Metathesis catalyst deactivation

These react with excess alkyne to give a cyclopentadienyl complex, via a metalabenzene 11.23. This is how the alkyne metathesis catalyst deactivates. 

A significant problem is the dehydrocoupling reaction, which proceeds only at low yields per pass and is accompanied by rapid deactivation of the catalyst. The metathesis step, although chemically feasible, requires that polar contaminants resulting from partial oxidation be removed so that they will not deactivate the metathesis catalyst. In addition, apparendy both cis- and /ra/ j -stilbenes are obtained consequendy, a means of converting the unreactive i j -stilbene to the more reactive trans isomer must also be provided, thus complicating the process. 

Table 3.19 lists examples of the preparation of nitrogen-containing heterocycles by RCM. As mentioned in Section 3.2.5.3, free amines can partly deactivate metathesis catalysts. With the highly reactive molybdenum catalyst 1 it is, however, possible to cyclize dienes containing a basic amino group. If the less reactive catalysts 2 or 3 are to be used, protonation or acylation of the amine can be used to reduce their nucleophilicity. This will generally lead to higher yields with smaller amounts of catalyst. 

New applications continue to demonstrate the enormous versatility of RCM for organic synthesis. Examples include triple ring closing (Eq. 48) and alkyne metathesis, an example being that of cross-metathesis that provides an efficient synthetic strategy for prostaglandin E2 (Eq. 49). Amines and alcohols deactivate metathesis catalysts, but their protection as ethers, esters, and amides allows them to be incorporated into the designated transformation. 

Both the intramolecular and the intermolecular secondary metathesis reactions affect the polymerisation kinetics by decreasing the rate of polymerisation, because a fraction of the active sites that should be available as propagation species are involved in these non-productive metathesis reactions. The kinetics of polymerisation in the presence of metal alkyl-activated and related catalysts shows in some cases a tendency towards retardation, again due to gradual catalyst deactivation [123]. Moreover, several other specific reactions can influence the polymerisation. Among them, the addition of carbene species to an olefinic double bond, resulting in the formation of cyclopropane derivatives [108], and metallacycle decomposition via reductive elimination of cyclopropane [109] deserve attention. 

These highly activated aqueous ROMP catalysts can be applied to the polymerization of monomers hitherto reluctant to polymerize in aqueous solution. This can be illustrated with the following example. When either 7-oxanorbomene-2,3-dicarboxylic acid, 46, or its dipotassium salt, which posses both the 1,4-bridging epoxide and the dicarboxylate moieties, is allowed to react with a wide range of metathesis catalysts, only catalyst deactivation is observed, Eq. (48) [79]. 

The overall metathesis activity of this class of ruthenium-carbene catalysts is determined by the relative magnitudes of several rate constants (i) the rate constant of phosphine dissociation (fej), which dictates the rate at which the precatalyst complex enters the catalytic cycle (ii) the ratio of k i/k2. which dictates the rate of catalyst deactivation (by re-coordination of phosphine) versus catalytic turnover (by coordination of olefmic substrate and subsequent steps) and (iii) the rate constant of metallacyclobutane formation (k ), which dictates the rate of carbon-carbon bond formation. 

I2 in THF-H2O. The self-cleavable linker 10, which provides the thermodynamically stable cyclopentene both on solid supports and on the products, prevents catalyst deactivation due to covalent attachment of the ruthenium catalyst to either the solid-support or the products via C=C double bonds during the metathesis reaction. 

One problem in catalysis is gradual loss of activity of the catalyst. There are many reasons underlying the deactivation of heterogeneous metathesis catalysts [42]. The most important causes of catalyst deactivation are (i) intrinsic deactivation reactions, such as the reductive elimination of metallacyclobutane intermediates,... 

The same neopentylidene-alkoxo complexes react with cis-2-pentene to give the two initial metathesis products (4,4-dimethyl-2-pentene and 5,5-dimethyl-3-hexene) and catalyze the metathesis of cis-2-pentene to 2-butenes and 3-hexenes. Furthermore, propylene and ethylene appear in the reaction medium as the catalyst deactivates. These latter olefins are formed by rearrangement of the ethylidene and propylidene intermediates, providing the mechanism for the metathesis chain termination step ... 

Catalyst deactivation and regeneration. The activity of a rhenium-based catalyst in the metathesis of unsaturated esters is unavoidably limited by the complexation of the ester group to the active site [8]. Moreover, there are many routes that lead to deactivation of the catalyst. Polar compounds such as H2O or free acids, alcohols and peroxides, which might be present as an impurity in the substrate(s), can act as catalyst poisons. Other possible routes for the deactivation of rhenium-based catalysts include (i) reduction of the rhenium below its optimum oxidation state (ii) adsorption of (polymeric) product molecules on the surface of the catalyst, blocking the active sites (iii) reductive elimination of the metallacyclobutane intermediate [59]. Even when the greatest care is taken, deactivation of the rhenium catalyst cannot be avoided. 

Catalyst deactivation. The molybdenum-based catalysts deactivate faster than the rhenium-based ones. Studies concerning the stability of the catalyst during continuous metathesis of propene showed a loss of activity due to an intrinsic deactivation mechanism. Because of the high stability of both [Mo]=CH2 and [Mo]=CHCH3, the deactivation of the catalyst is assigned to isomerization of the intermediate metallacyclobutane complexes, leading to inactive 7i-complexes, in a way analogous to that depicted in Scheme 2. This hypothesis is supported by in situ UV/vis spectroscopic studies [67]. 

The deactivation of this tandem, dual-catalytic system has been investigated. It was found that the low TONs obtained under these reaction conditions were mainly the result of the decomposition of the olefin metathesis catalyst. It was found that adding an additional amount of olefin metathesis catalyst at the end of a catalytic run reinitiated the process [18]. Therefore, to improve the catalytic efficiency, olefin metathesis catalysts as robust as the Ir-pincer dehydrogenation catalyst and more active at temperatures above 125 °C needed to be developed. A collaboration with Schrock and coworkers allowed for the synthesis of 40 different... 

Engineering of the Tandem, Dual-Catalyst System s Alkane Metathesis Heterogeneous olefin metathesis systems were studied to improve the operability ofthe Mo and W catalysts at the elevated temperatures required for the (de)hydrogenation steps. It is well documented that the immobilization of catalysts onto a solid support can increase the lifetimes, and thus the efficiency, of alkane metathesis systems. Catalyst immobilization limits catalyst-catalyst interactions and the bimolecular decomposition reactions responsible for tandem alkane metathesis system deactivation [137, 138]. Further reading on this topic can be found in... 

Leduc A-M, Salameh A, Soulivong D, Chabanas M, Basset J-M, Coperet C, Solans-Monfort X, Clot E, Eisenstein O, Bohm VPW, Roper M (2008) p-H transfer fitnn the metallacyclobutane a key step in the deactivation and byproduct formation for the well-defined silica-supported rhenium alkylidene alkene metathesis catalyst. J Am Chan Soc 130 6288-6297... 

This method is specific for metallacyclopentanes. The alkene-coupling process is favored by metal reduction. A typical synthetic strategy is the in situ reduction of a metal halide precursor in the presence of the alkene see, for example, the synthesis of 79 in Scheme 34.1 An alkylidene precursor may also lead to a metallacycle with elimination of the car-bene ligand as in the synthesis of 81, representing a deactivation pathway for alkene metathesis catalysts. Ilie two alkenes may be generated in situ in the coordination sphere by rearrangement processes, such as intramolecular hydrogen transfer from an alkyl-vinyl precursor. I ... 

However, the RCM methodology has also several drawbacks. First, the basic amino group in the substrate, if present, deactivates the catalyst. Therefore, before the reaction, it should be reversibly masked as, for example, a carbamate, amide, or ammonium salt. Second, the metathesis catalysts are still very expensive - a fact, that cannot be ignored when planning the synthesis on a larger scale. Moreover, the pollution of the products with ruthenium, even many steps after the metathesis reaction. 

It is important to remove all oxygen, dienes and acetylenes from the feed to the metathesis reactor. Furthermore, the C4 stream needs to contain the minimum practical level of butene-1 and isobutene to minimize the metathesis reaction of the C4 hydrocarbons, which results in the formation of C5 and Ce olefins. The higher olefins lead to polymer formation and catalyst deactivation. An excess of ethylene normally suppresses the C4 reactions. A typical steam cracker C4 stream, which has been subjected to selective hydrogenation to remove impurities and fractionation to provide a suitable butene-2 rich raffinate-2, or the butene-2 rich efflnent from an MTBE unit, can provide a suitable feed for the metathesis reactor. The catalyst operating cycle with a rhenium catalyst is usually fairly long. Regular catalyst regeneration may be necessary and the catalyst can last for several years. 

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