Decomposition catalysts

The exact mechanism of catalyst decomposition for these complexes is not rigorously known. Several general points need to be described, however. First, the decomposition of the ruthenium carbenes is second order in catalyst. Thermolytic half lives for some of these complexes have been measured and some of these data is tabulated in Tab. 6.6. 

the methylidene complex shows different behavior than the other carbenes. The decomposition of the methylidene is not affected by the presence of excess phosphine and is first order in catalyst. The decomposition in this case appears to be due to activation of C-H bonds in the L ligand. 

CI bimolecular unknown inorganic products q, Ru= P possibly bimetallic species 

Simple alkenes can give turnover numbers in the order of several 100,000 with tungsten or molybdenum based catalysts, including the in situ prepared catalysts (e.g. WC16, PhOH, SnBu4), provided that the alkene is thoroughly purified. A convenient purification method is percolation of the alkene over neutral alumina to remove peroxides. 

From these studies it is evident that the intermediates obtained during an organic synthesis will likely contain impurities at the percentage level that may make the use of similar catalyst levels necessary, unless still better catalysts that are more resistant to alcohols and water will be developed. 

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Well, I think it s better to add a bit of H2O2 to the toluene + safrol + catalyst to prevent catalyst decomposition, for example, with 100 cc of safrol, lOcc of H2O2, and then add the rest dropwise. 

The following conditions are stipulated the catalyst decomposition rate constant must be one hour or greater the residence time of the continuous reactor must be sufficient to decompose the catalyst to at least 50% of the feed level the catalyst concentration must be greater than or equal to 0.002 x Q, where the residence time, is expressed in hours. An upper limit on the rate of radical formation was also noted that is, when the rate of radical formation is greater than the addition rate of the primary radicals to the monomers, initiation efficiency is reduced by the recombination of primary radicals. 

In practice, 1—10 mol % of catalyst are used most of the time. Regeneration of the catalyst is often possible if deemed necessary. Some authors have advocated systems in which the catalyst is bound to a polymer matrix (triphase-catalysis). Here separation and generation of the catalyst is easy, but swelling, mixing, and diffusion problems are not always easy to solve. Furthermore, triphase-catalyst decomposition is a serious problem unless the active groups are crowns or poly(ethylene glycol)s. Commercial anion exchange resins are not useful as PT catalysts in many cases. 

When potassium fluoride is combined with a variety of quaternary ammonium salts its reaction rate is accelerated and the overall yields of a vanety of halogen displacements are improved [57, p 112ff. Variables like catalyst type and moisture content of the alkali metal fluoride need to be optimized. In addition, the maximum yield is a function of two parallel reactions direct fluorination and catalyst decomposition due to its low thermal stability in the presence of fluoride ion [5,8, 59, 60] One example is trimethylsilyl fluoride, which can be prepared from the chloride by using either 18-crown-6 (Procedure 3, p 192) or Aliquot 336 in wet chlorobenzene, as illustrated in equation 35 [61],... 

Independently, Caddick et al. reported microwave-assisted amination of aryl chlorides using a palladium-N-heterocyclic carbene complex as the catalyst (Scheme 99) [lOlj. Initial experiments in a domestic microwave oven (reflux conditions) revealed that the solvent is crucial for the reaction. The Pd source also proved very important, since Pd(OAc)2 at high power in DMF gave extensive catalyst decomposition and using it at medium and low power gave no reaction at all. Pd(dba)2/imidazohum salt (1 mol% catalyst loading) in DME with the addition of some DMF was found to be suitable. Oil bath experiments indicated that only thermal effects are governing the amination reactions. 

Subsequently, the scope of the reaction was extended to N-nucleophiles 82. Because the inherent basicity of the substitution products 83 imposed some problems concerning catalyst decomposition, the addition of catalytic amoimts of piperidine hydrochloride (pip-HCl) proved to be necessary. Under optimized reaction conditions different aromatic amines 82 were allylated with almost exclusive regioselectivites in favor of the ipso substitution products 83 (eq. 1 in Scheme 20) [64]. 

The most important undesired metallic impurities are nickel and vanadium, present in porphyrinic structures that originate from plants and are predominantly found in the heavy residues. In addition, iron may be present due to corrosion in storage tanks. These metals deposit on catalysts and give rise to enhanced carbon deposition (nickel in particular). Vanadium has a deleterious effect on the lattice structure of zeolites used in fluid catalytic cracking. A host of other elements may also be present. Hydrodemetallization is strictly speaking not a catalytic process, because the metallic elements remain in the form of sulfides on the catalyst. Decomposition of the porphyrinic structures is a relatively rapid reaction and as a result it occurs mainly in the front end of the catalyst bed, and at the outside of the catalyst particles. 

Diboration of terminal alkenes has also been studied with other d " metals (Fig. 2.12) including the Ag and Au complexes 75-77 and the Pt" complexes 78-79. Styrene is diborylated with 100% selectivity and good conversions in THF (46% for 75 and 94% for 77 at 5 mol%, 60 h) using equimolecular amounts of (Bcat)j. The difference in activity between the Ag and Au complexes has been ascribed to the increased lability of the Ag-NHC bond, which may lead to catalyst decomposition under the reaction conditions, hi both catalytic systems it is believed that the active species involves only one coordinated NHC ligand. Complex 77 is less active than 74 and 75, possibly due to steric reasons. The enantioselectivity of 77 in the diboration of prochiral alkenes is very low [63]. 

Developments in NHC chemistry have been considerable, and an exponential increase in pubhcations has occurred since the mid 1990s. Numerous reviews have appeared providing comprehensive experimental and theoretical desaiptions of advances in the field [1-13], and very recently a volume of Coordination Chemistry Reviews and complete issues of Dalton Transactions and European Journal of Inorganic Chemistry have been dedicated to NHC chanistry [14], An excellent review on the topic of catalyst decomposition was pubhshed in 2004 [7], and consequently the present chapter will... 

Hence, P-C bond-cleavage followed by isomerization is responsible for the formation of side products. Furthermore, due to destabilization of the catalyst complex, deactivation occurs and palladium black is formed, which is a notorious disadvantage of Pd-phosphine catalysts in general. Catalyst decomposition and the formation of side products causes additional separation and catalyst recovery problems. These problems have been solved by the discovery of novel catalyst complexes, which are active and stable at temperatures of over 250 °C (Cornils and Herrmann, 1996). 

Product purification not only includes recovery of the catalyst. It also consists of removal of co-catalysts, decomposition products of the ligands, unconverted reactants, and byproducts. The latter two, of course, are not specific for homogeneous catalysis. 

The application of olefin metathesis to the synthesis of piperidines continues to be widely employed. The use of ring closing metathesis (RCM) in the synthesis of fluorovinyl-containing a,P-unsaturated lactams 148 and cyclic amino acid derivatives 149 is shown below. A key improvement in these reactions is the addition of the Grubbs 2nd generation catalyst (G2) in small portions during the reaction to compensate for catalyst decomposition that occurs at elevated reaction temperatures <06EJOl 166>. 

During a 33 h continuous hydroformylation run using this set-up, no catalyst decomposition was observed and Rh leaching into the scC02/product stream was less than 1 ppm. The selectivity for the linear nonanal was found to be stable over the reaction time with n/iso = 3.1. During the continuous reaction, alkene, CO, H2 and C02 were separately fed into the reactor containing the ionic liquid catalyst solution. Products and unconverted feedstock dissolved in SCCO2 were removed from the ionic liquid. After decompression the liquid product was collected and analysed. 

Similar reactions, performed under low pressures of carbon monoxide, yield alkanes, without any carbonylation products (Equations (50) and (51)). A CO atmosphere prevented catalyst decomposition. 

The research group of Van Leeuwen reported the use of carbosilane de-ndrimers appended with peripherial diphenylphosphino end groups (i.e. 25, Scheme 26) [37]. After in situ complexation with allylpalladium chloride, the resultant metallodendrimer 25 was used as catalyst in the allylic alkylation of sodium diethyl malonate with allyl trifluoroacetate in a continuous flow reactor. Unlike in the batch reaction, in which a very high activity of the dendrimer catalyst and quantitative conversion of the substrate was observed, a rapid decrease in space time yield of the product was noted inside the membrane reactor. The authors concluded that this can most probably be ascribed to catalyst decomposition. The product flow (i.e. outside the membrane reactor)... 

Catalyst decomposition ( die-out ) during the catalytic reaction is a common phenomenon also distorting the kinetic measurements. If the decomposition reaction obeys a rate equation in a well-behaved manner, one can include the decomposition reaction in the kinetics, but usually one will prefer the use of a stable catalyst. Catalyst decomposition is an import issue in applied catalysis although it has received relatively little attention in literature as far as homogeneous catalysis is concerned [5],... 

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