Reaction metathesis

In many reactions between two compounds in aqueous solution, the positive and negative ions appear to change partners to form two new eompounds, with no change in oxidation numbers. Such reactions are called metathe s reacdons. 

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We can represent such reactions by the following general equation, where A and B represent positive ions (cations) and X and Y represent negative ions (anions)  

For example, when we mix silver nitrate and sodium chloride solutions, solid silver chloride is formed and sodium nitrate remains dissolved in water  

Metathesis reactions result in the removal of ions from solution this removal of ions can be thought of as the driving force for the reaction—the reason it occurs. The removal of ions can occur in various ways, which can be used to classify types of metathesis reactions  

Broadly speaking, metathesis covers a class of reactions where an interchange of carbon atoms between pairs of double bonds takes place. We have already seen application of metathesis (see Section 6.8.1) in Shell higher olefin process (SHOP). It is also used for the manufacture of the specialty polymer Vestenamer from cyclooctene (see reaction 7.2.3). As shown by reactions 7.3.1 and 7.3.2, in both these cases the highlighted carbon atoms are exchanged between the pair of double bonds. 

The early metathesis catalysts used in industry were multicomponent systems such as WClg/EtAlCl, MoO/SiO, and WClg/EtAlCl /EtOH. 

The structures of the active sites and/or catalytic intermediates in these heterogeneous or quasi-heterogeneous catalytic systems were not known. 

Thanks to the development of well-defined molybdenum and ruthenium alkylidene catalysts initiated by Schrock and Grubbs in the late 1990s, alkene metathesis became a key methodology in organic synthesis and in 

Fogg et al. introduced chelating aryloxide (pseudohalide) ligands onto complexes 12 instead of the usual chloro substituents. These catalysts 

Alternatively, Dorta and co-workers used SINap ligands in order to improve the activity and stability of second-generation ruthenium metathesis catalysts (14). Effects of NHC backbone substitution on imidazolinylidene-derived ligands were studied by Grubbs et who also replaced the imidazole-based N-heterocycles with their thiazole analogues.Because these modifications were also applied to oxygen-chelated alkylidene catalysts, they are discussed in more details in the next Section. 

In 2008, Butenschon and co-workers reported an unusual variation on the second-generation Hoveyda-Grubbs catalyst, in which the 2-isopropoxybenzy-lidene ligand was coordinated to a highly electron-withdrawing tricarbonylchro-mium moiety (19). In a series of RCM, CM and enyne metathesis reactions carried out to screen its catalytic properties, complex 19 matched or outperformed the results obtained with its parent 15 and related initiators. 

Last but not least, two ruthenium isopropoxybenzylidene complexes bearing triethoxysilyl-functionalised NHC ligands were synthesised and grafted onto silica gel. The resulting solid-supported catalysts (22 and 23) were found efficient for a number of metathesis reactions. They could be recycled a number of times with an eventual gradual decrease in activity, but most strikingly, they did not leach ruthenium under the experimental eonditions adopted, a feature of utmost importance for the development of pharmaeeutieal applieations. 

Butadiene and ethylene are codimerized with a soluble rhodium-phosphine complex as the catalyst. Very little has been reported on the mechanistic evidence for this reaction. However, a catalytic cycle as shown in Fig. 7.9 involving a rhodium hydride seems likely. Reducing rhodium trichloride with ethanol in the presence of a tertiary phosphine generates the hydride complex 7.32. The 1,4-hydride attack on the coordinated butadiene gives an rf-allyl complex. This is shown by the conversion of 7.33 to 7.34. Ethylene coordination to 7.34 produces 7.35. 

The latter undergoes insertion of ethylene into the rhodium-carbon cr bond to give 7.36. The formal mechanism for the formation of 7.36 is shown by reaction 7.10. Complex 7.36 undergoes /3-elimination to generate 7.37, which liberates 1,4-hexadiene and completes the catalytic cycle. 

Metathesis of alkenes is essentially a class of reactions where an interchange of C atoms between pairs of double bonds takes place. A few representative examples are shown by the reactions listed in Fig. 7.10. The industrial use of metathesis reactions so far has been limited mainly to exchange metathesis (Fig. 7.10, top, backward reaction) as in the SHOP process, and ring-open metathesis polymerization (ROMP). As already mentioned (Section 7.5), Vas-tenamer is a polymer made by Hulls by ROMP from cyclooctene. Similarly, the polymer from norborene by ROMP is manufactured by CdF Chemie and is sold by the trade name of Norsorex . 

The ruthenium-carbene complex 8.52 is an excellent commercially available example of the Grubbs catalyst. However, intramolecular five-membered ring compounds 8.53-8.55, which are activated by coordination with an ether oxygen atom, are much more active with respect to electron-deficient olefins and are also stable with respect to air as shown in Fig. 8.2. 

8 Applications of Five-Membered Ring Products as Catalysts. 

Many ruthenium-carbene five-membered ring compounds have recently been reported to show good activity for metathesis [51-71]. Many types of reactions, such as ring-closing metathesis, ring-opening metathesis, ctoss metathesis, enyne metathesis, and diyne metathesis, proceed with the help of these catalysts, as shown in Eqs. (8.15), (8.16), (8.17), (8.18), and (8.19) [58]. 

The ring-closing metathesis (Eq. (8.15) of an acyclic diene, for example, proceeds easily at room temperature with a high yield in the presence of the l,3-dimesityl-4,5-dihydroimidazole-2-ylidene ruthenium catalyst 8.53, as shown in Eq. (8.20) [51-54]. 

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Wallace C H, Rao L, Kim S-H, Heath J R, Nicol M and Kaner R B 1998 Solid-state metathesis reactions under pressure a rapid route to crystalline gallium nitride Appl. Phys. Lett. 72 596... 

Another interesting transformation is the intramolecular metathesis reaction of 1,6-enynes. Depending on the substrates and catalytic species, very different products are formed by the intramolecular enyne metathesis reaction of l,6-enynes[41]. The cyclic 1,3-diene 71 is formed from a linear 1,6-enyne. The bridged tricyclic compound 73 with a bridgehead alkene can be prepared by the enyne metathesis of the cyclic enyne 72. The first step of... 

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... 

Ca.ustlciZa.tlon, Time, particularly the high calcium type, reacts with carbonates such as Na2C02 and Li2C02 to form other hydroxides and carbonates through double decomposition or metathesis reactions as foUow ... 

The alkah metal perchlorates are either white or colorless, and have increasing solubiUty in water in the order of Na > Li > NH4 > K> Rb > Cs. The high solubiUty of sodium perchlorate, NaClO, makes this material useful as an intermediate for production of all other perchlorates by double metathesis reactions and controlled crystallization. 

In another process, strontium sulfate can be converted to strontium carbonate direcdy by a metathesis reaction wherein strontium sulfate is added to a solution of sodium carbonate to produce strontium carbonate and leave sodium sulfate in solution (6). Prior to this reaction, the finely ground ore is mixed with hydrochloric acid to convert the calcium carbonates and iron oxides to water-soluble chlorides. 

A different approach, taken by both Monsanto (58) and Gulf Research and Development Company (59), involved the oxidative coupling of two molecules of toluene to yield stilbene. The stilbene is then subjected to a metathesis reaction with ethylene to yield two molecules of styrene. 

A series of divalent lanthanide metal metaHaborane derivatives have been prepared by the redox reaction of metallic lanthanides and boron hydrides and by the metathesis reaction of boron hydride salts with LnCl2 where Ln = Sm, Eu, Yb (181,182). The species (CH3CN)3Yb[(p.-H)2B2QH22],... 

Disproportionation of Olefins. Disproportionation or the metathesis reaction offers an opportunity to convert surplus olefins to other desirable olefins. Phillips Petroleum and Institut Fransais du Petrc le have pioneered this technology for the dimerization of light olefins. The original metathesis reaction of Phillips Petroleum was intended to convert propylene to 2-butene and ethylene (58). The reverse reaction that converts 2-butene in the presence of excess ethylene to propylene has also been demonstrated (59). A commercial unit with a capacity of about 136,000 t/yr of propylene from ethylene via 2-butene has been in operation in the Gulf Coast since 1985 (60,61). In this process, ethylene is first dimerized to 2-butene foUowed by metathesis to yield propylene. Since this is a two-stage process, 2-butene can be produced from the first stage, if needed. In the dimerization step, about 95% purity of 2-butene is achieved at 90% ethylene conversion. 

Manufacture. Most chlorate is manufactured by the electrolysis of sodium chloride solution in electrochemical cells without diaphragms. Potassium chloride can be electroly2ed for the direct production of potassium chlorate (35,36), but because sodium chlorate is so much more soluble (see Fig. 2), the production of the sodium salt is generally preferred. Potassium chlorate may be obtained from the sodium chlorate by a metathesis reaction with potassium chloride (37). 

Cobalt(II) acetylacetonate [14024-48-7] cobalt(II) ethyUiexanoate [136-52-7] cobalt(II) oleate [14666-94-5] cobalt(II) linoleate [14666-96-7] cobalt(II) formate [6424-20-0], and cobalt(II) resinate can be produced by metathesis reaction of cobalt salt solutions and the sodium salt of the organic acid, by oxidation of cobalt metal in the presence of the acid, and by neutralization of the acid using cobalt carbonate or cobalt hydroxide. 

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]. 

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 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 ... 

Other Ionic Impurities from Incomplete Metathesis Reactions... 

Apart from halide and protic impurities, ionic liquids can also be contaminated with other ionic impurities from the metathesis reaction. This is especially likely if the alkali salt used in the metathesis reaction shows significant solubility in the... 

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 ... 

Chlorotris(diethylamino)titanium24 is prepared directly from diethylamine, lithium and tilani-um(IV) chloride in the presence of styrene as reducing agent25. However, a metathesis reaction between tetrakis(diethylamino)titanium26 28 and titanium(lV) chloride gives a cleaner product and is thus preferred. Bromotris(diethylamino)titanium is prepared similarly7,29. 

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 ... 

Since then, the metathesis reaction has been extended to other types of alkenes, viz. substituted alkenes, dienes and polyenes, and to alkynes. Of special interest is the metathesis of cycloalkenes. This gives rise to a ring enlargement resulting in macrocyclic compounds and eventually poly-... 

Scott et al. (3) and Wasserman et al. (4) were the first to realize that this ring-opening polymerization, which had been known for several years, might be a special case of the metathesis reaction. 

The discovery of the metathesis reaction is also of importance from a theoretical and fundamental point of view, and has contributed to the development of new ideas about reactions of alkenes in the presence of transition metal compounds. 

All the information to date (see Section III) indicates that the metathesis reaction proceeds via the rupture and formation of carbon-carbon double bonds ... 

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. 

It has been shown that halogen-substituted alkenes can participate in the metathesis reaction, e.g. 5-bromo-l-pentene reacts with 2-pentene 11). A very interesting reaction is the conversion of methyl-9-octa-decenoate into 9-octadecene and dimethyl-9-octadecenedioate 12) ... 

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) ... 

It has been suggested that these polymers are mainly linear, which may be a consequence of intermolecular metathesis reactions with traces of acyclic alkenes, or of other consecutive reactions 19-22). 

Solid catalysts for the metathesis reaction are mainly transition metal oxides, carbonyls, or sulfides deposited on high surface area supports (oxides and phosphates). After activation, a wide variety of solid catalysts is effective, for the metathesis of alkenes. Table I (1, 34 38) gives a survey of the more efficient catalysts which have been reported to convert propene into ethene and linear butenes. The most active ones contain rhenium, molybdenum, or tungsten. An outstanding catalyst is rhenium oxide on alumina, which is active under very mild conditions, viz. room temperature and atmospheric pressure, yielding exclusively the primary metathesis products. 

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