Self olefin metathesis

As stated above, olefin metathesis is in principle reversible, because all steps of the catalytic cycle are reversible. In preparatively useful transformations, the equilibrium is shifted to one side. This is most commonly achieved by removal of a volatile alkene, mostly ethene, from the reaction mixture. An obvious and well-established way to classify olefin metathesis reactions is depicted in Scheme 2. Depending on the structure of the olefin, metathesis may occur either inter- or intramolecularly. Intermolecular metathesis of two alkenes is called cross metathesis (CM) (if the two alkenes are identical, as in the case of the Phillips triolefin process, the term self metathesis is sometimes used). The intermolecular metathesis of an a,co-diene leads to polymeric structures and ethene this mode of metathesis is called acyclic diene metathesis (ADMET). Intramolecular metathesis of these substrates gives cycloalkenes and ethene (ring-closing metathesis, RCM) the reverse reaction is the cleavage of a cyclo-... 

Figure 1.13 Self-selection of molecular boxes via olefin metathesis.

Fig. 1 Olefin metathesis reactions (a) ring-opening metathesis (ROM) and ring-closing metathesis (RCM), (b) self-metathesis (SM), (c) cross metathesis (CM), (d) ring-opening metathesis polymerization (ROMP), and (e) acyclic diene metathesis (ADMET) polymerization...

Anions have also been used as templates in the synthesis of catenanes and rotaxanes, as recently described by Beer and coworkers [15]. They used the self-assembly of diamides, known as anion receptor molecules, around a chloride anion to produce, for example, [2]- and [3]catenanes by olefin metathesis [16] (Scheme 5.4). In this special example, the template effect was increased by hydrogen bonding between the N-methyl group to the crown ether chain and by n-n stacking interactions. 

Fig. 27 Examples of thermodynamically controlled reactions employed in the near-quantitative synthesis of MIMs. (a) Disulfide-exchange reaction permits equilibration between a bis(ammo-nium) disulfide dumbbell and a crown ether macrocycle to yield a mixture of [2]- and [3]rotaxanes quantitatively [194], (b) Olefin metathesis at high concentration on a benzylic amide macrocycle greatly favors the catenated species [196]. (c) Self-correcting imine bonds allow for nearly quantitative selection of a [2]rotaxane from an appropriate dynamic combinatorial library [76], (d) A weak nucleophile (E) equilibrates the components of a donor-acceptor [2]catenane in a dynamic Sn2 reaction [205]...

A recent report by Miller and coworkers investigated the effects of remote functionality on the efficiency and stereochemical outcome of the olefin metathesis reaction [55]. Using a series of allyl- and homoallylamides, they demonstrated that both the yield of self-metathesis products and the ratio of cis- and trans-olefin isomers formed were strongly dependent on remote functionalities. Although it does not preclude the use of olefin metathesis in DCC experiments, it is an important factor that needs to be considered when designing olefin-based DCLs. Indeed, in an ideal scenario, one would expect the course of the reaction and product distribution in a DCL to be relatively insensitive to functionality remote from the reacting centers, which is unfortunately rarely the case. 

An alternative strategy for the functionalization of polyoxometalates relies on self-assembly processes. Up to now we failed to introduced a Schrock-type alkylidyne in the lacunar undodecaphosphotungstate. As the current trend in olefin metathesis reaction now favours ruthenium catalysts, such as the Grubbs s ones, we turn to ruthenium precursors and to more stable Lappert-type carbene fragments, stabilized in the a-position by nitrogen atoms. The reaction of [PWii039p- with the carbene precursor... 

C2H4 Self-homologation Olefin metathesis Norbornadiene dimerization... 

Furthermore, these second generation catalysts have opened up new classes of substrates for use in olefin metathesis reactions, such as CM between electron deficient olefins and alkyl olefins [103] and the self-metathesis of electron deficient olefins [104]. The example in Fig. 4.37 illustrates the high yields of cross products that can be obtained by using catalyst 2 in reactions of alkyl olefins with a,/I-unsaturated carbonyl compounds. 

The polarity of chlorinated solvents can also play a role in affecting the product distribution of an olefin metathesis reaction. Clark and Ghadiri [8] observed that the macrocyclic peptide 10 self assembles by inter molecular H-bonding in nonpolar solvents. The cylindrical conformation that resulted did not allow for successful dimerization to occur between macrocycles. When the cyclization of the cyclic peptide 10 was conducted with Ru catalyst 12 in chloroform (Scheme 12.5), the chloroform was proposed to disrupt the H-bonding within molecules. The new conformation produced in solution with the CHClj proved conducive to ring closure. The cyclic dimer 11 was obtained in 65% isolated yield as a mixture of cisicis, transitrans, and cisitrans isomers. 

A chain mechanism for olefin metathesis explains product-time distributions in reactions between cyclo-octene and acyclic olefins. Even at the start of the reaction between cyclo-octene, trans-hut-2-ene, and trans-oct-4-ene in the presence of the catalyst, a significant amount of tetradeca-2,10-diene was found. " Tetradeca-1,9-diene was the principal product of metathesis reactions between cyclo-octene and hex-l-ene in the presence of tungsten catalysts. Ethylene and dec-5-ene formed by self-metathesis of the hex-l-ene also underwent cross-metathesis with the cyclo-octene to give deca-1,9-diene and octadeca-5,13-diene further reactions gave higher members of these homologous series. ... 

The only oxide that has been used for catalyzed olefin metathesis at 25°C is Re207/Al203 (in the middle of the 1960s by British Petroleum), but it suffered from a low number of active sites, side reactions caused by the acid support and deactivation of the catalyst. On die other hand, the silica-supported rhenium catalyst [(SiO)(Re(C-f-Bu)(=CH-f-Bu)(CH2-f-Bu)] catalyzes the metathesis of propene at 25°C with an initial rate of 0.25 mol/(mol Re x s). The formation of 3,3-dimethyl-butene and 4,4-dimethylpentene in a 3 1 ratio results from cross metathesis between propene and the neopentyl idene ligand, and die ratio of cross-metathesis products matches the relative stability of the metallacyclobutane intermediates. Cross metathesis of propene and isobutene and self-metathesis of methyl oleate can also... 

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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