Ruthenium self-cross metathesis

Metathesis has been applied in oleochemistry for many years, but only fairly recently technical realization comes within reach [33, 34]. As typical catalysts, ruthenium carbene complexes of the Grubbs type are applied because of their very high activity (turnover numbers up to 200 000). In principle, oleochemical metathesis can be divided into two different types in self-metathesis the same fatty substrate reacts with itself and in cross-metathesis a fatty substrate reacts with, for example, a petrochemical alkene. The simplest case, the self-metathesis of methyl oleate forms 9-octadecene and dimethyl 9-octadecenedioate. The resulting diester can be used along with diols for the production of special, comparatively hydrophobic, polyesters. An interesting example of cross-metathesis is the reaction of methyl oleate with an excess of ethene, so-called ethenolysis. This provides two produds, each with a terminal double bond, 1-decene and methyl 9-decenoate (Scheme 3.3). 

Cross-metathesis of two different alkenes to give an acyclic alkene is complicated by the possible formation of not only the desired cross-metathesis product, but also self-metathesis products, each as a mixture of alkene isomers. However, some alkenes are amenable to efficient cross-metathesis to give the desired substituted alkene. This is particularly the case with alkenes that are slow to homod-imerize, such as a, -unsaturated carbonyl compounds or alkenes bearing bulky substituents. Hence, cross-metathesis of methyl acrylate with an alkene proceeds efficiently (2.116). The ruthenium catalyst reacts preferentially with the more electron-rich alkene 98, which then undergoes cross-metathesis with the acrylate or self-metathesis with another molecule of the alkene 98. The latter reaction is reversible and hence a high yield of the desired substituted acrylate results over time. The use of 1,1-disubstituted alkenes as partners in cross-metathesis provides a route to trisubstituted alkenes. This chemistry is therefore a useful alternative to conventional syntheses of alkenes, such as by the Wittig reaction. 

The first report of NHC-containing osmium compounds acting as catalysts came from Esteruelas and co-workers in 2005. Thus, cationic benzyli-dene complexes 44 were prepared by reaction of the corresponding 16-electron precursors [(NHC)OsCl(p-cymene)][OTf] (NHC = IMes or IPr) with phe-nyldiazomethane, and their potential as initiators for olefin metathesis was probed in the RCM of diethyl diallylmalonate, the ROMP of cyclooctene, and a variety of self- and cross-metathesis reactions (Equation (7.10)). Although they were not as efficient as standard ruthenium-benzylidene metathesis initiators, compounds 44 displayed, nevertheless, a fairly decent activity. More importantly, in addition to being the first NHC-Os catalytic application, this study constituted a rare example of osmium catalysed C-C bond formation. 

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