Olefin metathesis effect

Table 8-5 indicates the wide variety of catalysts that can effect this type of disproportionation reaction, and Figure 8-7 is a flow diagram for the Phillips Co. triolefm process for the metathesis of propylene to produce 2-butene and ethylene. Anderson and Brown have discussed in depth this type of reaction and its general utilization. The utility with respect to propylene is to convert excess propylene to olefins of greater economic value. More discussion regarding olefin metathesis is noted in Chapter 9. 

Both the Tebbe and Petasis reagents, Cp2CH2ClAlMe2 and Cp2TiMe2, effect the direct conversion of alkenic esters to dihydropyrans. This olefin metathesis has been successfully applied to the synthesis of complex polyether frameworks <96JA1565,96JA10335>. 

As noted above, while stndying the effect of the side chains of the NHC ligands on rntheninm olefin metathesis, it was fonnd that catalysts bearing smaller-sized aryl gronps were more snccessfnl for RCM reactions to form tetrasnbstitnted double bonds [43,45]. However, several attempts dnring the evolution failed, becanse these complexes were either difficnlt to synthesise or nnstable (Fig. 3.13). 

Catalytic olefin metathesis, in only a few years, has risen to be one of the most important and reliable processes in organic synthesis. Recently, several reports by Schrock and Hoveydallsbbond forming transformations efficiently and enan-tioselectively. A recent concise and enantioselective synthesis of exo-brevicomin by Burke utilizes chiral catalyst 91 (Scheme 13) to effect the desymmetriza-tion of 90 through a ring-closing metathesis.11531... 

Poly (acetylenes) [16], There are several catalysts available for polymerization of substituted acetylenes. Whereas Ziegler-Natta catalysts are quite effective for polymerization of acetylene itself and simple alkylacetylenes, they are not active towards other substituted acetylenes, e.g. phenylacetylenes. Olefin-metathesis catalysts (Masuda, 1985 Masuda and Higashimura, 1984, 1986) and Rh(i) catalysts (Furlani et al., 1986 Tabata, 1987) are often employed. In our experience, however, many persistent radicals and typical nitrogen-containing functional groups serve as good poisons for these catalysts. Therefore, radical centres have to be introduced after construction of the polymer skeletons. Fortunately, the polymers obtained with these catalysts are often soluble in one or other organic solvent. For example, methyl p-ethynylbenzoate can be polymerized to a brick-coloured amorph- See the Appendix on p. 245 of suffixes to structural formula numbers. 

Generally, in conclusion, it is worth noting that the molecular and immobilized complexes show very similar catalytic activity in terms of the initial TOP in olefin metathesis. However, the supported catalyst has a longer lifetime under catalytic conditions, which indicates that the effect of active-site isolation prevents some deactivation pathways such as dimerization of reachve intermediates [30]. 

Hoveyda synthesized recyclable ruthenium catalyst 104b for enantioselective olefin metathesis. This catalyst is very effective for AROM-CM and can be recovered after chromatography [Eqs. (6.74)-(6.76)]. The recovered catalyst can be reused without significant loss of enantioselectivity and with similar reactivity ... 

Vanadium precursors tend to promote addition polymerization of olefins on activation with aluminum co-catalysts. Nomura and coworkers recently reported the first example of catalytic olefin metathesis by a vanadium alkylidene complex.The well-defined, thermally stable V(v) alkylidene complex 7 effected ROMP of NBE at 80 °C. Cettain alkylvanadium complexes, including V(CH2Ph)2(=NAr )(OAr ) (Ar = 2,6-Pr 2C6H3 Ar = 2,6-Me2C6H3) and... 

When the ruthenium Schiff base olefin metathesis catalyst was used to polymerize cyclooctadiene Mw s > 100,000 Da were obtained. Although only 12 Schiff bases were identified as effective in these polymerizations, additional analogs are anticipated to be reported from the University of Ghent group. 

Some catalysts, most notably W(CO)6 in a CC14 solution44,45 and systems based on molybdena 46 when irradiated, are effective in photoinduced metathesis. In fact, a photoreduced silica-supported molybdena proved to be the most active olefin metathesis catalyst.47... 

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