Metathesis heterogeneous catalysts

Because this book is devoted to fine chemical synthesis by heterogeneous catalysis, homogeneously catalyzed metathesis reactions will not be discussed. After a short overview of heterogeneous metathesis catalysts, the synthesis of fine chemicals from normal (unfunctionalized) olefins in the presence of these heterogeneous catalysts will be discussed. This is followed by a discussion of fine chemicals synthesized from functionalized olefins. 

Other heterogeneous catalyst systems have also been developed for the metathesis of unsaturated esters and other functionalized olefins. They include M0CI5/ Si02/R4Sn, where R is alkyl (e.g. Me or Et) [14], and CH3Re03/Si02-Al203 

Many heterogeneous metathesis catalysts cannot catalyze the metathesis of functionalized olefins because of their intolerance to functional groups. Interference of the functional groups also reduces the activity of the catalysts that are active in this case. This increases the costs of the catalyst because much higher catalyst/substrate ratios must be used than are required for normal olefins. 

Re207/B20i/Si02-Al203/Bu4Sn MeUn 350 50 13 348 43 

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The treatment of equivalent amounts of two different alkenes with a metathesis catalyst generally leads to the formation of complex product mixtures [925,926]. There are, however, several ways in which cross metathesis can be rendered synthetically useful. One example of an industrial application of cross metathesis is the ethenolysis of internal alkenes. In this process cyclic or linear olefins are treated with ethylene at 50 bar/20 80 °C in the presence of a heterogeneous metathesis catalyst. The reverse reaction of ADMET/RCM occurs, and terminal alkenes are obtained. 

In the Phillips neohexene process147 2,4,4-trimethyl-2-pentene (8) is converted by cleavage with ethylene to neohexene (9) used in the production of a perfume musk. The starting material is commercial diisobutylene. Since it is a mixture of positional isomers (2,4,4-trimethyl-2-pentene and 2,4,4-trimethyl-l-pentene) and the latter (7) participates in degenerative metathesis, effective utilization of the process requires the isomerization of 7 into 8. A bifunctional catalyst system consisting of an isomerization catalyst (MgO) and a heterogeneous metathesis catalyst is employed 131... 

Heterogeneous metathesis catalysts based on monolithic supports... 

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,... 

Very active heterogeneous metathesis catalysts for metathesis of linear olefins were formed by reactions of the Schrock type carbyne complexes Cl3(dme)W=C Bu (A), ( BuO)3W=C Bu (B) and Np3W=C Bu (C) with silicagel [4], Figure 3. 

Only two of the many Mo-based heterogeneous metathesis catalyst systems have been reported to bring about the metathesis of functionalized olefins, viz. MoCl5/Si02/R4Sn (R = Me, Et) [15] and Mo03/Si02, photoreduced at room temperature in a CO atmosphere, then activated with cyclopropane. The latter system has been systematically studied. 

Wittig like reactions with ketons (19) (Scheme 6) The heterogeneous carbene complex Si02/Np2WC(H)tBu is one of the most active heterogeneous metathesis catalyst (Table 1). 

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