Chauvin-Herisson mechanism

For didactic reasons we will not show mechanistic proposals that have been abandoned, because the carbene mechanism now7 is well established. It is referred to as the Chauvin-Herisson mechanism, as they were the first to propose the intermediacy of metal-alkylidene species [5], Their proposal was based on the observation (simplified for our purposes) that initially in the ringopening polymerisation of cyclopentene in the presence of 2-pentene a mixture of compounds was obtained rather than the single products if a pair-wise reaction of cyclopentene (or higher homologues) and 2-pentene would occur. Figure 16.3 shows the results. 

Figure 5.23 (a) Chauvin-Herisson mechanism for alkene metathesis with well-defined (b) alkylidene and (c) metallacyclobutane initiators... 

Scheme 36. Chauvin-Herisson mechanism for alkene metathesis.

A mechanism for olefin metathesis reactions, which is now generally accepted, was first proposed in 1970 by Herisson and Chauvin [4]. It is outlined... 

A major objection to these mechanisms was raised by Herisson and Chauvin,64 who found that cross-metathesis between a cycloalkene and an unsymmetric alkene resulted in a statistical distribution of cross-products even at very low conversion, whereas a simple pairwise mechanism would lead to a single product. It is also important to point out that cyclobutanes are not isolated as intermediates and are unreactive under metathesis conditions.30... 

In 1970, Chauvin and Herisson presented a study of the co-metathesis of cycloalkene/alkene mixtures using a WOCLj/SnBuj pro-catalyst mixture [12]. Whilst the fully quantitative analysis of the product mixtures was made complicated by the range of techniques that were required for the low, medium and high molecular weight products (mono alkenes, telomers and polymers), it became clear that product ratios were not consistent with what would be predicted by either mechanism in Scheme 12.14. The analysis and associated mechanistic interpretation were seminal and worthy of consideration in some detail here. The key point is that both mechanisms in Scheme 12.14 are pairwise, i.e. each turnover of the catalyst cycle involves two alkenes that undergo concerted alkylidene exchange. When a single alkene, e.g. pent-2-ene (C5), is considered, the products of alkylidene metathesis... 

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

Metathesis, which is reversible and can be catalyzed by a variety of organometallic complexes, has been the subject of considerable investigation, and many reviews on this topic have been published.In 1970, Herisson and Chauvin proposed that these reactions are catalyzed by carbene (alkylidene) complexes that react with alkenes via the formation of metallacyclobutane intermediates, as shown in Figure 14-20. This mechanism, now known as the Chauvin mechanism, has received considerable support and is believed to be the pathway of the majority of transition metal-catalyzed olefin metathesis reactions. 

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. 

In 1971, Herisson and Chauvin12 performed an experiment described in equation 11.7. They isolated three major products, 2 (C10), 3 (C9), and 4 (Cn), upon tungsten-catalyzed CM of cyclopentene and 2-pentene. Product 2 is expected as the direct result of pairwise metathesis of the two starting materials the other two products could result from subsequent reactions of 2 with 2-pentene. At equilibrium, all products would be present in nearly statistical distribution according to the pairwise mechanism. What troubled Herisson and Chauvin, however, was the observation that, upon quenching the reaction well before equilibrium could be achieved, a statistical distribution of the three products was already present. 

Mechanism 3 shows a pathway that was strongly influenced by the results of Herisson and Chauvin and is outlined in Scheme 11.2. Two key intermediates in this pathway are an alkene-metal carbene complex (5) and a metallacyclobu-tane (6), formed through concerted cycloaddition of the M=C and C=C bonds. A highly significant feature of the mechanism, caused by the unsymmetrical structure of 6, is its explanation of randomization early in the course of reaction. The Herisson-Chauvin mechanism does not require a specific pair of alkenes to interact directly for metathesis to occur, hence the name non-pairwise mechanism. 

J. L. Herisson and Y. Chauvin, Makromol. Chern., 1971, 141, 161. This paper does not propose the discrete intermediates, 5 and 6, but it does suggest that carbene complexes could interact with alkenes separately in a non-pairwise manner such that a new alkene and a new carbene complex could form after a bond reorganization. The mechanism shown in Scheme 11.2 is the non-pairwise mechanism that was elucidated after much work by other chemists, and it is discussed later in this section and in Section 11-1-2. 

The early development of Mechanism 3 was bold for its day because Fischer carbene complexes had just been discovered a few years earlier, and alkylidenes were not yet known. The carbene complexes prepared before 1971 also did not catalyze olefin metathesis. With the discovery of Schrock carbene complexes and the demonstration that some alkylidenes could promote metathesis, the non-pairwise mechanism became more plausible (Section 11-1-2). It was, however, the elegant work of Katz and co-workers that provided early substantial support for the Herisson-Chauvin mechanism. 

First, Katz13 conducted an experiment similar to that of Herisson and Chauvin (equation 11.8), which he termed the double cross metathesis. If Mechanism 2 were operative, the product ratios [8]/[7] and [8]/[9] should be zero when concentrations were extrapolated back to the very beginning of the reaction (t0), because 8—the double cross product would have to form after the symmetrical products 7 and 9. 

Casey and Burkhardt reasoned that the metathesis mechanism influenced by Chauvin and Herisson s work was sufficient to explain their results. Complex 13 is interesting in that the pentacarbonyl substitution makes it similar to a traditional Fischer carbene complex, yet it also possesses Schrock character because both substituents at Ccarbene are hydrocarbon fragments. 

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 the case of cycloocta-1,5-diene the series of cyclic oligomers have the formula (C4Hg) (n = 4-13) with no tendency for the even-numbered members of the series to predominate, one of the observations that led Herisson and Chauvin to the proposal of the metal carbene mechanism. 

Chauvin and Herisson found in 1970, that the initial product distribution in the cross metathesis of cyclopentene and 2-pentene is not in accordance with such a simple pairwise mechanism [30,50]. Therefore, they proposed a novel non-pairwise mechanism with metal carbene complexes as intermediates (5) [50]. 

Herisson and Chauvin in 1971 [44], were the first to propose generally accepted mechanism, widely supported with key experimental evidences by Casey [47], Katz [48] and Grubbs groups [49] as shown in Scheme 9.31. 

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