Kinetics cross-metathesis

While alkane metathesis is noteworthy, it affords lower homologues and especially methane, which cannot be used easily as a building block for basic chemicals. The reverse reaction, however, which would incorporate methane, would be much more valuable. Nonetheless, the free energy of this reaction is positive, and it is 8.2 kj/mol at 150 °C, which corresponds to an equihbrium conversion of 13%. On the other hand, thermodynamic calculation predicts that the conversion can be increased to 98% for a methane/propane ratio of 1250. The temperature and the contact time are also important parameters (kinetic), and optimal experimental conditions for a reaction carried in a continuous flow tubiflar reactor are as follows 300 mg of [(= SiO)2Ta - H], 1250/1 methane/propane mixture. Flow =1.5 mL/min, P = 50 bars and T = 250 °C [105]. After 1000 min, the steady state is reached, and 1.88 moles of ethane are produced per mole of propane consmned, which corresponds to a selectivity of 96% selectivity in the cross-metathesis reaction (Fig. 4). The overall reaction provides a route to the direct transformation of methane into more valuable hydrocarbon materials. 

There thenfollowed reports by Katz [13] and Grubbs [14] and their co-workers on studies that aimed to simplify and confirm the analysis. The key remaining issue was whether a modified pairwise mechanism, in which another alkene can coordinate to the metal and equilibrate with the product prior to product displacement, would also explain the appearance of the anomalous cross-over products early in the reaction evolution. However, a statistical kinetic analysis showed that for a 1 1 mixture of equally reactive alkenes, the kinetic ratio of cross-metathesis should be 1 1.6 1 for the pairwise mechanism and 1 2 1 for the Chauvin mechanism. Any equilibration (substrate or product) would, of course, cause an approach towards a statistical distribution (1 2 1) and thus allow no distinction between the mechanisms. 

Many reviews deal with the main mechanistic aspects of the metathesis reaction [10]. There are three basic metathesis reactions (apart from polymerization reactions) the ring-closing metathesis (RCM), the cross-metathesis (CM) and the ringopening metathesis (ROM) [11]. Among the fundamental aspects that govern the reaction course, the thermodynamic versus kinetic issue is particularly important when considering the application of RCM to the construction of macrocydes. 

The molybdenum complex Mo(NAr)(CHCMe2Ph)[(3)-Me2SiBiphen] 43 was used for catalytic asymmetric olefin metathesis reactions such as desymmetrization of trienes, kinetic resolution of allylic ethers, tandem catalytic asymmetric ring-opening metathesis/cross-metathesis. Interestingly, tandem catalytic asymmetric ring-opening... 

Taylor reported kinetically controlled cross metathesis of homoallylic alcohols and allyl trimethyl silane with (4a) gave products with high E-olefin selectivity and good yields via a five-membered chelate intermediate (equation 20). ... 

On Mo03/) -Al203 the cis/trans isomeratization of but-2-ene follows second-order kinetics, but is accompanied by isomerization to but-l-ene and secondary cross-metathesis reactions (Davie 1972a Engelhard 1982e). 

Totally intramolecular enyne metathesis Diels-Alder sequences have been demonstrated (e.g., the conversion of 244 into 246). Enyne RCM of substrate 247 affords stereochemically pure 248 due to kinetic resolution during metathesis.A process equivalent to enyne metathesis that proceeds through a non-carbene mechanism has been demonstrated using platinum chloride as a catalyst. In addition to routine observations, some other interesting side-reactions have been noted (Scheme 29). Competitive cross-metathesis with ethylene and intramolecular enyne metathesis were observed in the treatment of substrate 249 with catalyst 16 and ethylene.Cyclopropane ring opening was observed in the attempted enyne metathesis of substrate 253, resulting in alkylidenecyclopentene 254. 

The fifth class of olefin metathesis in Scheme 21.1 is cross-metathesis. Selective cross metathesis is a developing type of metathesis reaction. Expulsion of ethylene usually provides the driving force for cross metathesis. In principle a statistical mixture of olefins would be formed by cross metathesis, as shown in the first reaction of Scheme 21.1. However, kinetic preferences for the reaction of a hindered carbene complex with an unhindered olefin or a kinetic preference for the reaction of an electron-rich carbene with an electron-poor olefin can provide the selectivity needed for the formation of one olefin over other potential homo- and cross-metathesis products. 

In a subsequent report, the CAAC-based metathesis catalysts were examined for selectivity in the formation of Z E olefins, as well as their activity for ethenolysis [12]. Both of these processes require kinetic selectivity to produce the thermodynamically less-favored Z and terminal olefins, respectively. It was discovered that the CAAC catalysts displayed improved conversion to the Z olefin EIZ= 1.5—2.5 after 70% conversion) for the cross metathesis of cis-l,4-diacetoxy-2-butene with allylbenzene, relative to that observed using the classical NHC- and phosphine-based systems ElZ = 3-4) at comparable conversion (Scheme 4.3). 

The overall mechanistic picture that these experiments paint is summarized in Scheme 10.25. For clarity, the processes involved in catalyst activation (i.e., reactions of neophylidene 11) have been omitted. Ethylene opens two pathways that result in isomerization of the metal center direct epimerization of the metal-methylidene species via an unsubstituted metallacycle and interception of the substrate-bound intermediate via cross metathesis to generate a metal methylidene. At low conversions, when the concentration of ethylene in the system is low, these degenerate processes are not kinetically significant, and the initial enantioselectivity is low. As RCM proceeds and ethylene is generated, the rate of epimerization is increased, which, in turn, increases the enantiomeric excess of the cyclized product. These processes also provide an explanation for why the ultimate stereochemical outcome is not dependent on the diastereomer of the catalyst used. 

Using the same concept, Hoveyda and co-workers recently employed molybdenum derived chiral complexes to develop a net catalytic enantioselective, cross metathesis process based on either the kinetic resolution of racemic allylic alcohols or the asym-... 

Metathesis can usefully be divided into a number of types, depending on the nature of the substrates and products in the catalytic reaction. A reaction such as Eq. 12.1 is a sinq>te metathesis. With two substrates we have the reverse version, a cross metathesis (Eq. 12.3). With some choices of R and R the cross product can be favored kinetically. This happens in Eq. 12.4 wh e one alkene, r-BuCH=CH2, present in excess, is too bulky to metathesize with itself, and the cross product is formed in 93% yield. ... 

The kinetic selectivity of the CAAC-based catalysts was investigated by probing the EjZdiastereoselectivity in the cross-metathesis of cA-l,4-diaceto-2-butene with allylbenzene (Equation (5.4)). Compared to the commercially available Grubbs catalysts, 66-68 afforded lower EjZ ratios (3 1 at 70%... 

Extension of the previous systems to general cross metathesis had another hurdle to overcome isomerization of the kinetically formed Z-alkene to the E-alkene that is lower in energy. This is due to the fact that there is no release of strain in the substrate to manipulate as in ROCM and the reactions are under thermodynamic control. 

In the last synthesis from this series,the (5)-conhgured a-hydroxyphosphonate building block 70 was required (Scheme 47.17). In this case, the desired carbonate could be obtained directly from the enzymatic kinetic resolution as it was the unreactive enantiomer. The synthesis involved similar key steps as before, namely, a cross metathesis, followed by a stereospecific intramolecular palladium-catalyzed allylic substitution that furnished the furan ring in 78. In contradistinction to the previous syntheses, the phosphonate group was not removed by ozonolysis, but instead it was employed in the Wittig reaction, to produce fragments 80 and 81 for the synthesis of amphidinolides F and C. 

The ring-opening metathesis polymerization of dicyclopentadiene was monitored by ultrasonic spectroscopy.16 The thermoset poly(dicyclopentadiene) is formed by ringopening and cross-linking in a reaction injection molding system. A reaction cell with a plastic window was constructed for use with pulse echo ultrasonic spectroscopy. Realtime measurements of density, longitudinal velocity, acoustic modulus and attenuation were monitored. Reaction kinetics were successfully determined and monitored using this technique. 

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