Kinetics Olefin metathesis

Although the molybdenum and ruthenium complexes 1-3 have gained widespread popularity as initiators of RCM, the cydopentadienyl titanium derivative 93 (Tebbe reagent) [28,29] can also be used to promote olefin metathesis processes (Scheme 13) [28]. In a stoichiometric sense, 93 can be also used to promote the conversion of carbonyls into olefins [28b, 29]. Both transformations are thought to proceed via the reactive titanocene methylidene 94, which is released from the Tebbe reagent 93 on treatment with base. Subsequent reaction of 94 with olefins produces metallacyclobutanes 95 and 97. Isolation of these adducts, and extensive kinetic and labeling studies, have aided in the eluddation of the mechanism of metathesis processes [28]. 

Asymmetric Synthesis Using a Chiral Molybdenum Catalyst In olefin metathesis, a double bond is cleaved and a double bond is formed. Thus, a chiral carbon center is not constructed in the reaction. To realize the asymmetric induction by ring-closing metathesis, there are two procedures a kinetic resolution and desym-metrization of symmetric prochiral triene. Various molybdenum complexes are synthesized in order to explore the viabihty of these approaches (Figure 6.2). 

Such cases are not uncommon, but full quantitative treatments are rare, since often relatively large amounts of Y must be added to obtain measurable effects. Complications may then arise from the effects of the added Y on the nature of the medium (see Chapters 2 and 3). These are particularly notable when Y and I are charged, as is often the case. Under those circumstances, maintenance of the constant ionic strength of the medium with a known non-participating ionic species is essential. The classic case of common ion depression in solvolysis of benzhydryl chloride is dealt with in Chapter 2. A more recent example of this kind of treatment with neutral reactants occurs in the elucidation of the mechanism of olefin metathesis [20], catalysed by the ruthenium methylidene 9, Scheme 9.6. With ca. 5% of 9, disappearance of diene 10 was clearly not first order. However, reactions run in the presence of large excesses of phosphine 11 were much slower and showed first-order kinetics. The plot of kQ K against 1/ [ 11 ] was linear, consistent with dissociation of 9 to yield an active catalytic species prior to engagement with the diene, with k t [11] 3 > fc2[diene]. Because first-order kinetics were observed under these conditions, determination of order with respect to the catalytic species (as well as the diene) was simplified, and an outline for the mechanism could be constructed (see also Chapter 12 for more detailed consideration of catalysed olefin metathesis). 

Olefin metathesis does not generate stereogenic centers, however, the reaction may be employed in the desymmetrization of prochiral (poly)olefins of the kinetic resolution of racemates. In the example depicted in Scheme 17, a trialkene is desymmetrized, and the preference for the cyclization reaction with one of the two symmetry-equivalent C = C double bonds leads to the enantioselective formation of the reaction product, a chiral dihydrofuran. The following principal conclusions can be drawn from this study ... 

Natural product synthesis and medicinal chemistry exist in a symbiotic relationship with the development of synthesis methodology. Noy-ori s asymmetric hydrogenations, Sharpless olefin oxidations, Grubbs olefin metathesis, Buchwald-Hartwig couplings and Jacobsen s hydrolytic kinetic resolution are illustrious examples with many practical applications. The key to the success of the above-mentioned reactions is that they have provided reliable shortcuts to more traditional synthetic... 

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... 

The olefin metathesis system is used, with the physical properties and reaction kinetics being taken from the literature (Okasinski and Doherty 1998). The reaction is considered only to occur in the liquid phase with a negligible heat of reaction and ideal vapor-liquid equilibrium behavior at atmospheric pressure. The specifications for column operation are taken from Hoffmaster and Hauan (2006). The goal is to convert a pure pentene feed into product streams of butene and hexene with a purity of at least 98 mole percent using a feed flow of 2 kmol/h and a distillate to feed ratio of 0.5. 

Wagener, K.B. Lehman, S.E. Comparison of the kinetics of acyclic diene metathesis promoted by Grubbs ruthenium olefin metathesis catalysts. Macromolecules 2002, 35, 48-53. 

This chapter will present some of the history of ADMET and olefin metathesis in general, although the emphasis will be on the mechanism and kinetics of ADMET polymerization. The general mechanism for olefin metathesis will be presented before any of the specific catalyst structures are introduced or discussed in order to provide the reader with a firm basis upon which to compare the various popularly used catalysts for ADMET polymerization. In addition, procedural information will be given at the end of the chapter to give the reader an idea of what is specifically involved in a typical ADMET polymerization. 

Olefin metathesis has been extensively written on in both books and journals [1-10]. This chapter will focus on ADMET. Of particular interest are the issues of catalysis, mainly functional group tolerance, kinetics, and mechanistic details. The development of late-transition metal catalysts has enormously expanded the scope of ADMET, so particular attention will be given to the well-defined ruthenium-based olefin metathesis catalysts. Pertinent information pertaining to catalysts of Group VI metals will also be provided. Important procedural aspects of ADMET will be presented in conclusion. 

Berezin, M. Y., Ignatov, V. M., Belov, P. S., Elev, I. V., Shelimov, B.N., Kazansky, V. B. (1991) Mechanism of Olefin Metathesis and Active Site Formation on Photoreduced Molybdenum Catalysts. 5. Metathesis of Unsaturated Fatty Acid Esters, Kinet. Ratal. 32, 379-389. 

The kinetics of the olefin metathesis of 2-pentene by (pyri-dine)2Mo(NO)2Cl2 and organoaluminum halides have been measured (56) as first order in the metal and variable order in olefin (seemingly first order at high olefin concentration and up to order 1.7 at low olefin concentration) and were originally interpreted to support the conventional mechanism, but they now also seem in accord with the metal-carbene chain mechanism. 

When metathesis is effected with tra i-2-pentene, rather than cis-, and (diphenylcarbene)pentacarbonyltungsten is the initiator, the 2-butene and 2-hexene products are largely trans. The stereospecificity (73-83% trans) is not as great as for cw-olefin metathesis, but it is appreciable (63). The ratios of the stereoisomers in the products are close to the equilibrium ratios, but they probably are not determined by the products equilibrating, for in the short time the metathesis was run to determine the stereochemistry of the initial product, the precursor, tranj-2-pentene, underwent only negligible isomerization. The stereochemistries therefore are determined by the kinetics, which in turn should be affected by conformational factors similar to those in Scheme... 

Ruthenium catalysts are reactive only towards olefins. As a result, it is possible to introduce functional groups into the monomer prior to polymerizations. This was demonstrated by Hilf and Kilbinger [183] They demonstrated that small ring vinyl lactones and carbonates are efficient quenchers for the olefin metathesis polymerization. The slow kinetics of the reaction can be overcome by an excess of the reagent. The rapid termination of the polymerization reaction yields highly functionalized polymers with narrow molecular weight distribution ... 

The pentacarbonylrhenium halides, first prepared by Hieber, are starting materials for the syntheses of many novel rhenium carbonyl compounds. Photochemical, vibrational, and kinetic " properties of these molecules have been studied. A rhenium carbonyl halide-alkyl aluminum halide system polymerizes acetylene and is a useful olefin-metathesis catalyst. " ... 

Studies have been conducted on the factors that accelerate the rate of tiie sequential [2+2] and retro-[2+2] reactions that constitute the olefin metathesis process. The ruthenium system of Grubbs has been amenable to kinetic analysis. The starting complex is a 16-electron species, but the 14-electron species formed by ligand dissociation reacts with olefin in the [2+2] process. Therefore, faster reactions will occur with systems in which dissociation of an ancillary ligand is favored and in which ttie remaining ancillary ligand on the 14-electron intermediate promotes the [2+2] addition, relative to reassociation of the dissociated ligand. 

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. 

The phosphine dissociation rates ( d) of 1 and 2 do not track with their olefin metathesis activities, since 2 is significantly more active than 1 for both polymerization [5] and ring closing metathesis reactions. Instead, correlates directly with the initiation rates of the two catalysts. The kinetics of initiation was investigated by monitoring the reaction of 1 or 2 with ethyl vinyl ether using NMR and/or UV-vis spectroscopy [10]. For both catalysts, this reaction showed saturation kinetics, and the values of the initiation rate constant k ) at saturation were identical (within error) to the values of / d at the same temperature. These results implicate a dissociative olefin metathesis mechanism as depicted in Scheme 2. In this mechanism, phosphine dissociation to form a 14-electron intermediate L(Cl)2Ru=CHPh (3) is followed by trapping with olefinic substrate. 

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