The Olefin Metathesis Reaction

Co(acac)2 also catalyzes cross coupling of organozinc reagents under these conditions.289 

Several transition metal complexes can catalyze the exchange of partners of two double bonds. Known as the olefin metathesis reaction, this process can be used to close or open rings, as well to interchange double-bond components. 

Intermolecular metathesis Ring-closing metathesis Ring-opening metathesis 

The catalysts are metal-carbene complexes that react with the alkene to form a metal-locyclobutane intermediate.290 If the metallocyclobutane breaks down in the alternative path from its formation, an exchange of the double-bond components occurs. 

The most commonly used catalyst is the benzylidene complex of RuC12[P(c — C6Hn)3]2, F, which is called the Grubbs catalyst, but several other catalysts are also reactive. Catalyst H, which is known as the second-generation Grubbs catalyst, is used extensively. 

The word metath esis, with the accent on the second syllable, is derived from the Greek meta (change) and tithemi (place). As a grammatical term it means the transposition of sounds or letters in a word. In chemistry it refers to the interchange of atoms between two molecules. In olefin chemistry it describes the (apparent) interchange of carbon atoms between a pair of double bonds. 

Olefin metathesis reactions fall into three broad groups in which the overall chemistry (but not the mechanism) is represented by eqns. (l)-(3). The reactions are generally reversible and, with the right catalyst system, equilibrium can be attained in a matter of seconds, even with substrate/catalyst ratios of lO a truly remarkable reaction. 

Catalyst systems for olefin metathesis are described in detail in Ch. 2 and almost invariably contain a transition metal compound. These are sometimes effective by themselves but often require the presence of a second compound (cocatalyst), and sometimes a third (promoter). The systems most commonly used are based on the chlorides, oxides, or other easily accessible compounds of Mo, Ru, W, or Re. Os or Ir compounds are sometimes used, and occasionally Ti, V, Cr, Co, Nb, Rh, or Ta compounds. Typical cocatalysts are EtAlCl2, R3AI and ICjSn (R = Ph, Me, Et, Bu), while promoters often contain oxygen, e.g. O2, EtOH, PhOH. There is now much evidence, detailed in Ch. 3, to show that such first-generation catalyst systems act 

The most important advance over the past 15 years has been the preparation of numerous well-defined metal carbene complexes which can act directly as initiators of all types of olefin metathesis reaction. These second-generation catalysts allow much closer control and better understanding of the mechanism of the olefin metathesis reaction. The initiating and propagating species can be closely monitored and in some cases the intermediate metallacyclobutane complexes can also be observed. Well-defined metallacyclobutane complexes also can sometimes be used as initiators. 

The forward reaction (1) is an example of cross-metathesis between two different olefins and provides a route to styrene. The reverse of (1) is a self-metathesis reaction such reactions may be either productive as in (4), or non-productive (also called degenerate) as in (5). 

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Olefin metathesis is the transition-metal-catalyzed inter- or intramolecular exchange of alkylidene units of alkenes. The metathesis of propene is the most simple example in the presence of a suitable catalyst, an equilibrium mixture of ethene, 2-butene, and unreacted propene is obtained (Eq. 1). This example illustrates one of the most important features of olefin metathesis its reversibility. The metathesis of propene was the first technical process exploiting the olefin metathesis reaction. It is known as the Phillips triolefin process and was run from 1966 till 1972 for the production of 2-butene (feedstock propene) and from 1985 for the production of propene (feedstock ethene and 2-butene, which is nowadays obtained by dimerization of ethene). Typical catalysts are oxides of tungsten, molybdenum or rhenium supported on silica or alumina [ 1 ]. 

Scheme 2 Different modes of the olefin metathesis reaction cross metathesis (CM), ringclosing metathesis (RCM), ring-opening metathesis (ROM), acyclic diene metathesis polymerization (ADMET), and ring-opening metathesis polymerization (ROMP)...

The olefin metathesis reaction was also a key feature of the synthesis of epothilone A completed by a group at the Technical University in Braunschweig, Germany (Scheme 13.61). This synthesis employs a series of stereoselective additions to create the correct substituent stereochemistry. Two enantiomerically pure starting materials... 

Samuel Danishefsky s group at the Sloan Kettering Institute for Cancer Research in New York has also been active in the synthesis of the natural epothilones and biologically active analogs. One of their syntheses also used the olefin metathesis reaction (not shown). The synthesis in Scheme 13.62 used an alternative approach to create the macrocycle, as indicated in the retrosynthetic scheme. The stereochemistry at C(6), C(7), and C(8) was established by a TiCl4-mediated cyclocondensation (Step A). The thiazole-containing side chain was created by reaction sequences F and G. The... 

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... 

The olefin metathesis reaction apparently has rarely been applied to allene synthesis so far, or at least the potential of this important process has not been exploited... 

A special application of bimetallic ruthenium complexes was found in the olefin metathesis reaction vide infra) The two metal centers were closely attached to one another through /r-halide anions. The labile assembly was the key feature to the formation of highly active catalysts. 

Giger, T. Wigger, M. Audetat, S. Benner, S. A. Libraries for receptor-assisted combinatorial synthesis (RACS). The olefin metathesis reaction. Synlett 1998, 688-691. 

The epothilone synthesis in Scheme 13.49 has been used as the basis for a combinatorial approach to epothilone analogs. 167 The acyclic precursors were synthesized and attached to a solid support resin by steps A-E in Scheme 13.58. The cyclization and disconnection from the resin were then done by the olefin metathesis reaction. The aldol condensation in step D is not highly stereoselective. Similarly, olefin metathesis gives a mixture of E- and Z-stereoisomers so that the product of each combinatorial sequence is a mixture of four isomers. These were separated by thin-layer chromatography prior to bioassay. In this project, reactants A (3 variations), B (3 variations), and C (5 variations) were used, generating 45 possible combinations. The stereoisomeric products increase this to 180 (45 x 4). 

Both W(CO)5[C(C6Hs)2] and the analogous di-p-tolylmethylene complex have been used in model studies of the olefin metathesis reaction.2 3 In contrast to heteroatom-stabilized carbene complexes such as W(CO)s [C(OCH3)(C6Hs)], pentacarbonyl(diphenylmethylene)tungsten(0) reacts with alkenes to give cyclopropanes and 1,1-diphenylalkenes.2 The compound W(CO)5 [C(C6H5)2] is the best reported catalyst for the metathetical polymerization of 1-methylcyclo-butene.4... 

R.H. Grubbs, The olefin metathesis reaction, Prog. Inorg. Chem., 24 1— 50,1978. 

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... 

The olefin metathesis reaction is a relatively recent development in the chemistry of olefins. This reaction is in essence the catalyzed redistribution of alkylidene groups between olefins (54, 55). 

The role of carbenes and metal carbene complexes in transition metal-catalyzed processes is suspected of being quite extensive (61). For example, the role of carbenes in the olefin metathesis reaction as described in the previous section is probably important (55, 60). It is quite possible that the o-v rearrangement is important in these reactions also, but this has not been investigated in detail. 

The olefin metathesis reaction was so named by Calderon1 in 1967 following the discovery that it involved the total cleavage of the C=C bond and the apparent exchange of alkylidene moieties between two alkene molecules (equation 1). 

There are several ways of using the olefin metathesis reaction to generate copolymers. Occasional reference has been made earlier to the formation of copolymers. Here we give further illustrations. For the ADMET copolymerization of linear dienes, see Section VII.C. 

Olefin metathesis provides the principal synthetic context for metallacyclobutane reactivity this catalytic reaction proceeds by the transient, and reversible, formation of a metallacyclobutane intermediate from a metal alkylidene and an alkene. The olefin metathesis reaction has been exhaustively reviewed and is not directly discussed here... 

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