Chauvin proposal

Chauvin proposed that the metal-alkylidene catalyst forms an intermediate four-membered ring with an alkene, as shown in Mechanism 8-11. Then the ring breaks apart, either to give the starting alkene and catalyst or to give a new alkene that has traded one alkylidene group with the catalyst. 

More than half a century ago it was observed that Re207 and Mo or W carbonyls immobilized on alumina or silica could catalyze the metathesis of propylene into ethylene and 2-butene, an equilibrium reaction. The reaction can be driven either way and it is 100% atom efficient. The introduction of metathesis-based industrial processes was considerably faster than the elucidation of the mechanistic fundamentals [103, 104]. Indeed the first process, the Phillips triolefin process (Scheme 5.55) that was used to convert excess propylene into ethylene and 2-butene, was shut down in 1972, one year after Chauvin proposed the mechanism (Scheme 5.54) that earned him the Nobel prize [105]. Starting with a metal carbene species as active catalyst a metallocyclobutane has to be formed. The Fischer-type metal carbenes known at the time did not catalyze the metathesis reaction but further evidence supporting the Chauvin mechanism was published. Once the Schrock-type metal carbenes became known this changed. In 1980 Schrock and coworkers reported tungsten carbene complexes... 

In 1970, Chauvin proposed a mechanism in which the catalytically active species was a transition metal carbene complex. The carbene mechanism was proposed to account for the products observed when cyclopentene and pent-2-ene were metathesized in the presence of the catalysts WOCLi/Bu4Sn or WOCl4/Et2AlCl (equation 4). The products were observed in a 1 2 1 ratio, even as the initial products. A simple pairwise mechanism, however, would have predicted that compound (1) will be the only product initially formed. 

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. 

It would not be long before enlightenment was achieved as definitive evidence for the original Chauvin proposal came in late 1974... 

In 1964 Fischer prepared the first stable metal carbene complexes [11], such as W[=C(OMe)Me](CO)5. Six years later Herisson and Chauvin proposed that olefin metathesis reactions were initiated and propagated by complexes of this type in a chain reaction involving the intermediate formation of metallacyclobutane complexes, eq. (4) where [Mt] is a transition-metal atom surrounded by various ligands [12]. 

In the year 2005, Nobel Prize in Chemistry was awarded to Yves Chauvin, Robert H. Grubbs, and Riehard R. Schrock for the development of the metathesis method in organie synthesis [41-43], It is a general view that, this reaction revolutionized different fields of synthetic chemistry. Chauvin proposed the carbene mechanism to explain this reaction [44] and Schrock prepared the first well-defined highly active metathesis catalysts [45], while, Grubbs developed ruthenium based active and air stable catalysts, which are tolerant to various functional groups in the presence of olefins [46]. 

As an alternative to the cyclobutane mechanism there have also been proposed mechanisms involving carbene complexes2 which cannot be considered as a more detailed description of the quasi-cyclobutane intermediate, as in the case of the tetramethylene complex. H6risson and Chauvin 88) proposed the following scheme for the transalkylidenation step ... 

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

Scheme 3.5 General mechanism for olefin metathesis, proposed by Chauvin in 1971...

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. 

Some of these intermediates are analogous to those proposed by Chauvin in olefin metathesis ( Chauvin s mechanism ) [36]. They can be transformed into new olefins and new carbene-hydrides. The subsequent step of the catalytic cycle is then hydride reinsertion into the carbene as well as olefin hydrogenation. The final alkane liberation proceeds via a cleavage of the Ta-alkyl compounds by hydrogen, a process already observed in the hydrogenolysis [10] or possibly via a displacement by the entering alkane by o-bond metathesis [11]. Notably, the catalyst has a triple functionality (i) C-H bond activation to produce a metallo-carbene and an olefin, (ii) olefin metathesis and (iii) hydrogenolysis of the metal-alkyl. 

Operating within the framework of the Chauvin mechanism, the main consideration for the reaction mechanism is the order of events in terms of addition, loss and substitution of ligands around the ruthenium alkylidene centre. Additionally, there is a need for two pathways (see above), both being first order in diene, one with a first-order dependence on [Ru] and the other (which is inhibited by added Cy3P) with a half-order dependence on [Ru]. From the analysis of the reaction kinetics and the empirical rate equation thus derived, the sequence of elementary steps via two pathways was proposed, one non-dissociative (I) and the other dissociative (II), as shown in Scheme 12.20. The mechanism-derived rate equation is also shown in the scheme and it can thus be seen how the constants A and B relate to elementary forward rate constants and equilibria in the proposed mechanism. 

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

Several mechanisms were proposed to explain the catalytic metathesis reactions, but the mechanism published by Yves Chauvin in 1971 has come to be accepted as correct. We can think of an alkene as two alkylidene groups bonded together. Similarly, the Schrock and Grubbs catalysts are like a metal atom bonded to one alkylidene group. 

The mechanism of the alkene metathesis reaction is now very well understood and is shown in Scheme 1. The initial mechanistic proposal of a pairwise reaction (the pairwise mechanism) of two alkenes at a transition metal center in a pseudocyclobutane transition metal complex has been discarded in favor of the carbene mechanism (the Chauvin Mechanism) of Scheme 1. ... 

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. 

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. 

Tsong, T. Y. Liu, D. S. Chauvin, F. Astumian, R. D. Resonance electroconformational coupling A proposed mechanism for energy and signal transduction by membrane proteins. Biosci. Rep. 9, 13-27. 

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. 

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