Olefin metal-carbene mechanism

The cross-metathesis of cyclopentene with imsymmetrical olefins is catalyzed by WOCl4/Bu4Sn or WOCl4/Et2AlCl. Herisson (1971) observed that the products of reaction with pent-2-ene consisted of three series of compounds Q M Q , Q M Q, and Q M Q (n = 1-4), where Q = ethylidene, = propylidene, and M represents n ring-opened units of cyclopentene M. These series were formed in the statistical ratio 1 2 1 even in the initial products. Similar results were obtained with cyclooctene, cycloocta-1,5-diene, and cyclododeca-l,5,9-triene in place of cyclopentene. It was these observations that led to the proposal of the metal carbene mechanism, since direct exchange between the double bonds of the reactant molecules would yield only the unsymmetrical series. The formation of the three series of compounds is accounted for in terms of reactions (l)-(6). 

Referring to the ADMET mechanism discussed previously in this chapter, it is evident that both intramolecular complexation as well as intermolecular re-bond formation can occur with respect to the metal carbene present on the monomer unit. If intramolecular complexation is favored, then a chelated complex, 12, can be formed that serves as a thermodynamic well in this reaction process. If this complex is sufficiently stable, then no further reaction occurs, and ADMET polymer condensation chemistry is obviated. If in fact the chelate complex is present in equilibrium with re complexation leading to a polycondensation route, then the net result is a reduction in the rate of polymerization as will be discussed later in this chapter. Finally, if 12 is not kinetically favored because of the distant nature of the metathesizing olefin bond, then its effect is minimal, and condensation polymerization proceeds efficiently. Keeping this in perspective, it becomes evident that a wide variety of functionalized polyolefins can be synthesized by using controlled monomer design, some of which are illustrated in Fig. 2. 

When alkenes are allowed to react with certain catalysts (mostly tungsten and molybdenum complexes), they are converted to other alkenes in a reaction in which the substituents on the alkenes formally interchange. This interconversion is called metathesis 126>. For some time its mechanism was believed to involve a cyclobutane intermediate (Eq. (16)). Although this has since been proven wrong and found that the catalytic metathesis rather proceeds via metal carbene complexes and metallo-cyclobutanes as discrete intermediates, reactions of olefins forming cyclobutanes,... 

The mechanism of the catalytic metathesis reaction proceeds via reaction of the olefin substrate with a metal carbene intermediate, which may be generated in situ... 

Over the past 15 years the understanding of the mechanism of these reactions has been greatly enhanced through the preparation of metal carbene complexes, particularly of Mo, W and Ru, that are both electronically unsaturated (<18e) and coordinatively unsaturated (usually <6 ligands), and which can act directly as initiators of olefin metathesis reactions. The intermediate metallacyclobutane complexes can also occasionally be observed. Furthermore, certain metallacyclobutane complexes can be used as initiators. 

Olefin metathesis has become one of the most important large-scale technical processes for the manufacture of olefins in the petrochemical industry [123]. When cyclic olefins are used as substrates, high-molecular polymers, which are formed by the so-called ring-opening metathesis (ROM), have found applications as elastomers and plastics. Gas-phase studies on the mechanism of olefin metathesis had been confined to simple metal carbenes, for example [Mn=CH2]+, [Fe=CH2]+,and [Co=CH2]+ [124-127]. Most of the metatheses have been observed with deuterated ethylene. 

His proposal involved a metal carbene and a metallocyclobutane intermediate and was the first proposed mechanism consistent with all experimental observations to date. Later, Grubbs and coworkers performed spectroscopic studies on reaction intermediates and confirmed the presence of the proposed metal carbene. These results, along with the isolation of various metal alkyli-dene complexes from reaction mixtures eventually led to the development of well-defined metal carbene-containing catalysts of tungsten and molybdenum [23-25] (Fig. 2). After decades of research on olefin metathesis polymerization, polymer chemists started to use these well-defined catalysts to create novel polymer structures, while the application of metathesis in small molecule chemistry was just beginning. These advances in the understanding of metathesis continued, but low catalyst stability greatly hindered extensive use of the reaction. 

The reaction mechanism shown in Eq. (6) is now generally accepted. That is, the active species is thought to be a complex having a metal—carbon double bond (C=M), which is called a metal carbene. Recently it has been disclosed that metal carbenes mediate various reactions70). When a cycloolefin is employed instead of an olefin, then a polymer is obtained (Eq. (7)), and a similar reaction mechanism is valid (Eq. (8)). 

The mechanism for olefin metathesis is complex, and involves metal-carbene intermediates— intermediates that contain a metal-carbon double bond. The mechanism is drawn for the reaction of a terminal alkene (RCH=CH2) with Grubbs catalyst, abbreviated as Ru=CHPh, to form RCH = CHR and CH2 = CH2. To begin metathesis, Grubbs catalyst reacts with the alkene substrate to form two new metal-carbenes A and B by a two-step process addition of Ru=CHPh to the alkene to yield two different metallocyclobutanes (Step [1]), followed by elimination to form A and B (Steps [2a] and [2b]). The alkene by-products formed in this process (RCH=CHPh and PhCH=CH2) are present in only a small amount since Grubbs reagent is used catalytically. 

The understanding of the reaction mechanism is directly related to the role of the catalyst, i.e., the transition metal. It is universally accepted that olefin metathesis proceeds via the so-called metal carbene chain mechanism, first proposed by Herisson and Chauvin in 1971 [25]. The propagation reaction involves a transition metal carbene as the active species with a vacant coordination site at the transition metal. The olefin coordinates at this vacant site and subsequently a metalla-cyclobutane intermediate is formed. The metallacycle is unstable and cleaves in the opposite fashion to afford a new metal carbene complex and a new olefin. If this process is repeated often enough, eventually an equilibrium mixture of alkenes will be obtained. 

It was also found that the recycled aqueous catalyst solutions were actually more active than the original solutions. This increase in activity was attributed to the in situ formation of ruthenium(II) olefin complexes that were shown, after isolation, to be highly active ROMP catalysts. The mechanism involved in converting these olefin complexes to either metallacyclobutane or metal carbene species remains unknown. 

In support of the mechanisms of reaction (o), a elimination is well documented for early transition metal complexes. There exist examples of olefin additions to metal carbenes that yield a metallacycle as a product [reaction (q)] or required intermediate [reaction (r)]. ... 

Olefin additions to bridging alkylidenes yield dimetallacyclopentanes . These reactions also provide a mechanism for olefin metathesis, a topic not discussed here. Although addition of an olefin to a metal carbone, a 2n + In addition, would be symmetry forbidden in organic chemistry, ab initio calculations " of the conversion of a metal carbene-alkene to a metallocyclobutane show it to be a barrierless reaction. Metal d orbitals relax the symmetry restrictions for the In + 2n addition. The mechanism of reaction (p) has not been widely considered for the olefin polymerization, but it may be relevant to olefin dimerization and oligomerization—reaction (s), for example ... 

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