Chauvin mechanism, olefin metathesis

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. 

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...

The most important difference between Chauvin s mechanism for olefin metathesis and the mechanism for alkane metathesis is that the latter applies itself to the reverse reaction of cleavage of alkanes by methane (which has no single C-C bond, see below) and, especially, it is based on a metal hydrido-carbene in equi-Ubrium with a metal-alkyl. 

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

A recent report by Mayr of slow polymerization of PhC=CH by (PMe3)2Cl2(PhC=CPh)W=CHPh fulfills expectations based on the classic Chauvin mechanism for olefin metathesis (78). The presence of a carbene and a vacant coordination site are prerequisites for metallocyclo-butene formation with free alkyne. Mayr has both the carbene and the alkyne initially present in the catalyst, but there is no evidence for direct involvement of the cis alkyne in the actual polymerization mechanism. 

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. 

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. 

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. 

Fig. 4.17. The pairwise [2 + 2] mechanism and the Chauvin mechanism for olefin metathesis.

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

By analogy to olefin metathesis, alkyne metathesis occurs between a complex containing a metal-carbon triple bond - a metal carbyne (or alkylidyne) - and an alkyne substrate [66]. As illustrated in Fig. 4.21, this mechanism parallels the Chauvin mechanism for olefin metathesis after alkyne coordination to the metal center, [2 - - 2] cycloaddition between the metal carbyne and the alkyne yields a metallacyclobutadiene, which rearranges and fragments productively to afford a new carbyne and a new alkyne (Fig. 4.21) [54]. 

Initially, the most obvious mechanism for olefin metathesis was a land of molecular square dance in which two different olefin molecules join to form a cyclobutane ring and then change partners to form two new olefin molecules. While this thermal reaction is Woodward-Hoffmann forbidden, transition metals were initially perceived to allow violations of these rules. However, no cyclobutanes were detected in olefin metathesis reactions, nor did cyclobutanes produce olefins when placed into metathesis reaction mixtures. The breakthrough came in 1971 when Yves Chauvin (1930- ), at the French Petroleum Institute, made the concepmal link between the Phillips Petroleum reaction discovered in 1964 and metallocarbenes isolated in the same year by Ernest Otto Fischer (see chapter 7). Other important discoveries were made by Michael F. Lappert (1928- ) at Sussex, Charles P. Casey (1942- ) at Wisconsin, and especially Thomas J. Katz (1936- ) at Columbia. The mechanism involves formation of a metallocyclobutane (see the accompanying figure), from reaction of a metallocarbene ( M=CR 2 ) with an olefin (R2C=CR2), that splits into a new olefin (R 2C=CR2) and a new metallocarbene ( M=CR2 ). 

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 commonly accepted mechanism for the olefin metathesis reaction was proposed by Chauvin. It involves a [2-1-2] cycloaddition reaction between transition metal al-kylidene complex and the olefin to form an intermediate metallocyclobutane. This metallocycle then breaks up in the opposite fashion to afford a new alkylidene and new olefin. If this process is repeated, evenmally, an equilibrium mixture of olefins will be obtained. 

The 2005 Nobel Prize in Chemistry was jointly awarded to Robert H. Grubbs (Caltech), Yves Chauvin (French Petroleum Institute), and Richard R. Schrock (MIT) for establishing olefin metathesis as a reaction of synthetic versatility and contributing to an understanding of the mechanism of this novel process. Olefin metathesis first surfaced in the late 1950s when industrial researchers found that alkenes underwent a novel reaction when passed over a heated bed of mixed metal oxides. Propene, for example, was converted to a mixture of ethylene and 2-butene (cis + trans). 

In Section 24.12, we introduced alkene (olefin) metathesis, i.e. metal-catalysed reactions in which C=C bonds are redistributed. The importance of alkene and alkyne metathesis was recognized by the award of the 2005 Nobel Prize in Chemistry to Yves Chauvin, Robert H. Grubbs and Richard R. Schrock for the development of the metathesis method in organic synthesis . Examples of alkene metathesis are shown in Figure 27.3. The Chauvin mechanism for metal-catalysed alkene metathesis involves a metal alkyli-dene species and a series of [2 + 2]-cycloadditions and cycloreversions (Figure 27.4). Scheme 27.6 shows the mechanism for alkyne metathesis which involves a high oxidation state metal alkylidyne complex, L M=CR. 

Several reaction mechanisms were proposed to explain the course of olefin metathesis. Most of the evidence supports a carbene mechanism involving metal complexes, originally suggested by Harrison and Chauvin [160-163]. A typical metathesis reaction of olefins can be illustrated as follows ... 

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. 

In general, olefin metathesis is an organic reaction that allows redistribution of fragments of alkenes by the scission and regeneration of C=C double bonds (Scheme 2.11). The metallic catalysts needed for this reaction have evolved rapidly over the past few decades. The impact of this topic in organic synthesis was demonstrated by the 2005 Nobel Prize in Chemistry awarded to Yves Chauvin, Robert H. Gmbbs, and Richard R. Schrock for elucidation of the reaction mechanism and discovery of highly efficient and selective metathesis catalysts. 

Olefin metathesis ( metathesis from the Greek change of position, transposition ) [1] is a key reaction in organic synthesis because it allows preparation of molecules that are crucial to promote advances in medicine, biology and materials science. The importance of this reaction was demonstrated by the award of the 2005 Nobel Prize in chemistry to Yves Chauvin, Robert H. Grubbs, and Richard R. Schrock for elucidation of the reaction mechanisms and discovery of various highly efficient and selective catalysts related to metathesis [2]. 

It should be noted that the equilibrium between a metallocyclobutane and a metal complex containing both an alkene and a carbene ligand provides a sufficient mechanism for olefin metathesis. Such a scheme has previously been proposed by Chauvin (Chauvin and Herrisson, 1971 Soufflet et al.. 

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