The metal carbene mechanism

Evidence for the mechanism is discussed in detail in Ch. 3. Here is a brief outline. The basic question concerns the role of the catalyst and especially the transition metal in the olefin metathesis mechanism. How does it facilitate the exchange of alkylidene moieties At first it was thought that two double bonds came together in the vicinity of the transition metal site and that the orbitals of the transition metal overlapped with those of the double bonds in such a way as to allow exchange to 

Stable metal carbene complexes, such as W[=C(OMe)Me](CO)5, were first prepared by Fischer, E.O. (1964) and many hundreds are now known. Some, like 1, react with other compoimds in such a way as to indicate that the carbene ligand is electrophilic others, such as 2, react in the opposite way, indicating that the carbene ligand is nucleophilic (Grubbs 1978 Parshall 1980). 

Catalyzed olefin metathesis reactions are chain reactions with high turnover numbers. In some cases a metal carbene complex can exchange its alkylidene 

Of particular interest is the fact that many olefin metathesis catalyst systems are of the Ziegler—Natta type. This raises the question of the relationship between the mechanism of olefin metathesis and that of Ziegler-Natta polymerization this aspect is discussed in Ch. 4. 

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A further confirmation of the metal carbene mechanism is provided by enyne intramolecular metathesis reactions such as that depicted in equation 61. The C=C bond in the substrate becomes the single bond attaching the alkenyl group to the phenanthrene ring system634,635. 

Isotope labeling studies by Grubbs [20] and Katz [21] presented further proof for the metal carbene mechanism. 

Two mechanisms have been proposed for acetylene and substituted acetylene polymerization by transition metal catalysts one is the metal-alkyl mechanism and the other is the metal-carbene mechanism. In general, it has been proposed that the polymerization of acetylenes by Ziegler-Natta catalysts proceeds by the metal-alkyl mechanism, while the metal-carbene mechanism has been accepted for the polymerization of substituted acetylenes by metathesis catalysts whose main components are halides or complexes of group 5 and 6 transition metals. The latter will be discussed in Section III. 

The metal-carbene mechanism also accounts for a number of other experimental results. 

The cross-metathesis of cyclopentene with imsymmetrical olefins is catalyzed by WOCl4/Bu4Sn or WOCl4/Et2AlCl. Herisson (1971) observed that the products of reaction with pent-2-ene consisted of three series of compounds Q M Q , Q M Q, and Q M Q (n = 1-4), where Q = ethylidene, = propylidene, and M represents n ring-opened units of cyclopentene M. These series were formed in the statistical ratio 1 2 1 even in the initial products. Similar results were obtained with cyclooctene, cycloocta-1,5-diene, and cyclododeca-l,5,9-triene in place of cyclopentene. It was these observations that led to the proposal of the metal carbene mechanism, since direct exchange between the double bonds of the reactant molecules would yield only the unsymmetrical series. The formation of the three series of compounds is accounted for in terms of reactions (l)-(6). 

A more stringent test is to react cyclooctene with a mixture of but-2-ene and oct-4-ene. If the metal carbene mechanism is correct, one may expect to find the C14 product of double cross-metathesis, i.e. MeCH=CH(CH2)6CH==CHPr, and an initial value of 4.0 for the product of the two ratios (C14/C12) and (C14/C16). The observed ratio is 4.05 0.05 for cis reactants and 4.11 0.09 for trans reactants (Katz 1977a). 

Cross-metathesis reactions between undeuterated and deuterated dienes, such as octa-1,7-diene (Grubbs 1976) and 2,2 -divinylbiphenyl (Katz 1976b) have also been studied as a means of testing the metal carbene mechanism. The initial proportions of ethene-tfo, -d2, and -d4 formed in these reactions support this mechanism. Likewise for the cross-metathesis between c/5,cw-deca-2,8-dienes... 

The first important conclusion from these experiments is that the unsymmetrical telomer (C14) is formed from the start, which would not be possible on the pairwise mechanism. The results may be interpreted in terms of the metal carbene mechanism shown in Scheme 15.3. In order to obtain useful expressions from this scheme, it is necessary to make an assumption similar to that of eqn. (7) for Scheme 15.2, namely that the rate constant ratios are related by eqn. (19). One then obtains eqns. (20) and (21) from a steady-state treatment of Scheme 15.3. Multiplying the two together gives eqn. (22). The product of the two intercepts in Fig. 15.7 is 4.5 0.6. A more accurate value of 4.1 0.1 is obtained by extrapolating the quantity [C 4] /[Ci2][Ci6] to zero conversion, confirming the prediction of eqn. (22) and justifying eqn. (19) a posteriori. 

In the case of cycloocta-1,5-diene the series of cyclic oligomers have the formula (C4Hg) (n = 4-13) with no tendency for the even-numbered members of the series to predominate, one of the observations that led Herisson and Chauvin to the proposal of the metal carbene mechanism. 

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