Chauvin mechanisms

Scheme 1 Chauvin mechanism for ring-closing metathesis [4]...

There thenfollowed reports by Katz [13] and Grubbs [14] and their co-workers on studies that aimed to simplify and confirm the analysis. The key remaining issue was whether a modified pairwise mechanism, in which another alkene can coordinate to the metal and equilibrate with the product prior to product displacement, would also explain the appearance of the anomalous cross-over products early in the reaction evolution. However, a statistical kinetic analysis showed that for a 1 1 mixture of equally reactive alkenes, the kinetic ratio of cross-metathesis should be 1 1.6 1 for the pairwise mechanism and 1 2 1 for the Chauvin mechanism. Any equilibration (substrate or product) would, of course, cause an approach towards a statistical distribution (1 2 1) and thus allow no distinction between the mechanisms. 

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. 

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. 

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... 

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 mechanism described in Scheme 1 (the Chauvin mechanism) is the accepted mechanism of alkene metathesis, and its validity has been demonstrated in two ways. First, classical kinetic studies, including isotopic labeling and crossover experiments performed using poorly defined catalysts, conclusively demonstrated that the carbene mechanism was consistent with the experiments, while the pairwise mechanism was not. More recently, the synthesis of isolable carbene complexes that catalyze the reaction has allowed a more direct observation of the reaction. Each individual step in the Chauvin mechanism has now been observed spectroscopically for several of the well-defined catalyst systems. 

The mechanism of the reaction remained mysterious for many years. Several early papers, the first by Chauvin, suggested the correct solution, but this was not generally accepted until much later. The question was whether the two alkenes bound to the metal and underwent rearrangement (called the pairwise mechanism), or whether the alkenes reacted one at a time (the nonpairwise mechanism). The Chauvin Mechanism, equation (41), is now the accepted pathway, and was a particularly imaginative suggestion at a time when both the required metalacyclobutane formation and fragmentation reactions and nonhetroatom substituted carbenes were unknown. 

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 metal-carbene chain-mechanism concept (Chauvin mechanism) has been strengthened by many studies of mechanistic details. The origin of the initial metal-carbene complex has received considerable attention, as has the metallocyclobutane-alkyUdene interconversion. Spectroscopic, kinetic, and... 

Mangohas raised an objection to the Chauvin mechanism. His analysis and calculation based on basic principles of thermodynamics indicates that more cyclopropane should be present in metathesis reactions than has been observed, i.e, at equilibrium 20% ethylene converts to cyclopropane. However, arguments that Mango s analysis is in error have been presented. Grubbs notes that the formation of cyclopropane is a chain-termination step and, since the initiation of metal carbenes is very slow compared to the catalytic reaction itself, the concentration of cyclopropane cannot be greater than the metal carbene. Grubbs concludes that the Chauvin mechanism is not inconsistent with thermodynamic calculations and remains as the mechanism most compatible with a large body of other experimental... 

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. 

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-... 

Fig. 26.4 A catalyic cycle for ring-closure metathesis (RCM) showing the Chauvin mechanism which involves [2 + 2]-cycloadditions and cycloreversions.

---