Special Metathesis Reactions

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. 

Semenov s empirical formulation represents a special case of a Polanyi relationship in which the activation energy for an exothermic hydrocarbon metathesis reaction is given by (Semenov, 1958)... 

Berces, T., and Marta, F., Activation energies for metathesis reactions of radicals, in Chemical Kinetics of Small Organic Molecules IV Reactions in Special Systems (Alfassi, Z. B., ed.). CRC Press, Boca Raton, Florida, 1988. 

A special application of bimetallic ruthenium complexes was found in the olefin metathesis reaction vide infra) The two metal centers were closely attached to one another through /r-halide anions. The labile assembly was the key feature to the formation of highly active catalysts. 

Later on, Calderon et al. [42,43] recognised that the ring-opening polymerisation of cyclic olefins is a special case of the more general alkene metathesis reaction, e.g. as for propylene ... 

In these alkylidene complexes the M=C bond is significantly shorter than normal M-C single bonds, and the bonding is illustrated by formula (I). The partial double bond character confers special properties and such carbenes are intermediates in olefin metathesis reactions... 

A second type of metathesis reaction involves metals multiply bonded to nonmetals (equation 68). In one sense, this corresponds to a special case of dinuclear reductive elimination of neutral molecules in which multiply bonded species are involved. This reaction can occur in both directions. Multiply bonded metal-metal complexes can be cleaved by multiply bonded main group species. 

The ROMP reaction is a special example of the alkene metathesis reaction as shown in equation (2), and the mechanism of the ROMP reaction is shown in Scheme 3. The first step in the reaction involves coordination of the substrate... 

In the case of reactions such as valence isomerization, metathesis reactions of alkenes and alkynes, oligomerization or cyclooligomerization of olefins, metallacycloalkanes are of special importance. Their catalytic efficiency depends on the ease of the M—C bond cleavage, which is the result of reductive elimination of the organic substrate or of /J-hydrogen transfer. Also a- or / -C—C bond rupture has been reported. Heterocycles with an aliphatic carbon skeleton and a donor atom adjacent to the metal are suitable model compounds for the study of individual catalytic steps and structural properties. In connection with the activation of C—H bonds, cyclometa-lation has become a very general reaction and was reviewed in 1977. ... 

Studies on these carbene complexes, especially those of the Schrock type, have attracted special interest in connection with the mechanism of catalytic olefin metathesis reactions. The formation of metallacyclobutane intermediate from the oxidative cycloaddition reaction between carbene complex and olefin was found to be an important key step in the catalytic cycle (eq. (5)). 

Derivatives of oleic acid that imdergo cross-metathesis with simple olefins are listed in Table 9.4, while Table 9.5 gives some examples of cross-metathesis reactions of simple olefins with other functional olefins. Such cross-metathesis reactions may provide useful routes to speciality chemicals such as synthetic perfumes, insect pheromones (Crisp 1988), prostaglandin intermediates (Dalcanale 1985), etc. (see Mol 1982, 1991). Of special interest are ethenolysis reactions, which allow the synthesis of compoimds with terminal double bonds. Ethenolysis (and cross-metathesis with lower olefins) has been investigated extensively for... 

Equally important as recycling, problems involved with the use of certain reagents can often be completely avoided if they are attached to a resin. For instance, the ruthenium alkylidene catalysts typically employed to initiate metathesis reactions often decompose into darkly colored, metal-containing by-products, unwanted materials that sometimes require special protocols to remove and which are... 

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