Mechanism of olefin metathesis

The metallacyclobutane mechanism of olefin metathesis has been discussed in Sections 1.3 and 3.1.7. For metathesis of acetylenes carbyne complexes are generally required (Figure 3.44), and both heterogeneous and homogeneous catalytic systems have been developed for this purpose. 

Scheme 11.5 Four-step mechanism of olefin metathesis coordination, [2t-2] cycloaddition, [2+2] cycloreversion and de-coordination [4].

Figure 3.38. Mechanism of olefin metathesis and strategies for the cleavage of alkenes from polymeric supports by olefin metathesis.

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). 

Fig. 2 Mechanism of olefin metathesis proposed by Herisson and Chauvin in 1971 [7]...

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. 

Scheme 2 Mechanism of olefin metathesis and proposed cycle for RCM...

Vyboishchikov, S. F., Buhl, M., Thiel, W. Mechanism of olefin metathesis with catalysis by ruthenium carbene complexes density functional studies on model systems. Chem.— Eur. J. 2002, 8, 3962-3975. 

Schulz and Achtsnit (77) state that this transfer process cannot be a secondary cracking, as this would lead to a different distribution pattern. Moreover, the absence of cracking under these conditions had been established separately (20). They, therefore, conclude that a metathesis reaction must take place. This is a very interesting suggestion, as the mechanism of olefin metathesis is well established. There seems to be general agreement in the literature on metathesis (65, 69) that the mechanism involves metal carbenes and a metallocyclobutane intermediate, for instance,... 

Reactivity characteristic of alkylidene complexes of tantalum is that the a-carbon is susceptible to electrophilic attack, in contrast to the electron-deficient a-carbon of Fischer-type carbene complexes of group 6 transition metals [62]. Based on this unique property of the alkylidene metal-carbon double bond, a range of new types of reactions has been developed. The discovery of the alkylidene complexes of tantalum was a key to understanding the mechanism of olefin metathesis, and they continue to play important roles in C—H bond activation, alkyne polymerization, and ring-opening metathesis polymerization. 

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. 

Of particular interest is the fact that many olefin metathesis catalyst systems are of the Ziegler—Natta type. This raises the question of the relationship between the mechanism of olefin metathesis and that of Ziegler-Natta polymerization this aspect is discussed in Ch. 4. 

The metal carbene/metallacyclobutane mechanism of olefin metathesis, as outlined in Section 1.3, was first proposed by Herisson and Chauvin in 1971. By 1975 the evidence in its favour had become so compelling that the earlier pairwise mechanism had been totally discarded. From 1980 onwards well-defined carbene complexes of Ta, Mo, W, Re, and Ru were discovered which would act as initiators without the need for activation by heat, light, or cocatalyst. This in turn led to the spectroscopic detection of the propagating metal-carbene complexes in many systems, to the detection of the intermediate metallacyclobutane complexes in a few cases, and in one case to the detection of the metal-carbene-olefin complex that precedes the formation of the metallacyclobutane complex. In no individual case have all three intermediates been detected at most two have been observed, sometimes one, more often none. After 1980 metallacyclobutane complexes of Ti and Ta were found which would act as initiators at 60°C, but where the intermediate metal carbene complexes could not be detected. 

In this chapter we present the historical development of the metal carbene/ metallacyclobutane mechanism of olefin metathesis. 

Fig. 4.18. Labeling experiment to confirm the Chauvin mechanism of olefin metathesis.

While the basic Chauvin mechanism of olefin metathesis has been appreciated and acknowledged for some time, the studies on both Schrock and Grubbs type catalysts show that the knowledge of the mechanistic steps necessary to access Ghauvin-metathesis intermediates, and details concerning the structures and geometries of these intermediates, can lead to ever-improving and selective catalysts for this important polymerization reaction. 

The mechanism of olefin metathesis was originally worked out in the early 1970s by H risson and Chauvin. The mechanism involves the nonpairwise cleavage of C=C bonds that occurs in a [2-I-2] cycloaddition reaction between a carbene and an alkene to form an intermediate metallacyclobutane, as shown in Figure 19.23. The metallo-cyclobutane can open in either direction, such that an equilibrium mixture of alkenes results with the product distribution dictated by the thermodynamic stabilities of the different alkenes. Two of the more important organometallic catalysts for olefin metathesis are shown in Figure 19.24. 

Although it is now fully accepted that olefin metathesis ocurrs by this sequence of steps, the mechanism of olefin metathesis was not clear when it was first discovered. A series of papers were published with clever experiments to distinguish between "pairwise" and "non-pairwise" mechanisms. Pathway A of Scheme 21.3 is a "non-pairwise" mechanism because the olefin reacts with a metal complex containing only half of the second olefin reactant. In contrast, paths B and C contain all of both olefins within the coordination sphere of the metal. Several experiments to distinguish between these paths were reported by Grubbs, Casey, and Katz. ... 

With the general mechanism of olefin metathesis established by experimental work, early theoretical studies focused on the details of several of the steps outlined above. Ligand exchange to form the initial olefin complex could occur by either an associative or dissociative mechanism. Experimental evidence from Grubbs and coworkers [2] pointed to a dissociative process. The structure of the active olefin complex was also a matter of uncertainty, as both bottom-bound (trans to L) and side-bound (cis to L) complexes have been reported (Chapter 8). Finally, the detailed structure and reactivity of the metallacyclobutane have been the focus of several theoretical investigations, as this intermediate was not initially experimentally observed (Chapter 8). 

Mechanisms of Olefin Metathesis Catalyst Decomposition and Methods of Catalyst Reactivation... 

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