of the optimal catalysts.
207 citations
TL;DR: In this paper, the detailed mechanisms for the borylation of α,β-unsaturated carbonyl compounds, acrolein and methylacrylate, catalyzed by phosphine copper(I) boryl complexes were studied with the aid of density functional theory calculations.
191 citations
TL;DR: An efficient multi-gram scale synthesis protocol of a variety of P,N ligands is described in this article, where the synthesis is achieved in a two-step reaction, first the amine is deprotonated and subsequently the chlorophosphine is added to yield the corresponding P-N ligand.
Abstract: An efficient multi-gram scale synthesis protocol of a variety of P,N ligands is described. The synthesis is achieved in a two-step reaction. First, the amine is deprotonated and subsequently the chlorophosphine is added to yield the corresponding P,N ligand. Deprotonation of the amine is normally achieved with n-BuLi at low temperature, but for the preparation of ligands with a 2,2′-dipyridylamino backbone and phosphines with a high steric demand KH has to be employed in combination with reaction temperatures of 110 °C for the salt metathesis step. The reaction of two equivalents of a selected P,N ligand with one equivalent of the iridium complex [IrCl(cod)]2 (cod=1,5-cyclooctadiene) affords P,N ligand-coordinated iridium complexes in quantitative yield. X-Ray single crystal structure analysis of one of these complexes reveals a monomeric five-coordinated structure in the solid state. The iridium complexes were used to form catalysts for the N-alkylation of aromatic amines with alcohols. The catalyst system was optimized by studying 8 different P,N ligands, 9 different solvents and 14 different bases. Systematic variation of the substrate to base and the amine to alcohol ratios as well as the catalyst loading led to optimized catalytic reaction conditions. The substrate scope of the developed catalytic protocol was shown by synthesizing 20 different amines of which 12 could be obtained in isolated yields higher than 90%. A new efficient catalyst system for the selective monoalkylation of primary aromatic and heteroaromatic amines with primary aromatic, heteroaromatic as well as aliphatic alcohols has been established. The reaction proceeds with rather moderate catalyst loadings.
174 citations
TL;DR: In this paper, the development of metathesis in oleochemistry is discussed, covering self-metathesis as well as more recent developments in the field of cross metatathesis that lead to desired platform chemicals.
Abstract: Olefin metathesis, awarded with the Nobel Prize in Chemistry 2005 for Chauvin, Grubbs and Schrock, has emerged as a powerful tool for organic as well as polymer chemistry. In oleochemistry, this reaction is well known and has been applied for many decades. Examples include the functionalization of the double bonds of different oleochemicals or the (direct) polymerization of plant oils via metathesis. More recent developments, particularly the development of better and more robust catalysts, allow for highly efficient cross-metathesis reactions opening new possibilities for the direct introduction of chemical functionalities. Within this contribution, the development of metathesis in oleochemistry will be discussed, covering self-metathesis as well as more recent developments in the field of cross-metathesis that lead to desired platform chemicals.
161 citations
TL;DR: An overview of the various complexes belonging to the Ruthenium-indenylidene family is provided and their use in various applications is summarized.
153 citations
TL;DR: The first examples of unsymmetrical olefin cross-metathesis reactions in water, involving water-insoluble substrates, at room temperature and using commercially available catalysts are reported.
147 citations
TL;DR: The resulting positron-emitting nanoparticles are to be used as in vivo molecular imaging agents for use in tumor imaging.
Abstract: Nanoparticles containing fluorine-18 were prepared from block co-polymers made by ring-opening metathesis polymerization (ROMP). Using the fast initiating ruthenium metathesis catalyst (H2IMes)(pyr)2(Cl)2RuCHPh, narrow polydispersity, amphiphilic block copolymers were prepared from a cinnamoyl-containing, hydrophobic norbornene monomer and a mesylate-terminated, PEG-containing hydrophilic norbornene monomer. Self-assembly into micelles and subsequent crosslinking of the micelle cores by light-activated dimerization of the cinnamoyl groups yielded stable nanoparticles. Incorporation of fluorine-18 was achieved by nucleophilic displacement of the mesylates with the radioactive fluoride ion with 31% incorporation of radioactivity. The resulting positron-emitting nanoparticles are to be used as in vivo molecular imaging agents in tumor imaging.
130 citations
TL;DR: Quantitative energy-resolved collision-induced dissociation cross-sections by tandem ESI-MS provide absolute thermochemical data for phosphine binding energies in first- and second-generation ruthenium metathesis catalysts of 33.4 and 36.9 kcal/mol, respectively.
Abstract: Quantitative energy-resolved collision-induced dissociation cross-sections by tandem ESI-MS provide absolute thermochemical data for phosphine binding energies in first- and second-generation ruthenium metathesis catalysts of 33.4 and 36.9 kcal/mol, respectively. Furthermore a study of the ring-closing metathesis in the second-generation system to liberate norbornene by forming the 14-electron reactive intermediate from the intramolecular π-complex gives an estimate of the olefin binding energy to the 14-electron complex of around 18 kcal/mol, assuming a loose transition state. The results reported here are in remarkably good agreement with the latest DFT calculations using the M06-L functional.
128 citations
TL;DR: An efficient and general Au(III)-catalyzed tandem reaction, heteroenyne metathesis and Nazarov cyclization, of 1,3-enynyl ketones to produce fused tri- and tetracyclic enones has been developed.
124 citations
TL;DR: It was determined that while increasing the tether length of the catalyst leads to enhanced rates of polymerization, shorter tethers were found to facilitate intramolecular chain transfer and release of catalyst from the polymer.
Abstract: A series of cyclic Ru-alkylidene catalysts have been prepared and evaluated for their efficiency in ring-expansion metathesis polymerization (REMP). The catalyst structures feature chelating tethers extending from one N-atom of an N-heterocyclic carbene (NHC) ligand to the Ru metal center. The catalyst design is modular in nature, which provided access to Ru complexes having varying tether lengths, as well as electronically different NHC ligands. Structural impacts of the tether length were unveiled through 1H NMR spectroscopy as well as single-crystal X-ray analyses. Catalyst activities were evaluated via polymerization of cyclooctene, and key data are provided regarding propagation rates, intramolecular chain transfer, and catalyst stabilities, three areas necessary for the efficient synthesis of cyclic poly(olefin)s via REMP. From these studies, it was determined that while increasing the tether length of the catalyst leads to enhanced rates of polymerization, shorter tethers were found to facilitate intramolecular chain transfer and release of catalyst from the polymer. Electronic modification of the NHC via backbone saturation was found to enhance polymerization rates to a greater extent than did homologation of the tether. Overall, cyclic Ru complexes bearing 5- or 6-carbon tethers and saturated NHC ligands were found to be readily synthesized, bench-stable, and highly active catalysts for REMP.
TL;DR: The preparation of a long-chain aliphatic alpha,omega-diene from plant oil derivatives and its subsequent polymerization through acyclic diene metathesis (ADMET) is described and the application of a poly(ethylene glycol) methyl ether acrylate as the chain stopper led to the preparation of ABA triblock copolymers in a one-step, one-pot procedure.
Abstract: The preparation of a long-chain aliphatic alpha,omega-diene from plant oil derivatives and its subsequent polymerization through acyclic diene metathesis (ADMET) is described. The ADMET bulk polymerization of the thus-obtained monomer, undecyl undecenoate, was investigated and optimized by applying ruthenium-based metathesis catalysts from Grubbs and Hoveyda-Grubbs, leading to high-molecular-weight polyesters. Moreover, by applying different amounts of methyl 10-undecenoate as a chain stopper in this ADMET step growth polymerization, the molecular weight of the resulting polyester could be tuned in a range from approximately 10 to 45 kDa. Finally, the application of a poly(ethylene glycol) methyl ether acrylate as the chain stopper led to the preparation of ABA triblock copolymers in a one-step, one-pot procedure.
TL;DR: It is demonstrated that balancing the steric demands at both the hydrosilane and alkoxysilanes, and the careful control of reaction conditions, permits clean condensation reactions to occur in the absence of competing metathesis processes.
Abstract: Few routes to well-defined 3D silicone structures exist because of their susceptibility to depolymerization/metathesis in the presence of acids or bases. The Lewis acid B(C6F5)3 can be employed to condense hydrosilanes with alkoxysilanes, producing siloxanes and alkanes (R3SiH+R‘OSiR‘ ‘3 → R3SiOSiR‘ ‘3 + R‘H). We demonstrate that balancing the steric demands at both the hydrosilane and alkoxysilanes, and the careful control of reaction conditions, permits clean condensation reactions to occur in the absence of competing metathesis processes. The resulting linear or highly branched siloxane compounds can be rapidly and easily assembled into explicit, complex 3D silicone structures in high yield.
TL;DR: A new family of ruthenium-based olefin metathesis catalysts bearing a series of thiazole-2-ylidene ligands has been prepared, and theosphine-free catalysts of this family are more stable than their phosphine-containing counterparts, exhibiting pseudo-first-order kinetics in the ring-closing of diethyldiallyl malonate.
Abstract: A new family of ruthenium-based olefin metathesis catalysts bearing a series of thiazole-2-ylidene ligands has been prepared. These complexes are readily accessible in one step from commercially available (PCy_3)_2Cl_2Ru CHPh or (PCy_3)Cl_2Ru CH(o-iPrO−Ph) and have been fully characterized. The X-ray crystal structures of four of these complexes are disclosed. In the solid state, the aryl substituents of the thiazole-2-ylidene ligands are located above the empty coordination site of the ruthenium center. Despite the decreased steric bulk of their ligands, all of the complexes reported herein efficiently promote benchmark olefin metathesis reactions such as the ring-closing of diethyldiallyl and diethylallylmethallyl malonate and the ring-opening metathesis polymerization of 1,5-cyclooctadiene and norbornene, as well as the cross metathesis of allyl benzene with cis-1,4-diacetoxy-2-butene and the macrocyclic ring-closing of a 14-membered lactone. The phosphine-free catalysts of this family are more stable than their phosphine-containing counterparts, exhibiting pseudo-first-order kinetics in the ring-closing of diethyldiallyl malonate. Upon removing the steric bulk from the ortho positions of the N-aryl group of the thiazole-2-ylidene ligands, the phosphine-free catalysts lose stability, but when the substituents become too bulky the resulting catalysts show prolonged induction periods. Among five thiazole-2-ylidene ligands examined, 3-(2,4,6-trimethylphenyl)- and 3-(2,6-diethylphenyl)-4,5-dimethylthiazol-2-ylidene afforded the most efficient and stable catalysts. In the cross metathesis reaction of allyl benzene with cis-1,4-diacetoxy-2-butene increasing the steric bulk at the ortho positions of the N-aryl substituents results in catalysts that are more Z-selective.
TL;DR: The N, N'-diphenyl-substituted NHC complex proved to be one of the most efficient catalysts in RCM to form tetrasubstituting olefins.
TL;DR: The first enantioselective total synthesis of clavirolide C, a member of the dolabellane family of diterpenes isolated from Pacific soft coral Clavularia viridis, is disclosed and the development of a new protocol for NHC is spawned.
Abstract: The first enantioselective total synthesis of clavirolide C, a member of the dolabellane family of diterpenes isolated from Pacific soft coral Clavularia viridis, is disclosed. The total synthesis features the application of chiral amino acid based ligands in Cu-catalyzed asymmetric conjugate addition (ACA) reactions and a relatively rare application of catalytic ring-closing metathesis to access an 11-membered ring structure. The total synthesis effort has spawned the development of a new protocol for NHC.Cu-catalyzed ACA of alkylaluminum reagents to beta-substituted cycloalkenones. The enantioselective clavirolide C synthesis requires 17 steps (longest linear sequence), affords the target molecule in 3.5% overall yield, and confirms the stereochemical assignment for the natural product.
TL;DR: Conventional hydrophobic ruthenium complexes catalyze a similar array of metathesis reactions in mixtures of water and organic solvents, but this strategy has enabled cross meetingathesis on the surface of a protein.
TL;DR: These results represent the first highly efficient diastereoselective RuO4-catalyzed oxidation of various olefins with chiral auxiliary substituted acrylamides and a variety of structurally diverse enantiomerically enriched diols.
Abstract: Sequential catalysis has evolved as a powerful concept within the past years and allows the more efficient use of catalytically active expensive transition metals in organic synthesis. In this paper we present the stereoselective cross-metathesis−dihydroxylation of various olefins with chiral auxiliary substituted acrylamides. The chiral information (i.e., the auxiliary) introduced in the metathesis reactions allows for a stereoselective subsequent RuO4-catalyzed dihydroxylation. The sequence is concluded by an unusual kinetic resolution of the diastereomeric diols obtained in the oxidation reaction. As a consequence a variety of structurally diverse enantiomerically enriched diols are obtained. To the best of our knowledge the results summarized in this paper represent the first highly efficient diastereoselective RuO4-catalyzed oxidation.
TL;DR: A series of ruthenium-based olefin metathesis catalysts coordinated with unsymmetrical N-heterocyclic carbene (NHC) ligands has been prepared and fully characterized, and all new catalysts demonstrate similar or higher activity than the second-generation rUThenium catalysts and afford improved E/Z ratios of the desired cross-product at conversion above 60 %.
Abstract: A series of ruthenium-based olefin metathesis catalysts coordinated with unsymmetrical N-heterocyclic carbene (NHC) ligands has been prepared and fully characterized. These complexes are readily accessible in one or two steps from commercially available [(PCy_3)_2Cl_2Ru=CHPh]. All of the complexes reported herein promote the ring-closing of diethyldiallyl and diethylallylmethallyl malonate, the ring-opening metathesis polymerization of 1,5-cyclooctadiene. and the cross metathesis of allyl benzene with cis-1,4-diac-etoxy-2-butene, in some cases surpassing in efficiency the existing second-generation catalysts. Especially in the cross metathesis of allyl benzene with cis-1,4-diacetoxy-2-butene, all new catalysts demonstrate similar or higher activity than the second-generation ruthenium catalysts and, most importantly, afford improved E/Z ratios of the desired cross-product at conversion above 60%. The influence of the unsymmetrical NHC ligands on the initiation rate and the activation parameters for the irreversible reaction of these ruthenium complexes with butyl vinyl ether were also studied. Finally, the synthesis of the related chlorodicarbonyl(carbene) rhodium(l) complexes allowed for the study of the electronic properties of the new unsymmetrical NHC ligands that are discussed in detail.
TL;DR: Olefin cross metathesis was used to efficiently functionalize polyolefin dendrimers, polymers, and gold nanoparticles using the second-generation Grubbs catalyst to efficiently achieve an iterative divergent dendritic construction.
Abstract: Olefin cross metathesis was used to efficiently functionalize polyolefin dendrimers, polymers, and gold nanoparticles using the second-generation Grubbs catalyst. In these structures, the tethers were lengthened to prevent the facile cross metathesis that otherwise predominates in polyolefin dendrimers having short tethers. This synthetic strategy allows the one-step access to polyacid, polyester, and polyferrocenyl structures from polyolefins. Cross metathesis is also used to efficiently achieve an iterative divergent dendritic construction. All the cross metathesis reactions were monitored by 1H NMR showing the chemio-, regio-, and stereoselectivity. MALDI-TOF mass spectrometry was a very useful technique to confirm the efficiency of this synthetic strategy.
TL;DR: The second generation Hoveyda-Grubbs metathesis catalyst was successfully immobilized on silica in pellet and powder form following a practical and fast synthesis procedure.
Abstract: The second generation Hoveyda-Grubbs metathesis catalyst was successfully immobilized on silica in pellet and powder form following a practical and fast synthesis procedure. The activity of the solid system is truly heterogeneous, efficient in various metathesis reaction types and stable for at least 4000 TON. Ru contamination of the products was very low (ppb levels). The successful use of the robust system has been demonstrated in a continuous reactor set-up.
TL;DR: The development of the first thermally stable, truly UV-triggerable precatalysts for ROMP and their application in surface functionalization is reported.
Abstract: Metathesis chemistry and, in the context of polymer chemistry, ring-opening metathesis polymerization (ROMP) have gained a strong position in chemistry and materials science. ROMP is strongly associated with two classes of well-defined metal alkylidene based initiators, molybdenumbased Schrock and ruthenium-based Grubbs type initiators. Despite the tremendous achievements in catalyst development, both families of initiators are still experiencing ongoing, vivid development. Most Grubbs type initiators work at room temperature or require only gentle warming to work properly. More recently, an increasing number of reports on latent Ru-based initiators has appeared. Such precatalysts are of particular interest in technical applications of ROMP, since they allow for premixing, that is, the preformulation of a monomer/precatalyst mixture, its storage over a longer period of time even at elevated temperatures (usually less than 45 8C), and, most importantly, the shaping and profiling of such mixtures prior to polymerization (“curing”). Numerous latent Grubbs type initiators have been reported recently; however, all these precatalysts are triggered thermally. By contrast, surface modification and functionalization require UV-triggerable precatalysts. Few such systems have been reported to date. The synthesis of photoactive Schrock type tungsten-based compounds as well as ruthenium and osmium arene compounds of the general formula [Ru(p-cymene)Cl2(PR3)] and [Os(p-cymene)Cl2(PR3)] (R= cyclohexyl, etc.) were first reported by van der Schaaf et al. They also investigated the photoinduced polymerization of different functionalized norbornenes and 7-oxanorbornenes using various [Ru(solvent)n]X2 complexes, (X= tosylate, trifluoromethanesulfonate) as well as Ru half-sandwich and sandwich complexes. Noels and co-workers reported on the visiblelight-induced ROMP of cyclooctene using [RuCl2(IMes)(pcymene)] (IMes= 1,3-dimesitylimidazol-2-ylidene). Some of these systems were also used in ring-closing metathesis reactions. Most of the systems available to date, however, have significant disadvantages. They either show low activity, resulting in low polymer yields (less than 30%) in the photochemically triggered process, or the irradiation wavelength necessary to trigger ROMP is 360 nm or higher. In the latter case, the initiatorsA thermal stability is generally poor, thus discouraging their application in photoinduced ROMP. Thus, none of the systems reported to date was entirely thermally stable above or even at room temperature. Therefore, these systems do not fulfill the requirements of a truly latent photocatalyst. Herein, we report the development of the first thermally stable, truly UV-triggerable precatalysts for ROMP and their application in surface functionalization. We commenced our investigations with [Ru(IMesH2)(CF3CO2)(tBuCN)4)] CF3CO2 (PI-1) and [Ru(IMes)(CF3CO2)(tBuCN)4)] CF3CO2 (PI-2), which were prepared from [Ru(CF3CO2)2(L)(p-cymene)] [31,32] (L= IMes or IMesH2, 1,3-dimesityl-4,5-diyhdroimidazolin-2-ylidene) by reaction with excess tBuCN. Both compounds can be handled in air. H and C NMR spectroscopy data and elemental analysis reveal the presence of one N-heterocyclic carbene (NHC) ligand, two inequivalent trifluoroacetate groups, and four tBuCN ligands, suggesting cationic Ru complexes. The structures of PI-1 and PI-2 were confirmed by X-ray analysis; the structure of PI-1 is shown in Figure 1 (see also the Supporting Information). Upon mixing of either PI-1 or PI-2 with monomers 3–8 (Scheme 1), no reaction was observed at room temperature within 24 h. Even highly reactive (distilled) dicyclopentadiene (4) did not react with PI-1 or PI-2 at room or elevated temperature (RT
TL;DR: The importance of heteroaromatic motifs in medicinal chemistry and biology, as well as the efficiency and wealth of metathesis transformations, have resulted in significant success in this rapidly developing area.
Abstract: Olefin metathesis has been established as an important and general reaction in synthetic organic chemistry. Recently, it has attracted interest as a powerful tool for the construction of aromatic heterocycles. The importance of heteroaromatic motifs in medicinal chemistry and biology, as well as the efficiency and wealth of metathesis transformations, have resulted in significant success in this rapidly developing area.
TL;DR: Three novel "boomerang" precatalysts bearing different aminocarbonyl functions are reported, and comparative kinetic studies show that this functional group allows for a control of the catalytic activity in metathesis transformations.
Abstract: Three novel "boomerang" precatalysts bearing different aminocarbonyl functions are reported. Comparative kinetic studies show that this functional group allows for a control of the catalytic activity in metathesis transformations. The scope of the more active catalyst is investigated and shows a good tolerance to various substrates in ring-closing metathesis, enyne metathesis, and cross metathesis. ICP-MS analyses illustrate the good affinity of this catalyst for silica gel, as levels of Ru contamination lower than 6 ppm are detected in the final products.
TL;DR: These species have the same reactivity as the initial surface complex [(≡SiO)W(≡NAr)(=CHtBu)(2,5-Me2NC4H2)], which shows that they are the key intermediates of alkene metathesis.
Abstract: Grafting of [W(≡NAr)(=CHtBu)(2,5-Me2NC4H2)2] on a silica partially dehydroxylated at 700°C (SiO2- (700)) generates the corresponding monosiloxy complex [(≡SiO)W(≡NAr)(=CHtBu)(2,5-Me2NC4H2)] as the major species (≈90%) along with [(≡SiO)W(≡NAr)(CH2tBu)(2,5-Me2NC4H2)2], according to mass balance analysis, IR, and NMR studies. This heterogeneous catalyst displays good activity and stability in the metathesis of propene. Very importantly, solid state NMR spectroscopy allows observation of the propagating alkylidene as well as stable metallacyclobutane intermediates. These species have the same reactivity as the initial surface complex [(≡SiO)W(≡NAr)(=CHtBu)(2,5-Me2NC4H2)], which shows that they are the key intermediates of alkene metathesis.
TL;DR: In this paper, the synthesis of complexes of the general formula Cl2Ru(SIMes)(L)(3-phenylinden-1-ylidene) (5, L = PCy3; 6, L= py; and 7, PPh3) from Cl 2Ru(PR3)2(3-PN-1ylidenes) (2a, R = Ph; 2b, R= Cy) is reported.
Abstract: The synthesis of complexes of the general formula Cl2Ru(SIMes)(L)(3-phenylinden-1-ylidene) (5, L = PCy3; 6, L = py; and 7, L = PPh3) from Cl2Ru(PR3)2(3-phenylinden-1-ylidene) (2a, R = Ph; 2b, R = Cy) is reported. This family of olefin metathesis catalysts was fully characterized (1H, 13C and 31P NMR spectroscopy and elemental analysis) and provided excellent activity in the ring-opening metathesis polymerization of 1,5-cyclooctadiene and the ring-closing metathesis of diethyl diallylmalonate. Comparison with the corresponding benzylidene-containing catalysts, 1a,c and 8b, established the decisive role of the carbene ligand on the procedure of the reaction and led to the observation of an unusual catalytic phenomenon, here called “self-inhibition”.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)
TL;DR: DFT calculations on molecular model systems show that byproduct formation and deactivation start by a beta-H transfer trans to the weak sigma-donor ligand at the metallacyclobutane intermediate having a square-based pyramid geometry.
Abstract: The surface complex [(≡SiO)Re(≡CtBu)(═CHtBu)(CH2tBu)] (1) is a highly efficient propene metathesis catalyst with high initial activities and a good productivity. However, it undergoes a fast deactivation process with time on stream, which is first order in active sites and ethene. Noteworthy, 1-butene and pentenes, unexpected products in the metathesis of propene, are formed as primary products, in large amount relative to Re (>>1 equiv/Re), showing that their formation is not associated with the formation of inactive species. DFT calculations on molecular model systems show that byproduct formation and deactivation start by a β-H transfer trans to the weak σ-donor ligand (siloxy) at the metallacyclobutane intermediate having a square-based pyramid geometry. This key step has an energy barrier slightly higher than that calculated for olefin metathesis. After β-H transfer, the most accessible pathway is the insertion of ethene in the Re−H bond. The resulting pentacoordinated trisperhydrocarbyl complex rear...