Metathesis examples

When decomposition processes occur under catalytically relevant conditions they can lead to a decrease or loss of activity, or an erosion of enan-tioselectivity if chiral ligands are employed. Even when small amounts of decomposition are observed, itcanbe problematic if the non-NHC-containing catalyst is more aetive than the NHC-containing ones. Referring again to the metathesis example, ruthenium hydride complexes have been implicated in deleterious alkene isomerisation processes during metathesis reactions. ... 

There are two main classes of RCM reactions that are important in the synthesis of THP rings (Scheme 54). Class 1 involves the ring closure of ether 199 bearing allylic and homoallylic functionalities to afford 3,4-dihydropyrans 198, which upon simple reduction leads to THP 202. Class 2 involves an RCM of homoallylic acrylate substrate 203 to provide unsaturated lactone 204, which can be further functionalized to THP 202. All of the RCM metathesis examples in this survey utilize either the first-generation Grubbs catalyst (G-I) or the second-generation Grubbs catalyst (G-II), shown in Fig. 2. 

An alternative to elucidating the active sites on a surface is to synthesize them. For example, a new catalyst for metathesis of alkanes. 

A precipitation reaction occurs when two or more soluble species combine to form an insoluble product that we call a precipitate. The most common precipitation reaction is a metathesis reaction, in which two soluble ionic compounds exchange parts. When a solution of lead nitrate is added to a solution of potassium chloride, for example, a precipitate of lead chloride forms. We usually write the balanced reaction as a net ionic equation, in which only the precipitate and those ions involved in the reaction are included. Thus, the precipitation of PbCl2 is written as... 

Tetrakis-Cp uranium complexes are readily prepared via metathesis of UCl and KCp in refluxing benzene. These complexes are a relatively rare example of a pseudotetrahedral complex with four Tj -Cp rings, (rj-ring) (25). The Cp derivative has been shown to react with CO and CO2 to give acyl and carboxylato complexes. This complex also reacts with alkyl haUdes to afford the U(IV) complex, Cp UX (X = halide). 

Olefin Metathesis. The olefin metathesis (dismutation) reaction (30), discovered by Eleuterio (31), converts olefins to lower and higher molecular weight olefins. For example, propylene is converted into ethylene and butene... 

Metathesis is the rupture and reformation of carbon-carbon bonds—for example, of propylene into ethylene plus butene. Catalysts are oxides, carbonyls, or sulfides of Mo, W, or Re. 

To date a number of reactions have been carried out in ionic liquids [for examples, see Dell Anna et al. J Chem Soc, Chem Commun 434 2002 Nara, Harjani and Salunkhe Tetrahedron Lett 43 1127 2002 Semeril et al. J Chem Soc Chem Commun 146 2002 Buijsman, van Vuuren and Sterrenburg Org Lett 3 3785 2007]. These include Diels-Alder reactions, transition-metal mediated catalysis, e.g. Heck and Suzuki coupling reactions, and olefin metathesis reactions. An example of ionic liquid acceleration of reactions carried out on solid phase is given by Revell and Ganesan [Org Lett 4 3071 2002]. 

Only few examples of functionalized copper reagents have been prepared The perfluoroallyleopper reagent was prepared via metathesis of the corresponding perfluoroallylcadmium compound [146] (equation 159) Reaction of the allyl... 

Sulfur imides with a single NR functionality, S5NR (6.12), SeNR (6.13) (R = Oct), " SgNH (6.14), ° and S9NH (6.15) ° are obtained by a methodology similar to that which has been used for the preparation of unstable sulfur allotropes, e.g., S9 and Sio. Eor example, the metathesis reaction between the bis(cyclopentadienyl)titanium complexes 6.8-6.10 and the appropriate dichlorosulfane yields 6.14 and 6.15 (Eq. 6.4). °... 

Metathesis reactions are sometimes the reverse of those in aqueous systems because of the differing solubility relations. For example because AgBr forms the complex ion [Ag(NH3)2]" " in liquid NH3 it is readily soluble, whereas BaBr2 is not, and can be precipitated ... 

The fluorination reactions considered so far can be categorized as metathesis, oxidation or substitution, Occasionally reductive fluorination is the preferred route to a lower fluoride. Examples are ... 

When a mixture of alkenes 1 and 2 or an unsymmetrically substituted alkene 3 is treated with an appropriate transition-metal catalyst, a mixture of products (including fi/Z-isomers) from apparent interchange of alkylidene moieties is obtained by a process called alkene metathesis. With the development of new catalysts in recent years, alkene metathesis has become a useful synthetic method. Special synthetic applications are, for example, ring-closing metathesis (RCM) and ring-opening metathesis polymerization (ROM) (see below). 

The synthetic utility of the alkene metathesis reaction may in some cases be limited because of the formation of a mixture of products. The steps of the catalytic cycle are equilibrium processes, with the yields being determined by the thermodynamic equilibrium. The metathesis process generally tends to give complex mixtures of products. For example, pent-2-ene 8 disproportionates to give, at equilibrium, a statistical mixture of but-2-enes, pent-2-enes and hex-3-enes ... 

Table 2.1-1 Examples of ionic liquids prepared by anion metathesis.

The choice of the anion ultimately intended to be an element of the ionic liquid is of particular importance. Perhaps more than any other single factor, it appears that the anion of the ionic liquid exercises a significant degree of control over the molecular solvents (water, ether, etc.) with which the IL will form two-phase systems. Nitrate salts, for example, are typically water-miscible while those of hexaflu-orophosphate are not those of tetrafluoroborate may or may not be, depending on the nature of the cation. Certain anions such as hexafluorophosphate are subject to hydrolysis at higher temperatures, while those such as bis(trifluoromethane)sulfonamide are not, but are extremely expensive. Additionally, the cation of the salt used to perform any anion metathesis is important. While salts of potassium, sodium, and silver are routinely used for this purpose, the use of ammonium salts in acetone is frequently the most convenient and least expensive approach. 

In addition to the applications reported in detail above, a number of other transition metal-catalyzed reactions in ionic liquids have been carried out with some success in recent years, illustrating the broad versatility of the methodology. Butadiene telomerization [34], olefin metathesis [110], carbonylation [111], allylic alkylation [112] and substitution [113], and Trost-Tsuji-coupling [114] are other examples of high value for synthetic chemists. 

Acyclic diene molecules are capable of undergoing intramolecular and intermolec-ular reactions in the presence of certain transition metal catalysts molybdenum alkylidene and ruthenium carbene complexes, for example [50, 51]. The intramolecular reaction, called ring-closing olefin metathesis (RCM), affords cyclic compounds, while the intermolecular reaction, called acyclic diene metathesis (ADMET) polymerization, provides oligomers and polymers. Alteration of the dilution of the reaction mixture can to some extent control the intrinsic competition between RCM and ADMET. 

Olefin metatheses are equilibrium reactions among the two-reactant and two-product olefin molecules. If chemists design the reaction so that one product is ethylene, for example, they can shift the equilibrium by removing it from the reaction medium. Because of the statistical nature of the metathesis reaction, the equilibrium is essentially a function of the ratio of the reactants and the temperature. For an equimolar mixture of ethylene and 2-butene at 350°C, the maximum conversion to propylene is 63%. Higher conversions require recycling unreacted butenes after fractionation. This reaction was first used to produce 2-butene and ethylene from propylene (Chapter 8). The reverse reaction is used to prepare polymer-grade propylene form 2-butene and ethylene ... 

In this process, which has been jointly developed by Institute Francais du Petrole and Chinese Petroleum Corp., the C4 feed is mainly composed of 2-butene (1-butene does not favor this reaction but reacts differently with olefins, producing metathetic by-products). The reaction between 1-butene and 2-butene, for example, produces 2-pentene and propylene. The amount of 2-pentene depends on the ratio of 1-butene in the feedstock. 3-Hexene is also a by-product from the reaction of two butene molecules (ethylene is also formed during this reaction). The properties of the feed to metathesis are shown in Table 9-1. Table 9-2 illustrates the results from the metatheses reaction at two different conversions. The main by-product was 2-pentene. Olefins in the range of Ce-Cg and higher were present, but to a much lower extent than C5. 

Another example is the metathesis of cyclooctene, which produces poly-octenylene, an elastomor known as trans-polyoctenamer ... 

The metathesis reaction of alkenes constitutes a major development in the field of hydrocarbon chemistry in recent years. The first examples of the heterogeneously and the homogeneously catalyzed metathesis of linear alkenes have been published by Banks and Bailey (I) and Calderon et al. (2), respectively. By this reaction, linear alkenes are converted with high selectivity into equimolar amounts of two new alkenes, according to ... 

Various types of unsaturated hydrocarbons have been reported to undergo metathesis reactions by contact with appropriate catalysts. A short survey is given below. It is to be expected that in the near future still more examples will be found. 

Recently, a few examples of the metathesis of alkenes carrying functional groups have been reported. According to a patent, acrylonitrile reacts with propene to crotononitrile (cis and trans) and ethene 10) ... 

The metathesis of acyclic alkadienes and higher polyenes may involve both inter- and intramolecular processes. An example of an intermolecular reaction is the conversion of 1,5-hexadiene into 1,5,9-decatriene and ethene ... 

Deeatriene may, of course, react further to 1,5,9,13-tetradeca-tetraene, 1,5,9,13,17-octadecapentaene, etc. (18). Even the conjugated system 1,3-butadiene participates in metathesis reactions (14). An example of an intramolecular process is the reaction of 1,7-octadiene, which gives cyclohexene and ethene (13, 15) ... 

The metathesis of alkyl- or aryl-substituted cycloalkenes provides a route to certain perfectly alternating copolymers. For example, metathesis of 5-methylcyclooctene leads to a polymer that may be considered as a... 

Typical Examples of Catalyst Systems for the Homogeneous Metathesis of Pentene... 

Of course, even in the case of acyclic alkenes reaction enthalpy is not exactly zero, and therefore the product distribution is never completely statistically determined. Table V gives equilibrium data for the metathesis of some lower alkenes, where deviations of the reaction enthalpy from zero are relatively large. In this table the ratio of the contributions of the reaction enthalpy and the reaction entropy to the free enthalpy of the reaction, expressed as AHr/TASr, is given together with the equilibrium distribution. It can be seen that for the metathesis of the lower linear alkenes the equilibrium distribution is determined predominantly by the reaction entropy, whereas in the case of the lower branched alkenes the reaction enthalpy dominates. If the reaction enthalpy deviates substantially from zero, the influence of the temperature on the equilibrium distribution will be considerable, since the high temperature limit will always be a 2 1 1 distribution. Typical examples of the influence of the temperature are given in Tables VI and VII. 

Analyses of rate measurements for the decomposition of a large number of basic halides of Cd, Cu and Zn did not always identify obedience to a single kinetic expression [623—625], though in many instances a satisfactory fit to the first-order equation was found. Observations for the pyrolysis of lead salts were interpreted as indications of diffusion control. More recent work [625] has been concerned with the double salts jcM(OH)2 yMeCl2 where M is Cd or Cu and Me is Ca, Cd, Co, Cu, Mg, Mn, Ni or Zn. In the M = Cd series, with the single exception of the zinc salt, reaction was dehydroxylation with concomitant metathesis and the first-order equation was obeyed. Copper (=M) salts underwent a similar change but kinetic characteristics were more diverse and examples of obedience to the first order, the phase boundary and the Avrami—Erofe ev equations [eqns. (7) and (6)] were found for salts containing the various cations (=Me). 

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