Metathesis Products Research Articles

Cross metathesis of unsaturated epoxides for the synthesis of polyfunctional building blocks.

The cross metathesis of 1,2-epoxy-5-hexene (1) with methyl acrylate and acrylonitrile was investigated as an entry to the synthesis of polyfunctional compounds. The resulting cross metathesis products were hydrogenated in a tandem fashion employing the residual ruthenium from the metathesis step as the hydrogenation catalyst. Interestingly, the epoxide ring remained unreactive toward this hydrogenation method. The saturated compound resulting from the cross metathesis of 1 with methyl acrylate was transformed by means of nucleophilic ring-opening of the epoxide to furnish a diol, an alkoxy alcohol and an amino alcohol in high yields.

Exceptionally Stable and Efficient Solid Supported Hoveyda-Type Catalyst

The ammonium tagged Hoveyda-type catalyst bearing sterically enlarged N-heterocyclic carbene ligand was synthesized and supported on SBA-15. The obtained heterogeneous olefin metathesis catalyst forms unprecedentedly stable Ru-methylidenes and provides products of ring-closing and cross metathesis with turnover numbers (TONs) up to 35 000 and turnover frequencies (TOFs) up to 1590 min–1. The catalyst proved to be truly recyclable and effective in a continuous flow mode.

1,1′‐Dialkynylferrocenes as Substrates for Bidirectional Alkyne Metathesis Reaction

AbstractIn the construction of ferrocene‐based molecular wires, alkyne metathesis was investigated as an alternative to palladium‐catalyzed coupling reactions for the synthesis of alkynyl ferrocenes. With a Mortreux catalyst system [Mo(CO)6, 4‐chloro‐ or 2‐fluorophenol], (1‐propynyl)ferrocenes gave the expected homo metathesis products in moderate yields. Because the Mortreux catalyst system is not compatible with 1,1′‐dialkynylferrocenes, a nitridomolybdenum catalyst introduced by Fürstner was tested in bidirectional homo and cross‐alkyne metathesis reactions. Although the homo alkyne metathesis products were obtained in poor yields, the cross‐alkyne metathesis reactions afforded the expected products in good overall yields.

A Novel Approval for Degradation of Polybutadiene and Synthesis of Diene‐Based Telechelic Oligomers via Olefin Cross Metathesis

Metathetic degradation of polybutadiene (PB) in presence of a ruthenium catalyst is accomplished. Well‐defined functionalized telechelic PB oligomers with a wide range of designed molecular weights (Mn) and polydispersity indexes (PDI) are achieved via the choice of chain transfer agents (CTAs), mole ratio of PB/CTA, mole ratio of PB/catalyst and the reaction time. Experiments are performed using Grubbs second generation catalyst (G2) in 1,2‐dichloromethane at 30 °C. It has been demonstrated that the degradation performance can be controllably tuned by using selected monomers. Additionally, when mono‐olefins are introduced, the Mn of metathesis products would be on order of a few hundred g mol−1 with narrow PDI (less than 1.50). The compositions and structures of the final product is determined using GC/MS analysis, which indicates that the butadiene repeating unit number of the telechelic oligomers, could be as low as two or even only one.

Silica-Supported Tungsten Carbynes (≡SiO)xW(≡CH)(Me)y (x = 1, y = 2; x = 2, y = 1): New Efficient Catalysts for Alkyne Cyclotrimerization

The activity of silica-supported tungsten carbyne complexes (≡SiO)xW(≡CH)(Me)y (x = 1, y = 2; x = 2, y = 1) toward alkynes is reported. We found that they are efficient precatalysts for terminal alkyne cyclotrimerization with high TONs. We also demonstrate that this catalyst species is active for alkyne cyclotrimerization without the formation of significant alkyne metathesis products. Additional DFT calculations highlight the importance of the W coordination sphere in supporting this experimental behavior.

Striking difference between alkane and olefin metathesis using the well-defined precursor [Si–O–WMe5]: indirect evidence in favour of a bifunctional catalyst W alkylidene–hydride

Metathesis of linear alkanes versus olefins using supported multifunctional catalyst gives a very striking different distribution of metathesis products.

ChemInform Abstract: Ring‐Closing Metathesis Reactions of Bis(enynes): Selectivity and Surprises.

A study of the ring-closing metathesis reactions of two bis(enynes) is presented. These substrates, which contain two alkenes and two alkynes, as well as a resident stereocenter, can potentially generate metathesis products resulting from many reaction pathways. In this contribution we present our results on these reactions, show how small changes in reaction conditions can lead to different product ratios, and attempt to provide a rationale for the outcomes.

A latent and controllable ruthenium-indenylidene catalyst for emulsion ROMP in water

A latent and switchable ruthenium indenylidene catalyst bearing an imidazole functionalized Schiff base ligand for emulsion ring opening metathesis polymerization (ROMP) reactions in water is reported. This catalyst shows latent behavior towards olefin metathesis reactions and can be activated by addition of acids. The activity of this catalyst is first tested using ring closing metathesis (RCM) and self-metathesis (SM) and later intensively studied for ring opening metathesis polymerization (ROMP) reactions. By varying the acid/ruthenium ratio, a high control of ROMP of 5-norbornene-2yl-acetate in both organic and aqueous media was achieved. In emulsion ROMP reactions, the molecular weight of the emulsion polymers can be controlled, while keeping the particle sizes between 11 and 21nm. In addition to its latent behavior, the catalyst displayed a high affinity to silica gel, which eases the separation of ruthenium residues from the metathesis products.

Synthesis of a Double-Spanned Resorc[4]arene via Ring-Closing Metathesis and Calculation of Aggregation Propensity

Ring-closing metathesis (RCM) catalyzed by a second-generation Grubbs catalyst has been used to synthesize resorc[4]arenes 2b-5b starting from undecenyl resorc[4]arene 1b fixed in the cone conformation. X-ray diffraction analysis of the major metathesis product, 3b (50% yield), revealed a cavity-shaped architecture resembling a basket, endowed with a large intramolecular space (∼10 Å) and a strong propensity to self-assemble as a supramolecular trio of heterochiral dimers. This prompted us to investigate the aggregation propensity of basket 3b in THF/water solution by UV-visible spectroscopy. The cavitation Gibbs free-energy change (ΔΔGcav = 4.78 kcal mol(-1)) associated with the self-assembly of macrocycle 3b was calculated as a measure of the solvophobic interactions involved in the process.

Concurrent Cross Metathesis and Enzymatic Oxidation: Enabling Off‐Equilibrium Transformations

Criss-cross catalysis: H. Zhao, J. F. Hartwig and co-workers have combined homogeneous alkene metathesis and biocatalysis in a concurrent fashion. A ruthenium–N-heterocyclic carbene (NHC) complex provides an equilibrating mixture of cross metathesis products. The selective simultaneous epoxidation by cytochrome P450 BM3 enables product yields well above the hypothetical two-step process.

Ring-closing metathesis reactions of bis(enynes): selectivity and surprises.

A study of the ring-closing metathesis reactions of two bis(enynes) is presented. These substrates, which contain two alkenes and two alkynes, as well as a resident stereocenter, can potentially generate metathesis products resulting from many reaction pathways. In this contribution we present our results on these reactions, show how small changes in reaction conditions can lead to different product ratios, and attempt to provide a rationale for the outcomes.

A comparison of low and high activity precatalysts: do the calculated energy barriers during the self-metathesis reaction of 1-octene correlate with the precatalyst metathesis activity?

The self-metathesis reaction of 1-octene with several well-known Grubbs-type precatalysts and the new Z-selective Grubbs precatalyst were studied with molecular modeling. The obtained Gibbs-free energy values for all the steps during the productive metathesis of 1-octene were compared to the values obtained for some low catalytic activity precatalysts. Determining how the Gibbs-free energy values of highly active precatalysts compare to that of low catalytic activity precatalysts gave a deeper insight into the mechanism. The questionable correlation of the theoretically observed trends with those obtained experimentally does point to the need to be very cautious when making assumptions from theoretical results without a sufficiently large dataset.

Triacylglycerol self-metathesis over highly chemoselective methyltrioxorhenium supported on ZnCl2-promoted mesoporous alumina

Since triolein is the main constituent of most oils such as olive and high oleic sunflower oils, self-metathesis of this triacylglycerol at mild reaction conditions was investigated via a heterogeneous approach, in the presence of ZnCl2-modified mesoporous alumina supported methyltrioxorhenium (MTO) as a catalyst. Both intra and intermolecular metathesis reactions were observed. While, 9-octadecene, dimers and trimers of the triacylglycerol were isolated, intermolecular cyclization metathesis products were also obtained. The MTO-based catalyst was found to be highly chemoselective affording only desired metathesis product formation. In contrast, more than 50% of triolein was converted to undesired solid product in the presence of the homogeneous Grubbs 2nd generation catalyst. Triolein metathesis products were effectively separated by chromatographic techniques and identified using mass spectrometric and NMR analyses. The reported results show that 3%MTO/ZnCl2–Al2O3-meso is an efficient and selective heterogeneous catalyst for bulky functionalized olefins metathesis such as triacylglycerols and edible oils, avoiding the use of costly homogeneous catalysts, or the use other rhenium-based catalysts with toxic promoters such as R4Sn.

Nucleophilic addition of carborane anion to Ir, Rh-coordinated Cp* ring: C–C bond formation accompanied by reduction of metal center

Reaction of carboranyl anion or its thiolate derivative with a fully-substituted Cp*-based group 9 (Ir, Rh) metal complex affords respectively a C-C bond formation complex and a salt metathesis product. This example represents the first nucleophilic addition attempt in the area of M-Cp* chemistry.

Evidence of a Lead Metathesis Product from Calcium Hydroxyapatite Dissolution in Lead Nitrate Solution

Calcium hydroxyapatite, CaHAp, synthesized by the precipitation method, was utilized to study the calcium-lead metathesis reaction on dissolution in a lead nitrate solution under reflux conditions to prepare larger lead hydroxyapatite, PbHAp, crystals from CaHAp. SEM images show development of crystalline PbHAp on the surfaces of CaHAp. The needle-like crystal morphology observed for PbHAp after 24 h reaction time developed into hexagonal-rod crystal morphology within 48 h reaction time. The largest PbHAp crystals obtained from 48 h reaction time have approximate size of 10 × 10 × 40 μm. Powder X-ray diffraction results show mixed phases of CaHAp and PbHAp due to difficulty in separating the PbHAp product from the CaHAp substrate. The PbHAp peaks observed after 24 h of reaction sharpen and increase in intensity after 48 h of reaction confirming that the PbHAp phase is the major product for the 48 h reaction time. EDX results of the crystalline products show high intensity Pb peaks with lead to phosphorous ratio (5 : 3) as expected for PbHAp. Lower intensity Ca peaks are also observed, consistent with incomplete coverage of the CaHAp growth substrate.

ChemInform Abstract: Catalytic Z‐Selective Cross‐Metathesis with Secondary Silyl‐ and Benzyl‐Protected Allylic Ethers: Mechanistic Aspects and Applications to Natural Product Synthesis.

AbstractThis method affords the metathesis products with good to excellent Z/E selectivity.

DFT study on the organocatalytic carbonyl–olefin metathesis

Organocatalytic carbonyl–olefin metathesis has been given a systematic theoretical study with density functional theory (DFT) method. The catalytic cycle includes three key steps: (a) (3+2) cycloaddition of (E/Z)-hydrazonium ion I (from the reaction of aldehyde with the hydrazine catalyst) with olefin to produce pyrazolidinium ion II; (b) proton transfer between two nitrogen atoms in II to give pyrazolidinium ion III; (c) cycloreversion of III to produce hydrazonium ion IV, whose hydrolysis leads to the metathesis product. It is found that the stereo-selectivity of product is determined by the first step. The catalyst with high steric effect can effectively improve the stereo-selectivity. The second step, namely the proton transfer, is the rate-determining step in the whole catalytic cycle. Water molecule can effectively facilitate this step. In the third step, olefins with high strain, such as cyclopropene substrate, favor the cycloreversion.

Structural assignment of a bis-cyclopentenyl-β-cyanohydrin formedviaalkene metathesis from either a triene or a tetraene precursor

The identity of the major product of Ru-catalysed alkene metathesis of two polyene substrates has been determined using density functional theory (DFT) NMR prediction, a (1)H-(1)H Total Correlated Spectroscopy (TOCSY) NMR experiment and ultimately by single-crystal X-ray crystallography. The substrates were designed as those that would potentially allow expedient access to the trans-decalin skeleton of the natural product (-)-euonyminol, but the product was found to be a bis-cyclopentenyl-β-cyanohydrin [1-(1-hydroxycyclopent-3-en-1-yl)cyclopent-3-ene-1-carbonitrile, C11H13NO] rather than the trans-2,3,6,7-dehydrodecalin-β-cyanohydrin.

Quantum chemical study of butane metathesis on mixed aluminum and cobalt chloride complexes

Activated complexes and routes of the model catalytic butane metathesis in the presence of mixed complexes of aluminum and cobalt chlorides were calculated by the quantum chemical DFT/PBE/TZ2p method. The earlier predicted bimetallic alkyl clusters containing Co-Co dimeric bonds are key intermediates leading to the formation of both isomerization products and products of alkane metathesis (pentane and propane molecules in the case of butane). The sequence of mutual rearrangements of the polynuclear complexes provides a possibility of cleavage and formation of C-C bonds in alkanes and metal-carbon bond formation. The full model of the catalytic cycle was constructed for the maximum multiplicity (7) of the catalytic system, and individual key regions related to C-C bond cleavage and formation were calculated with a lowered multiplicity (5 and 3). The obtained energy barriers of individual stages of the cyclic catalytic process of butane metathesis are not higher than 33 kcal mol−1 for multiplicity 7 and are lower by ∼15 kcal mol−1 for a lower multiplicity.

A Strategy for Hydroxide Exclusion in Nanocrystalline Solid-State Metathesis Products

We demonstrate a simple strategy to either prevent or enhance hydroxide incorporation in nanocrystalline solid-state metathesis reaction products prepared in ambient environments. As an example, we show that ZnCO3 (smithsonite) or Zn5(CO3)2(OH)6 (hydrozincite) forms extremely rapidly, in less than two minutes, to form crystalline domains of 11 ± 2 nm and 6 ± 2 nm, respectively. The phase selectivity between these nanocrystalline products is dominated by the alkalinity of the hydrated precursor salts, which may in turn affect the availability of carbon dioxide during the reaction. Thus, unlike traditional aqueous precipitation reactions, our solid-state method offers a way to produce hydroxide-free, nanocrystalline products without active pH control.