Chauvin-Herisson

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... 

J.L. Herisson and Y. Chauvin, Catalysis of olefin transformations by tungsten complexes. II telomerization of cyclic olefins in the presence of acyclic olefins, Makromol. Chem., 141 161-176,1971. 

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]...

These early results, along with a vast number of other data [45], establish that ring-opening metathesis polymerisation proceeds via a chain process [scheme (4) in Chapter 2] in which the structures of the active species fluctuate between metal alkylidenes (carbenes) and four-membered metallacycles (metallacyclo-butane)s, a concept that was first introduced by Herisson and Chauvin [46]. 

Introduction of ring-opening metathesis as a versatile polymerisation technique (ROMP) by Chauvin and Herisson Nobel Prize Chemistry to Paul J. Flory for his fundamental achievements, both theoretical and experimental, in the physical chemistry of the macromolecules Fully aromatic polyamides developed Aramids, being lyotropic liquid crystalline polymers of high strength, due to extended molecular chains (Morgan and Kwolek)... 

J.-L. Herisson, and Y. Chauvin, Catalysis of Olefin Transformations by Tungsten Complexes. Telomerization of Cyclic Olefins in the Presence of Acyclic Olefins, Makro-mol. Chem. 141, 161-167 (1971). 

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