Olefin self-metathesis reactions

The forward reaction (1) is an example of cross-metathesis between two different olefins and provides a route to styrene. The reverse of (1) is a self-metathesis reaction such reactions may be either productive as in (4), or non-productive (also called degenerate) as in (5). 

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-... 

In Ghosh s enantioselective total synthesis of the cytotoxic marine macrolide (+)-amphidinolide T1 (318) [143], the C1-C10 fragment 317 was constructed by CM of subunits 315 and 316 (Scheme 62). The reaction mediated by catalyst C (5 mol%) afforded in the first cycle an inconsequential 1 1 mixture of (E/Z)-isomeric CM products 317 in 60% yield, along with the homodimers of 315 and 316. The self-coupling products were separated by chromatography and exposed to a second metathesis reaction to provide olefins 317 in additional 36% yield [144]. 

Cross-metathesis, however, is usually a nonselective reaction. Transformation of two terminal alkenes in the presence of a metathesis catalyst, for instance, can give six possible products (three pairs of cis/trans isomers) since self-metathesis of each alkene and cross-metathesis occur in parallel. It has been observed, however, that terminal olefins when cross-metathesized with styrene yield trans-P-alkylstyrenes with high selectivity.5 A useful synthetic application of cross-metathesis is the cleavage of internal alkenes with ethylene called ethenolysis to yield terminal olefins ... 

A number of new processes exploiting metathesis have been developed by Phillips. A novel way to manufacture lubricating oils has been demonstrated.145 The basic reaction is self-metathesis of 1-octene or 1-decene to produce Ci4-C28 internal alkenes. The branched hydrocarbons formed after dimerization and hydrogenation may be utilized as lubricating oils. Metathetical cleavage of isobutylene with propylene or 2-butenes to isoamylenes has a potential in isoprene manufacture.136,146 High isoamylene yields can be achieved by further metathesis of C6+ byproducts with ethylene and propylene. Dehydrogenation to isoprene is already practiced in the transformation of isoamylenes of FCC C5 olefin cuts. 

Where there is no spacer group between the C=C bond and the functional group, productive self-metathesis does not occur, but cross-metathesis reactions with other olefins are still possible. Recent impressive examples of this are the cross-metathesis reactions of acrylonitrile (equation 19). The reaction occurs with a wide variety of R groups. For 15 different compounds the yield of the new nitrile after 3 h at room temperature is 40-90%, with the cis isomer always strongly preferred (75-90%). Only minor amounts of RCH2CH=CHCH2R are formed, and no NCCH=CHCN182. The fact that acrylonitrile... 

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...

Brandli and Ward prepared a library of internal olefins through self-metathesis and CM of a variety of derivatives of oleic acid, namely, methyl oleate, oleic acid, oleyl amine (which did not react), oleyl alcohol, and other olefins [42], These reactions were performed using 1 mol% of C3 and led to statistical product mixtures that were analyzed by GC-MS and NMR. 

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. 

Metathesis is a versatile reaction applicable to almost any olefinic substrate internal, terminal or cyclic alkenes, as well as dienes or polyenes. (Alkyne metathesis is a growing area, but will not be dealt with here.) The reaction is also known as olefin disproportionation or olefin transmutation, and involves the exchange of fragments between two double bonds. Cross metathesis (CM, Figure 1) is defined as the reaction of two discrete alkene molecules to form two new alkenes. Where the two starting alkene molecules are the same it is called self-metathesis. Ethenolysis is a specific type of cross metathesis where ethylene... 

Many other (cross-) metathesis reactions of functionalized olefins have been shown to be possible in the presence of rhenium-based catalysts, such as self-metathesis (or cross-metathesis with normal olefins) of allyl- and vinylsilanes, unsaturated nitriles, chlorides, bromides etc. The products of these reactions are not yet of use in fine chemistry, but this might be remedied by future developments in this area. 

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... 

The ease with which the geometry of the metal-carbene complexes can adjust to accommodate the incoming olefin may be an important factor in determining the rate and stereoselectivity in a given metathesis reaction (Lee, J.B. 1981). Extended Hiickel MO calculations on Ti(=CH2)L2, where L = H, Cl, Cp, have shown that the completely planar molecule is easily distorted into a flattish pyramid with Ti at the apex, ready to receive the incoming donor olefin (Gregory 1985). Similarly, calculations employing the self-consistent-field-Xa-scattered-wave method on Mo(=CH2)(=NH)(OMe)2 show that the reaction with ethene at the COO faee to... 

The RCM of diallylphenyl phosphane, eqn. (13), proceeds smoothly at 80°C in chlorobenzene (95% conversion in 5 h) when catalyzed by complex 1 of Ch. 3. The reaction occurs more readily than the self-metathesis of allyldiphenyl phosphane (10% conversion in 5 h). Steric hindrance evidently prevents the phosphorus atom from competing seriously with the olefinic bond for the vacant coordination position at the metal site (Leconte 1995a,b). 

The simplest example of a productive cross-metathesis reaction between acyclic olefins is that between ethene and but-2-ene reaction (1). In this case only one product is possible, apart from cis/trans isomerization of the but-2-ene the equilibrium mixture thus consists of four compounds. At the other extreme, the reaction of two unsymmetrical olefins, R CH=CHR and R CH=CHR , with R, R, R, R all different, can produce cis/trans isomers of four different unsymmetrical olefins by cross-metathesis as well as four symmetrical olefins by self-metathesis. Counting the cis/trans isomers of the reactants as well, this means that the equilibrium mixture will contain 20 different compounds. Side reactions, such as double-bond shift reactions, will complicate the situation still further. The main value of cross-metathesis reactions, apart from their use in the proof of mechanism, lies in their application to the synthesis of olefins that are otherwise expensive or difficult to prepare. A number of higher olefins, useful as insect sex attractants, have been made in this way. 

The cross-metathesis of ethene with higher olefins has been termed ethenolysis (Bradshaw 1967), and provides a useful means of reducing the extent of substitution of an olefin feedstock and of converting cyclic olefins into linear dienes, trienes, etc. Some examples for acyclic olefins are shown in Table 9.1. The reactions are best done at 50 bar in order to drive the reaction to the right and to minimize self-metathesis of the substrate. 

Reactions with the lower olefins are described in the preceding sections. The crossmetathesis of but-l-ene with cis- or trans-hut-2-ene on Re207/Al203 at 30-40°C is accompanied by cis/trans isomerization of the but-2-ene, but not by self-metathesis... 

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. 

In general, polar solvents also seem to have a beneficial effect on catalyzing challenging metathesis reactions. He and coworkers [13] have studied the self-metathesis of 1-hexene using the pyridine-containing, Ru-based catalyst 17 under neat conditions or in solvent (Scheme 12.7). The reaction was very efficient under neat conditions, where the conversion of 1-hexene to the desired olefin 19 was 85.7%. When using solvents, it was observed that the... 

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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