Mechanisms olefin metathesis

In a wide variety of cases, application of this model allows the desired cross product to be generated in high yield. In order to generate the product as a single stereoisomer, an understanding of the relationship between catalyst structure in Ru-based catalysts and cis-trans selectivity is necessary. This requires a closer examination of the olefin metathesis 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... 

Many stable tungstacyclobutane complexes are known, but few will initiate the metathesis of internal olefins or ROMP of cyclic olefins. Yet many will undergo exchange reactions with ethene or terminal olefins by a mechanism which must involve dissociation to a tungsten carbene complex. A great deal can therefore be learnt about the olefin metathesis mechanism from a study of such reactions. The following is a short summary. 

The phosphine dissociation rates ( d) of 1 and 2 do not track with their olefin metathesis activities, since 2 is significantly more active than 1 for both polymerization [5] and ring closing metathesis reactions. Instead, correlates directly with the initiation rates of the two catalysts. The kinetics of initiation was investigated by monitoring the reaction of 1 or 2 with ethyl vinyl ether using NMR and/or UV-vis spectroscopy [10]. For both catalysts, this reaction showed saturation kinetics, and the values of the initiation rate constant k ) at saturation were identical (within error) to the values of / d at the same temperature. These results implicate a dissociative olefin metathesis mechanism as depicted in Scheme 2. In this mechanism, phosphine dissociation to form a 14-electron intermediate L(Cl)2Ru=CHPh (3) is followed by trapping with olefinic substrate. 

The olefins that undergo metathesis include most simple and substituted olefins cycHc olefins give linear high molecular-weight polymers. The mechanism of the reaction is beheved to involve formation of carbene complexes that react via cycHc intermediates, ie, metaHacycles. Industrial olefin metathesis processes are carried out with soHd catalysts (30). 

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...

E.O. Fischer s discovery of (CO)sW[C(Ph)(OMe)D in 1964 marks the beginning of the development of the chemistry of metal-carbon double bonds (1). At about this same time the olefin metathesis reaction was discovered (2), but It was not until about five years later that Chauvln proposed (3) that the catalyst contained an alkylidene ligand and that the mechanism consisted of the random reversible formation of all possible metallacyclobutane rings. Yet low oxidation state Fischer-type carbene complexes were found not to be catalysts for the metathesis of simple olefins. It is now... 

Although the molybdenum and ruthenium complexes 1-3 have gained widespread popularity as initiators of RCM, the cydopentadienyl titanium derivative 93 (Tebbe reagent) [28,29] can also be used to promote olefin metathesis processes (Scheme 13) [28]. In a stoichiometric sense, 93 can be also used to promote the conversion of carbonyls into olefins [28b, 29]. Both transformations are thought to proceed via the reactive titanocene methylidene 94, which is released from the Tebbe reagent 93 on treatment with base. Subsequent reaction of 94 with olefins produces metallacyclobutanes 95 and 97. Isolation of these adducts, and extensive kinetic and labeling studies, have aided in the eluddation of the mechanism of metathesis processes [28]. 

For more information regarding this mechanism, see Ivin KJ,Mol JC (1997) Olefin Metathesis and Metathesis Polymerization. Academic Press, London... 

As mentioned in the introduction, early transition metal complexes are also able to catalyze hydroboration reactions. Reported examples include mainly metallocene complexes of lanthanide, titanium and niobium metals [8, 15, 29]. Unlike the Wilkinson catalysts, these early transition metal catalysts have been reported to give exclusively anti-Markonikov products. The unique feature in giving exclusively anti-Markonikov products has been attributed to the different reaction mechanism associated with these catalysts. The hydroboration reactions catalyzed by these early transition metal complexes are believed to proceed with a o-bond metathesis mechanism (Figure 2). In contrast to the associative and dissociative mechanisms discussed for the Wilkinson catalysts in which HBR2 is oxidatively added to the metal center, the reaction mechanism associated with the early transition metal complexes involves a a-bond metathesis step between the coordinated olefin ligand and the incoming borane (Figure 2). The preference for a o-bond metathesis instead of an oxidative addition can be traced to the difficulty of further oxidation at the metal center because early transition metals have fewer d electrons. 

Fig. 1.7. Mechanism of carbene-complex-catalyzed olefin metathesis.

The metallacyclobutane mechanism of olefin metathesis has been discussed in Sections 1.3 and 3.1.7. For metathesis of acetylenes carbyne complexes are generally required (Figure 3.44), and both heterogeneous and homogeneous catalytic systems have been developed for this purpose. 

The mechanism of ROMP is sketched in Figure 3.47. Because of the high demand for new polymers with improved properties, ROMP has received much attention since the discovery of olefin metathesis [396,816,817]. 

Well-Defined Ruthenium Olefin Metathesis Catalysts Mechanism and Activity, ... 

Recent mechanistic work has shown that 16 e Ru methylene complexes (such as bisphosphine 11) are slow to re-enter the catalytic cycle. Their reluctance to initiate can result in competitive decomposition see Mechanism and Activity of Ruthenium Olefin Metathesis Catalysts, M.S. Sanford, J.A. Love, R.H. Grubbs,/. 

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. 

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. 

Scheme 11.5 Four-step mechanism of olefin metathesis coordination, [2t-2] cycloaddition, [2+2] cycloreversion and de-coordination [4].

It is worthwhile commenting that the one method that has really seen an explosion in its use for heterocycle formation since the publication of CHEC-II(1996) is olefin metathesis, with the various metathesis catalysts now readily commercially available. It is beyond the scope of this review to discuss the mechanism and nature of metathesis catalysts and the reader is directed to alternative texts for this <2004CRV2199, 2004CRV2239>. 

Sanford, M. S. Love, J. A. Mechanism of Ruthenium-catalyzed Olefin Metathesis Reactions. In Handbook of Metathesis-, Grubbs, R. H., Ed. Wiley-VCH Weinheim, 2003 Vol. 1, pp 112-131. 

This mechanism is similar to the olefin metathesis reaction. When the molecule structure permits formation of 1,3-diadsorbed species, this reaction can occur via Ti-allyI adsorbed complexes.271... 

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