Chauvin mechanism, olefin

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 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 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. 

Scheme 6.2 (a) Chauvin mechanism of olefin olefins, (b) Nomenclature of positions for me-... 

The reaction is of great interest because the strongest bond in alkene, the C=C bond, is broken during the reaction. The widely accepted so-called Chauvin mechanism [42] suggests that transition-metal carbene complex acts as catalyst by undergoing a [2 -I- 2J cycloaddition reaction with olefin, via a metallacyclobutane intermediate ... 

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]. 

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. 

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. 

The reaction pathway shown is the widely accepted Chauvin mechanism " which involves a metalacycle formed by C—C bond formation within an Ti -olefin-carbene complex. It should be noted that the metal complex is not really a catalyst, but rather a promoter, because it is transformed into a new carbene. Theory suggests"" that the carbene fragment and the olefin should be parallel, as shown in the Scheme, for the C—C bond to form. It also suggests that a great deal of the added reactivity of the C systems is derived from the NHC enforcing this orientation on the carbene through steric interactions. 

The metallacyclobutane has a symmetric topology. Thus it can, analogously to the above process, shift in the perpendicular direction to give the other olefin and the other metal-carbene this is the Chauvin mechanism of the olefin metathesis reaction (see Chap. 15.2). 

Different types of olefin metathesis, all proceeding according to the Chauvin mechanism and catalyzed by Schrock-type or Grubbs-type metathesis catalysts. Tandem, domino and cascade metathesis reactions couple several of these reactions (in particular ROMP + ROM). 

The mechanism is suggested to proceed by a composite series of a-bond metatheses of C-H bonds and a- and (3-eliminations (rather than direct a-bond metathesis of C-C bonds). The a-elimination from a d metal-methyl or metal-alkyl species generates respectively HTa=CH2 or HTa=CHR, and the mechanism is proposed to then follow an alkene metathesis pathway with the olefins being generated by (3-elimination (including metallacyclobutane intermediates as in the Chauvin mechanism). 

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. 

As a result, it could be shown that both in the solid state and in solution the [ R2P(CH2)nPR2-K2P XRu=CHR]+cation dimerizes to form dicationic, dinuclear complexes [ R2P(CH2)nPR2-/c2P XRu=CHR 2]2+. In different crossover and trapping experiments, monomeric species could be found in solution. During the next pre-equilibrium step these monomeric complexes should form an olefin jt-complex. The product formation presumably proceeds via a metallacyclobutane according to the Chauvin metathesis mechanism (Figure 3.17) [48],... 

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

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. 

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