Metathesis secondary

Many of the studies concerning ring-opening metathesis by well-characterized metathesis catalysts have employed substituted norbornenes or norborna-dienes. Substituted norbornenes and norbornadienes are readily available in wide variety, and they usually react irreversibly with an alkylidene. Norbornene itself is the most reactive, and the resulting polynorbornene probably is the most susceptible to secondary metathesis. Formation of polynorbornene often is used as the test reaction for ROMP activity. ROMP by well-defined species has been reviewed relatively recently [30], so only highlights and selected background material will be covered here. 

The design and development of a Z-selective CM reaction faces two major challenges first, the thermodynamic preference for the formation of -olefms renders their Z-counterparts difficult to prepare second, any inherent kinetic Z-preference demonstrated by a catalyst can erode via secondary metathesis if the -pathway is not blocked. Consequently, most efforts to effect stereoselective olefin formation have focused on the preparation of -olefins. 

The CM of olefins bearing electron-withdrawing functionalities, such as a,/ -unsaturated aldehydes, ketones, amides, and esters, allows for the direct installment of olefin functionality, which can either be retained or utilized as a synthetic handle for further elaboration. The poor nucleophilicity of electron-deficient olefins makes them challenging substrates for olefin CM. As a result, these substrates must generally be paired with more electron-rich crosspartners to proceed. In one of the initial reports in this area, Crowe and Goldberg found that acrylonitrile could participate in CM reactions with various terminal olefins using catalyst 1 (Equation (2))." Acrylonitrile was found not to be active in secondary metathesis isomerization, and no homodimer formation was observed, making it a type III olefin. In addition, as mentioned in Section 11.06.3.2, this reaction represents one of the few examples of Z-selectivity in CM. Subsequent to this report, ruthenium complexes 6 and 7a were also observed to function as competent catalysts for acrylonitrile... 

Mechanistic studies revealed that alkyne metathesis and ring-opening metathesis polymerization of cycloalkynes proceed via metal carbyne complexes,217 218 which is also supported by theoretical studies.219 The polymerization of PhC=CMe with NbCIs or TaCIs yields a polymer that degrades to oligomers as a result of secondary metathesis reaction. A stable polymer, however, may be synthesized with TaCIs and Ph4Sn as a cocatalyst.220... 

Cis- and fraws-cyclooctene, 100 and 102 respectively, and their derivatives 103-107, all undergo ROMP295 also 10862,362,109 and 11062, 111-113362, 114363,115364,116365, 118362, 119 and 120366,367. Only 101295 and 117362 fail to polymerize, perhaps due to unfavourable choice of catalyst and conditions. The trans monomer 102 gives a 43% cis polymer very rapidly in the presence of MoCl2(PPh3)2(NO)2/EtAlCl2368 and is polymerizable by 18110. With a catalyst of type 10 secondary metathesis reactions of the double bonds in the polymer of 100 cause the cis content to fall from 75% to 25% as the reaction proceeds271. 

The ROMP of [2.2]paracyclophane-l,9-diene (128) yields poly(p-phenylenevinylene) (129) as an insoluble yellow fluorescent powder. Soluble copolymers can be made by the ROMP of 128 in the presence of an excess of cyclopentene387, cycloocta-1,5-diene388 or cyclooctene389. The UV/vis absorption spectra of the copolymers with cyclooctene show separate peaks for sequences of one, two and three p-phenylene-vinylene units at 290, 345 and about 390 nm respectively, with a Bernoullian distribution. The formation of the odd members of this series must involve dissection of the two halves of the original monomer units by secondary metathesis reactions. 

Secondary metathesis reactions are sometimes encountered during metathesis copolymerization, leading to a reshuffling of the units in the chain and eventually to a random distribution for example in the copolymerization of 248 and 258 using RUCI3 as catalyst, statistical copolymers are produced no matter whether the monomers are mixed initially or added sequentially576. See also the copolymers of 128 Section Vm.B.6. 

ADMET is a step growth polymerization in which all double bonds present can react in secondary metathesis events. However, olefin metathesis can be performed in a very selective manner by correct choice of the olefinic partner, and thus, the ADMET of a,co-dienes containing two different olefins (one of which has low homodimerization tendency) can lead to a head-to-tail ADMET polymerization. In this regard, terminal double bonds have been classified as Type I olefins (fast homodimerization) and acrylates as Type II (unlikely homodimerization), and it has been shown that CM reactions between Types I and II olefins take place with high CM selectivity [142], This has been applied in the ADMET of a monomer derived from 10-undecenol containing an acrylate and a terminal double bond (undec-10-en-l-yl acrylate) [143]. Thus, the ADMET of undec-10-en-l-yl acrylate in the presence of 0.5 mol% of C5 at 40°C provided a polymer with 97% of CM selectivity. The high selectivity of this reaction was used for the synthesis of block copolymers and star-shaped polymers using mono- and multifunctional acrylates as selective chain stoppers. 

On the other hand, intermolecular coordination of the in-chain C=C bond at the active site will lead to a secondary metathesis reaction according to the following scheme [121] ... 

The occurrence of this chain transfer reaction results in a cis-trans isomerisation of double bonds in the polymer chains however the cis or trans structure of these double bonds has no essential influence on their susceptibility to a backbiting reaction. An important implication of the intermolecular secondary metathesis reaction is, instead, the tendency of the molecular weight distribution in the resulting polymer to attain the equilibrium condition Mw/Mn = 2 [122]. 

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. 

The mole fraction of individual oligomers formed by secondary metathesis reactions decreases continuously with increasing degree of polymerisation, in agreement with the Jacobson Stockmaycr theory [124], It is obvious that the... 

Which cycloolefins (monocyclic, bicyclic or policyclic) will undergo secondary metathesis reactions during ring-opening metathesis polymerisation Give reasons why. 

As shown by H NMR, the activated catalyst mixture reacts with norbornene (or a series of methyl-substituted norbornenes) to be partially converted to a new carbene species [53]. From the ratio of product carbene and residual initiator carbene concentrations, it was estimated that the rate constant for propagation is at least 3 times that for initiation. The three species present in the equilibrium situation of Eq. (39) may all possess their own intrinsic activities, resulting in a more complex polymerization behavior. In addition, a substantial amount of secondary metathesis occurs, as shown by changes in both the cis content of the polymer and head/tail ratio of the substituted carbenes when the catalyst was left in solution (120 min at 20 °C) after consumption of the monomer. 

Both of these complexes can be used in ADMET polymerizations at temperatures up to approximately 55 °C, although decomposition certainly occurs over the time scale of a typical ADMET polymerization (days). A structure-reactivity study was performed on complexes 1 and 2 that revealed a number of features of these complexes [68]. Notably, 2 will polymerize dienes containing a terminal and a 1,1-disubstituted olefin, but never produces a tetrasubstituted olefin. One of the substituents of the 1,1-disubstituted olefin must be a methyl group. In contrast, complex 1 will not react with a 1,1-disubstituted olefin. The tungsten complex is more reactive towards internal olefins than external olefins [23, 63] indicating that secondary metathesis, or trans-metathesis, probably dominates the catalytic turnovers in ADMET with complex 1. 

Scheme 5 Secondary metathesis processes in CM. (See insert for color/color representation of this figure)...

These experiments demonstrate the challenge in achieving a catalyst that is selective for the Z-olefin. First, the catalyst must have a kinetic selectivity for the Z-olefin, potentially dictated by steric bulk of the ligands. All of the abovementioned catalysts are kinetically fJ-selective to a greater or lesser extent. Hence, the design of a catalyst enviromnent that can impose a steric or electronic preference for syn- over anft-metal-lacycles needs to be achieved. Secondly, secondary metathesis needs to be prevented or else this will lead to an increase in the more thermodynamically stable olefin, which is the E-olefin in the vast majority of cases. For NHC-containing catalysts, this has a greater effect on the final E/Z ratio than the initial kinetic selectivity, but selectivity is ultimately limited by the inherent catalyst preference. 

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