Non-productive metathesis

Both the intramolecular and the intermolecular secondary metathesis reactions affect the polymerisation kinetics by decreasing the rate of polymerisation, because a fraction of the active sites that should be available as propagation species are involved in these non-productive metathesis reactions. The kinetics of polymerisation in the presence of metal alkyl-activated and related catalysts shows in some cases a tendency towards retardation, again due to gradual catalyst deactivation [123]. Moreover, several other specific reactions can influence the polymerisation. Among them, the addition of carbene species to an olefinic double bond, resulting in the formation of cyclopropane derivatives [108], and metallacycle decomposition via reductive elimination of cyclopropane [109] deserve attention. 

This preference is not energetic enough to prohibit ADMET, however, and increasing the temperature of the reaction will reduce the preference for the a,a -(Jisubsti-tuted olefins. This may be one factor in the temperature dependence of the rate of ADMET of 1,9-decadiene. The Arrhenius plot for the polymerization of 1,9-decadiene with complex 6 or complex 10 is not hnear (Eig. 6.5), especially for complex 10, indicating that there are more than one temperature-dependent processes occurring, which probably include decomposition, phosphine dissociation, inherent metathesis activities of the Ij species, and ratio of productive and non-productive metathesis. 

W-based catalysts for the metathesis of terminal olefins are comparatively few in number. However, this is partly an illusion because systems such as WClg/EtAlCla/ EtOH, although not effective in the sense of yielding ethene and an internal olefin, cause rapid non-productive metathesis in which the products can only be distinguished from the reactants by isotopic labelling see Ch. 5. 

Some examples of cross-metathesis between isotopically labelled alkenes have been given in Chs. 3 and 5. They demonstrate not only the fission of the double bond but also the occurrence of non-productive metathesis. Here we summarize the information on all types of cross-metathesis reaction involving only acyclic compounds. Cross-metathesis between cyclic and acyclic compounds is discussed in Ch, 15. 

With an increase in the reaction time and conversion the thermodynamically stable E-double bonds are accumulated as a result of consumption of Z- double bonds. Evidently, stereo-content goes to equilibrium at the cost of non-productive metathesis of linear structures with secondary carbene centers. We have also observed that the stereo-content practically does not depend on a-olefm chain length (C4-C8). 

Cis-traiis isomerization has been proven by Bilhou et. al.l ) to occur with a rate comparable to that of the productive metathesis reaction this has been tal en as an indication that the cis-trans iso-merization can be regarded as a regenerative or non-productive metathesis reaction. 

Intermolecular enyne metathesis has recently been developed using ethylene gas as the alkene [20]. The plan is shown in Scheme 10. In this reaction,benzyli-dene carbene complex 52b, which is commercially available [16b], reacts with ethylene to give ruthenacyclobutane 73. This then converts into methylene ruthenium complex 57, which is the real catalyst in this reaction. It reacts with the alkyne intermolecularly to produce ruthenacyclobutene 74, which is converted into vinyl ruthenium carbene complex 75. It must react with ethylene, not with the alkyne, to produce ruthenacyclobutane 76 via [2+2] cycloaddition. Then it gives diene 72, and methylene ruthenium complex 57 would be regenerated. If the methylene ruthenium complex 57 reacts with ethylene, ruthenacyclobutane 77 would be formed. However, this process is a so-called non-productive process, and it returns to ethylene and 57. The reaction was carried out in CH2Cl2 un-... 

Products of this type were not isolated, however, when the non-immobilised metathesis substrates were allyl esters or ethers. In these cases, the combined effect of metathesis and TFA cleavage was simply to remove the allyl group. A modified protodesilylation mechanism was proposed to account for these results (Scheme 3). 

Interestingly, the complexes CpCp Hf(SiHPhSiHPhSiH2Ph)Cl and CpCp Hf(SiHPh SiH2Ph)Cl are observed to form concurrently with the small polysilanes. It seems that these complexes are not necessarily the intermediates in the dehydrogenative process as suggested in Scheme 6, but perhaps only side products formed via non-productive cr-bond metathesis reactions (equation ll)22. 

The stereochemical outcome of the metathesis reaction in ADMET with Grubbs catalysts determines the ratio of cis and trans olefins in the polymer, which affects the physical properties of the polymer. In addition, consideration of the formation of metallacyclobutane shows that a great majority of the metathesis events in an ADMET reaction are non-productive, and it is the occurrence of the less favored pathways that cause productive ADMET. 

On the basis of knowledge accumulated in olefin metathesis, a degenerate process was expected in which the Ta-Me surface species could also interact with ethane to form another molecule of ethane. This reaction was deconvoluted by the use of " C-monolabeled ethane. Upon contact of this reagent with 7 rapid evolution of non-labeled and C-dilabeled ethane was detected by GC-MS until a statistical distribution was reached (1 2 1 distribution of non-, mono- and di-labeled C-ethane, Scheme 9). This reaction occurs ca five time faster than the productive metathesis. 

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). 

Titanacyclobutane complexes have been explored as catalysts for the metathesis of alk-l-enes (non-productive reaction) (Lee, J.B. 1982), alk-2-enes and alkadienes (poor yields) (Straus 1985). The titanacyclobutane 19, formed by reaction of 17 with norbomene at 20°C, eqn. (17), initiates the ROMP of fiirther norbomene when the temperature is raised to 65°C. The polymer formed has a narrow molecular... 

W]=C/fEt (5 12.1) and [W]=C//2 ( 12.34 and 11.47) in the ratio 80 20 (Schrock 1980 Wengrovius 1980). [W]=CH2 is thus more reactive than [W]=CHEt so far as productive metathesis is concerned. The non-equivalence of the protons in [W]=C//2 and the fact that there is a single chemical shift for [W]=C/fEt show that, in this case, there is a barrier to rotation but only one distinguishable orientation of the carbene ligand with respect to the other ligands. In... 

Cross-metathesis reactions with lower olefins are discussed in the earlier sections. The reaction of hex-l-ene with hept-l-ene-l,l- /2 (Mocella 1976a) and with oct-1-ene-l,l-cf2 (McGinnis 1976) results both in exchange of methylene groups and in productive metathesis reaction (8). The non-productive reaction always far outstrips the productive reaction k/lc 3> 1), to an extent that depends on the catalyst (see Table 9.2). 

Table 9.2 Ratio of non-productive to productive metathesis in the reaction of hex-l-ene with oct-l-ene-l,l-< 2 (McGinnis 1976)...

In it, the Ru catalyst is still immobilized via the same stable, non-dissociative NHC-based anchor, as in 19, with the second-anchor position on the Hoveyda hgand providing additional stabihty. Upon a productive metathesis turnover, the styrenic Hoveyda hgand wiU disconnect off of the ruthenium via CM, allowing catalysis to occur, yet it wiU remain in relatively close proximity to the active site due to the fact that it is anchored to the silica support. This serves two purposes ... 

Another study of CH2 and CD2 exchange between 1-hexene and [l,l-D2]-l-pentene showed that this reaction was approximatively 10 times faster than productive metathesis in WCl —BuLi or WCl —. AlEtCl2 systems [35]. The structural selectivity of metathesis reactions can be summed up as follows Degenerate exchange of =CH2 groups between terminal olefin > cross metathesis between terminal and internal alkenes > metathesis of internal olefins > non-degenerate metathesis of terminal alkenes. 

From the foregoing, however, it should not be concluded that the approach of Mango and Schachtschneider is appropriate for the understanding of the metathesis reaction. The main difficulty is the supposition that the metathesis is a concerted reaction. If the reaction is not concerted, it makes no sense, of course, to correlate directly the orbitals of the reactants with those of the products. Recently, non-concertedness has been proved probable for several similar reactions, which were formerly believed to be concerted. For instance, Cassar et al. (84) demonstrated that the Rh catalyzed valence isomerization of cubane to sj/w-tricyclooctadiene proceeds stepwise. They concluded that a metallocyclic intermediate is formed via an oxidative addition mechanism ... 

Non-heteroatom-stabilised Fischer carbene complexes also react with alkenes to give mixtures of olefin metathesis products and cyclopropane derivatives which are frequently the minor reaction products [19]. Furthermore, non-heteroatom-stabilised vinylcarbene complexes, generated in situ by reaction of an alkoxy- or aminocarbene complex with an alkyne, are able to react with different types of alkenes in an intramolecular or intermolecular process to produce bicyclic compounds containing a cyclopropane ring [20]. 

An interesting way to control the stereoselectivity of metathesis-reactions is by intramolecular H-bonding between the chlorine ligands at the Ru-centre and an OH-moiety in the substrate [167]. With this concept and enantiomerically enriched allylic alcohols as substrates, the use of an achiral Ru-NHC complex can result in high diastereoselectivities like in the ROCM of 111-112 (Scheme 3.18). If non-H-bonding substrates are used, the selectivity not only decreases but proceeds in the opposite sense (product 113 and 114). 

Like styrene, acrylonitrile is a non-nucleophilic alkene which can stabilise the electron-rich molybdenum-carbon bond and therefore the cross-/self-metathe-sis selectivity was similarly dependent on the nucleophilicity of the second alkene [metallacycle 10 versus 12, see Scheme 2 (replace Ar with CN)]. A notable difference between the styrene and acrylonitrile cross-metathesis reactions is the reversal in stereochemistry observed, with the cis isomer dominating (3 1— 9 1) in the nitrile products. In general, the greater the steric bulk of the alkyl-substituted alkene, the higher the trans cis ratio in the product (Eq. 11). 

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