A tracer study of the metathesis of simple olefins over molybdena-alumina catalysts

Metathesis of perdeuterio- and perhydroethene, propene, 1-butene, and their mixtures with cis-2-butene was studied over a molybdena-alumina catalyst using recirculation and pulse systems. No metathesis was observed over a fresh unreduced catalyst, but after pretreatment with cis-2-butene the reactivity of olefins in metathesis reactions increased in the sequence of ethene ? 1-butene < propene < cis-2-butene. The activity was higher for the metathesis of propene or 1-butene after the catalyst was pretreated with the corresponding olefin instead of cis-2-butene, but no metathesis was induced by ethene. Metathesis of cis-2-butene was suppressed by the addition of equal amounts of olefins in the sequence: ethene ? 1-butene < propene. Studies of mixtures revealed that the rate of cis- to trans-2-butene isomerization by metathesis ～- the rate of degenerate metathesis of 1-butene or the cross metathesis of 1-butene with 2-butene.     

Metathesis as a versatile tool in oleochemistry

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.     

Katalysatorrecycling in der homogenkatalysierten Kreuzmetathese von Ölsäuremethylester und 4‐Octen

The recycling of homogeneous metathesis catalysts is a big challenge in alkene metathesis. Within this contribution the recycling of these catalysts is investigated in the cross metathesis of methyl oleate and 4‐octene as model reaction. Fluid‐fluid separation technique and temperature dependent solvent systems allow the separation of the homogeneously solved catalysts from the metathesis products. The catalyst phase was successfully recycled in multiple recycling runs only with minor loss of yield.     

长碳链烯烃歧化研究进展

目前,虽然直链烯烃歧化研究的重点仍是碳四烯烃歧化制丙烯技术,但是长碳链烯烃歧化研究已开始引起人们的重视。本文介绍了铼基催化剂、钼基催化剂和钨基催化剂催化长碳链烯烃歧化反应的研究进展。重点介绍了钨基催化剂上长碳链歧化研究状况,分别介绍了钨基催化剂上1-己烯自歧化、1-庚烯自歧化、1-辛烯自歧化的最优工艺条件研究以及预处理条件对钨基催化剂歧化性能的影响。同时分别介绍了含氧化合物对歧化催化剂中毒失活的研究以及钨基催化剂上结焦失活的研究状况。     

Synthesis of polyethylene/polyester copolymers through main chain exchange reactions via olefin metathesis

The synthesis and characterization of linear polyethylene/polyester copolymers produced by olefin cross metathesis and subsequent catalytic hydrogenation are described. The success of the scrambling reaction between 1,4-polybutadiene and unsaturated polyester via olefin cross metathesis was confirmed by ¹H NMR. Differential scanning calorimetric (DSC) measurements indicated that the size of the copolymer blocks varied gradually with the exchange ratio. Catalytic hydrogenation of the unsaturated copolymers with different exchange ratios was carried out using Pd–Al₂O₃. The crystallinity of the resulting hydrogenated polymers depended on the exchange ratios achieved during olefin cross metathesis. This suggests that the unique sequential structure imparted to the unsaturated polymers during metathesis was retained during the hydrogenation process.     

Cross‐metathesis of methyl 10‐undecenoate with diethyl maleate: Formation of an α,ω‐diester via a metathesis reaction network

The cross‐metathesis of methyl 10‐undecenoate 1 derived from castor oil as a renewable raw material with diethyl maleate 2 was investigated. These reactions were carried out with several phosphine and N‐heterocyclic carbene ruthenium catalysts. The reaction conditions were optimised for high conversions in combination with high cross‐metathesis selectivity. This single‐step and atom‐economic synthetic method illustrates an efficient and selective preparation procedure of linear α,ω‐dicarboxylic acid esters starting from renewable resources and comparatively inexpensive base chemicals. Further by‐products are hardly obtained due to their consumption in secondary metathesis reactions. Hence, a sustainable alternative for polyamide and polyester monomers is presented.     

Salicylaldimine Ruthenium Alkylidene Complexes: Metathesis Catalysts Tuned for Protic Solvents

Tuning the electronic and steric environment of olefin metathesis catalysts with specialized ligands can adapt them to broader applications, including metathesis in aqueous solvents. Bidentate salicylaldimine ligands are known to stabilize ruthenium alkylidene complexes, as well as allow ring‐closing metathesis in protic media. Here, we report the synthesis and characterization of exceptionally robust olefin metathesis catalysts bearing both bidentate salicylaldimine and N‐heterocyclic carbene ligands, including a trimethylammonium‐functionalized complex adapted for polar solvents. NMR spectroscopy and X‐ray crystallographic analysis confirm the structures of the complexes. Although the N‐heterocyclic carbene–salicylaldimine ligand combination limits the activity of these catalysts in non‐polar solvents, this pairing enables efficient ring‐closing metathesis of both dienes and enynes in methanol and methanol/water mixtures under air.     

Metathesis of Unsaturated Fatty Esters

The development of a homogeneous catalyst (WCl₆Sn(CH₃)₄) for the metathesis of unsaturated fatty esters encloses new routes in fat chemistry. By these reactions an exchange of alkylidene moities takes place, thus methyl oleate is converted into 9-octadecene and 9-octadecenedioic dimethyl ester. The metathesis of poly-unsaturated esters leads to a much larger number of products, namely polyenes, mono- and dicarboxylic esters, and cyclopolyenes. Specially the formation of 1,4-cyclohexadiene is of great interest. The metathesis of unsaturated fatty oils and the cometathesis with alkenes are also possible. Reactants of technical purity with a low acid value (< 0.1) can undergo metathesis. The reaction is essentially thermoneutral; the distribution of products is statistical and no completion of the reaction occurs in one way or an other. Thus separation of reaction products and unconverted material is necessary. By the metathesis of mono-unsaturated esters this can be done by fractional crystallization. A review is given of these and other applications on account of the latest research development.     

Metathesis of Unsaturated Fatty Esters

The development of a homogeneous catalyst [WCl₆/Sn(CH₃)₄) for the metathesis of unsaturated fatty esters encloses new routes in fat chemistry. By these reactions an exchange of alkylidene moities takes place, thus methyl oleate is converted into 9-octadecene and 9-octadecenedioic dimethyl ester. The metathesis of poly-unsaturated esters leads to a much larger number of products, namely polyenes, mono- and dicarboxylic esters, and cyclopolyenes. Specially the formation of 1,4-cyclohexadiene is of great interest. The metathesis of unsaturated fatty oils and the cometathesis with alkenes are also possible. Reactants of technical purity with a low acid value (<0.1) can undergo metathesis. The reaction is essentially thermoneutral; the distribution of products is statistical and no completion of the reaction occurs in one way or an other. Thus separation of reaction products and unconverted material is necessary. By the metathesis of mono-unsaturated esters this can be done by fractional crystallization. A review is given of these and other applications on account of the latest research development.     

Cross-linking of hexenyl silicone polymers via the olefin metathesis reaction

The olefin metathesis reaction was explored as a novel method to produce cross-linked silicone polymers. Although this reaction is well-known for non-silicone-containing monomers, there are no examples of this reaction in the presence of siloxanes. We have discovered a catalyst system for the reaction of silicone substrates comprised of a 1:4 molar ratio of WCl₆/Sn(CH₃)₄. No metathesis is seen with the traditional ratio of catalyst and cocatalyst. In a model system, 1-hexenylmethylbis(trimethylsiloxy)silane underwent self-metathesis in greater than 75% yield. The catalyst system also promoted metathesis of a hexenyl containing silicone polymer.     

Optimization of Ring‐Closing Metathesis: Inert Gas Sparging and Microwave Irradiation

A systematic study of a ring‐closing metathesis towards a tetrasubstituted double bond as part of a seven‐membered ring in a 5.7.5‐tricyclic guaianolide system is described. By combining two techniques, namely sparging an inert gas through the solution together with dielectric heating via microwave irradiation a high‐yielding ring‐closing metathesis reaction in this particularly challenging case was achieved. The results obtained compare favorably with conventional heating conditions or with microwave irradiation in a closed system. The key aspects seem to be that rapid microwave irradiation diminishes catalyst decay by allowing the required high reaction temperature to be reached quickly and homogeneously and thereby providing enough energy for a successful metathesis reaction, while inert gas sparging is purging off evolving ethylene to shift the equilibrium to the product.     

The Enhancement of Metathesis Catalysts via Alkylidene and N-Heterocyclic Carbene Ligand Optimization

Olefin metathesis has the potential to greatly improve the efficiency of organic synthesis. However, its commercial growth has been hampered by the difficulties of catalyst production, a complicated patent situation and expensive licenses. This work describes the design of robust and reproducible metathesis catalysts, deemed royalty free (RF), and their use for ring closing metathesis.     

Metathesis applied to unsaturated lipid compounds

The metathesis of unsaturated oleochemicals is an excellent tool to generate useful products based on easily available raw materials. Through various types of metathesis different classes of substances can be produced. Metathesis reactions are of particular interest because they are 100% atom‐efficient, that is no by‐products are formed. Also, metathesis reactions can be carried out under mild reaction conditions with high conversions of the fatty substrates and high yields of target products. With oleochemical metathesis it may become possible to replace some petrochemical processes and to generate products which are otherwise only poorly accessible. In this short review, several homogeneous metathesis reactions that may lead to products of industrial importance are presented and discussed.     

Olefin Metathesis on a TLC Plate as a Tool for a High‐Throughput Screening of Catalyst‐Substrate Sets

A methodology for screening either various catalysts for a given metathesis reaction, i.e., ring opening‐ring closing alkene metathesis (RO‐RCM) and cross‐metathesis (CM), or various substrates for a given pre‐catalyst on a thin layer chromatography (TLC) plate has been developed. As the substrates elute with the solvent, this TLC‐based system acts as a heterogeneous catalyst bed (“TLC reactor”). Selected promising catalyst candidates were screened on a TLC plate and their initial catalytic potential as observed in the TLC test was later fully confirmed in a classical heterogeneous reaction set‐up using standard commercially available silica (D11‐10). Reacting polyfunctional, natural product‐like substrates in our TLC reactor allows the simultaneous screening of various substrates and the convenient micro‐scale preparation and isolation of potentially biologically active products.     

Solid-state metathesis of KMgF3:Eu2+ phosphor

Solid-state metathesis is one of the important, but least explored, kind of SHS. Recently, solid state metathesis of some double fluorides of potassium with divalent metal has been demonstrated. This method is quite attractive as it is fast and simple. The reaction gets completed within ten minutes. A simple domestic microwave oven has been used to carry out the metathesis. Though the formation of double fluorides by metathesis is demonstrated, this itself is not sufficient for obtaining a luminescent material. It is equally, if not more, important to incorporate the activator ion during the synthesis. It will be highly convenient if the fluoride phosphors (and not simply the hosts) can be prepared by the solid state metathesis. We successfully prepared KMgF₃:Eu²⁺ phosphor by this method. The results are presented and discussed in this paper.     

烯烃复分解法制备端基官能化液体橡胶研究进展

用烯烃复分解法制备端基官能化液体橡胶具有分子量可控、易于引入多种官能团、反应条件较温和等优势。本文即综述了利用烯烃复分解法制备端羟基、端羧基、端酯基等官能化液体橡胶的研究新进展,并提示了其今后的发展方向。     

Cross‐metathesis with fatty acid derivatives: Scope,challenges and perspectives

Plant oils are an important renewable resource for the chemical industry and will help to reduce CO₂ emissions and contribute to a sustainable development of our future. Chemically efficient as well as cost‐effective reactions and processes are now being developed to achieve this goal, metathesis being one of them. This feature briefly describes the possibilities of cross‐metathesis reactions with oleochemicals, discussing general aspects of cross‐metathesis reactions, evaluating current pros and cons, and highlighting recent developments.     

Search for solutions to the reactivity and selectivity problems in enyne metathesis

Enyne metathesis is a powerful carbon-carbon bond-forming reaction to generate 1,3-dienes from an alkyne and an alkene. Different from the diene and diyne metathesis, the enyne metathesis suffers from both regio- and stereoselectivity problems, yet there is no general solution to these problems. This Account briefly describes the evolution of various new strategies and substrate platforms from these laboratories to address the reactivity and selectivity issues in the enyne metathesis processes.     

端羟基聚丁二烯液体橡胶制备技术的研究进展

对端羟基聚丁二烯（HTPB）液体橡胶的性能特点和应用领域等进行了概述,综述了HTPB的制备方法及其特点,同时对各类方法进行了对比,并提出了利用烯烃复分解法制备HTPB的优势以及烯烃复分解反应的特点,对其未来的发展方向进行了展望。     

Hoveyda-Grubbs complexes with boryl anions are predicted to be fast metathesis catalysts

A series of potential metathesis catalyst based on Hoveyda-Grubbs ruthenium system with the N-heterocyclic carbene replaced by N-heterocyclic carbene boryl anion were investigated using the M06 density functional approach. We show that these hypothetical metathesis precatalysts have two stable conformations depending on the position of the benzylidene moiety with respect to the NHC part of the complex. Estimates of the free Gibbs energy of initiation for these systems predict that they can be one of the fastest initiating metathesis catalysts, on the level of 3rd generation Grubbs complexes.