Olefin Metathesis Case Studies

Several features of this reaction are worth further consideration. First, one should recognize that although the then-available ruthenium alkylidenes such as 12 and 13 have a level of functional group tolerance compatible with 23, their reactivity as metathesis catalysts has, in general, proven too low for them to effect the formation of macrocyclic trisubstituted olefins. Accordingly, at the 

Indeed, upon reaction of 32 with catalyst 13 at ambient temperature, the desired dimer 33 was formed hy cross-metathesis in 58 % yield. Although the material return is modest, this result is still remarkable because there are so many seemingly deleterious coordinating functional groups near the terminal olefin, including two free hydroxy groups and a ketone. 

Guided by this set of constraction principles, the first retrosynthetic simplification consisted of excising the C12—C13 epoxide 

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Despite the numerous reports concerning NHC-Ru olefin metathesis initiators, a complex incorporating a carbene that has only one exocyclic substituent adjacent to the carbenic centre was not reported until 2008. Studies by Grubbs and co-workers led to the development of ruthenium-based catalysts bearing such carbene ligands, in this case incorporating thiazole-2-ylidenes [63] (Fig. 3.19). 

Adlhart and Chen [126] reported a QM/MM study of the olefin-metathesis reaction catalyzed by [L2(Cl)2Ru=CHPh] complexes L are phosphane groups in the case of the first-generation catalyst, whereas for the second-generation catalysts an N-heterocyclic carbene ligand replaces one of the phosphanes. A study of first- and second-generation metathesis catalysts... 

In order to design superior catalyst systems and expand the applications of these first generation catalysts, it was necessary to understand the fundamental mechanism of ruthenium-catalyzed olefin metathesis reactions. Initial investigations focused on the activity of 1 and its derivatives for the catalytic RCM of diethyl diallylmalonate (Eq. 4.14) [86]. These studies revealed that, in all cases, the overall catalytic activity was inhibited by the addition of free phosphine, and that the turnover rate was inversely proportional to the concentration of added phosphine. This indicated that phosphine dissociation was required for catalytic activity, and further suggested that olefin metathesis may be initiated by the substitution of a phosphine ligand with an olefin substrate. 

In this case, ADMET was chosen based on the mild reaction conditions involved and the functional group tolerance of the ruthenium metathesis catalyst. The results of this study highlighted the use of the many forms of olefin metathesis in the fabrication of complex structures that previously were synthetically unobtainable. 

There have been several theoretical studies on the olefin metathesis reaction and the structures of homogeneous catalysts [1-5]. However, we have not found examples of theoretical modelling of olefin metathesis active sites on heterogeneous catalysts. In the case of the heterogeneous molybdena catalysts, the active centres contain probably Mo [6,7], but other Mo valences are also possible (e.g., Mo [8]). 

Another methodology to achieve latent chelated olefin metathesis catalysts was adopted by Grubbs et al. [28]. They reported that exchange of the anionic ligands from chlorides to iodides caused a decrease in activity of the catalyst. While in most cases this change is detrimental, it opened another pathway to achieve latent ROMP initiators. Thus, iodide bound complexes 41 and 42, more sterically hindered for olefin association [29], were synthesized and studied for... 

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. ... 

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