Selectivity metathesis

Ongoing research efforts will lead to the arrival of even more efficient and selective metathesis catalysts with specifically tailored properties [196]. Due to the synergistic relationship between catalyst design and subsequent application in advanced synthesis [197], this progress will further expand the scope of metathesis and its popularity amongst the synthetic community. 

In contrast, cis-selective metathesis proceeds by a three-ligand sequence ... 

A triple RCM reaction cascade has been reported that allows for the construction of dihydrofurans with two adjacent dihydropyran or dihydrofuran rings from a common enantiomerically enriched acyclic precursor (Equation 55) <20010L1989>. Ring-size-selective metathesis reactions and the synthetic utility of enantiomerically pure l,5-hexadiene-3,4-diol, derived from D-mannitol, have been discussed in detail <2007ASC215>. 

The properties of silica- and/or alumina-supported molybdena catalysts for propene metathesis were studied by Handzlik and coworkers. These materials could be prepared by thermal spreading of Mo02(acac)2 with well-dispersed molybdenum in a wide range of its loading. The selective metathesis activity depends on the substrate and on the surface molybdenum concentration. For example, a higher activity is found for the molybdena-alumina system at high Mo loadings. 

In order to give perspective on the kinetic versus thermodynamic balance, the cis-trans selectivity of some commonly utilized Ru metathesis catalysts is presented. A number of catalysts with modified ligands that result in a distinct stereochemical preference are then compared with these original catalysts and their reactivity discussed. Finally, the successful implementation of ligand-driven selectivity has led to three families of Ru-based metathesis catalysts that can perform Z-selective metathesis. For each of these catalyst families, a model for the origin of Z-selectivity, the role of ligands in influencing stereochemistry and trends in their reactivity are examined. 

Cyclometallated Z-selective metathesis catalysts 2.5.1 Initial discovery... 

To date, three distinct strategies have been utilized to impose a preference for i yn-metal-lacycles in Ru-based olefin metathesis catalysts, resulting in three families of Z-selective catalysts. Promising initial reactivity has been observed with both thiophenolate- and dithiolate-based catalysts and both frameworks offer many opportunities for further tuning of activity and Z-selectivity. The cyclometallated catalysts have been further developed and have demonstrated high activity and Z-selectivity for a wide variety of substrates. However, in all cases, further improvements will be necessary to achieve Z-selective metathesis across the broad substrate scope demonstrated by previous generations of Ru-based catalysts. 

Olefin metathesis is a versatile reaction for the production of fine chemicals. Through metathesis, many different products, which are otherwise difficult to obtain, can be produced from readily available olefins in only a few reaction steps. With heterogeneous catalysts metathesis can be performed under mild reaction conditions and with high selectivity. Metathesis routes that use cheap raw material, such as esters from natural sources, and accessible heterogeneous catalysts are technologically viable. 

Unsupported WO3 is an insulator and does not catalyze the metathesis of propene at 200°C instead it catalyzes the dimerization (Tsuda 1985). Unsupported Nao2sW03 is a conductor and does catalyze the metathesis reaction (Kosaka 1986 Mori, T. 1986). On a series of unsupported non-stoichiometric samples of Na W03 and K WOj (0 maximum activity and selectivity for propene metathesis at 400°C is found at x 0.2, corresponding to the conductor/ insulator transition (Stevenson 1991). However, both metathesis and dimerization occur over conducting Ko,3Mo03 and insulating K0.33M0O3. Below 127°C metathesis is dominant over both oxides. In contrast to Na jW03 there seems to be no correlation between the selectivity (metathesis dimerization) of the reaction and the conductivity of the catalyst (Suzuki, M. 1987). 

One of the few catalysts to give reasonably selective metathesis to form ethene and hex-3-ene, reaction (6), is M0O3/AI2O3 doped with 2% alkali metal ions (Bradshaw 1967 Alkema 1968) or thallium ions (Kobylinski 1972). The suppression of the isomerization is more effective the larger the cation Cs+ > T > Rb" > K > Na" " > Li it also correlates with a diminished ability to adsorb ammonia. The polarizability of the cation thus appears to be an important factor in reducing the surface acidity which is the cause of isomerization. 

The different modes of activation for the different organoactinides are indeed very unusual. For both organoactinide-imido complexes, a selective metathesis with the alkyne 7i-bond is operative (yielding the hydroamination products), whereas for the thorium complex a competing protonolysis reaction also takes... 

In general, olefin metathesis is an organic reaction that allows redistribution of fragments of alkenes by the scission and regeneration of C=C double bonds (Scheme 2.11). The metallic catalysts needed for this reaction have evolved rapidly over the past few decades. The impact of this topic in organic synthesis was demonstrated by the 2005 Nobel Prize in Chemistry awarded to Yves Chauvin, Robert H. Gmbbs, and Richard R. Schrock for elucidation of the reaction mechanism and discovery of highly efficient and selective metathesis catalysts. 

Catalysts continue to be developed for particular alkene metathesis applications, such as stereoselective cross metathesis. These precatalysts are tasked with selective metathesis and turnover, but must maintain Z-selectivity throughout the reaction. New ruthenium(II) species featuring a Ru-C bond have been recruited for this purpose. In a short time, reactivity gains and improved initiation rates have been achieved in this new area by manipulation of the X-type ligand. 

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