Alkene metathesis metal carbene complexes

Keywords Metathesis Alkenes Catalysis Ruthenium Metal carbene complexes... 

Enyne metathesis is unique and interesting in synthetic organic chemistry. Since it is difficult to control intermolecular enyne metathesis, this reaction is used as intramolecular enyne metathesis. There are two types of enyne metathesis one is caused by [2+2] cycloaddition of a multiple bond and transition metal carbene complex, and the other is an oxidative cyclization reaction caused by low-valent transition metals. In these cases, the alkyli-dene part migrates from alkene to alkyne carbon. Thus, this reaction is called an alkylidene migration reaction or a skeletal reorganization reaction. Many cyclized products having a diene moiety were obtained using intramolecular enyne metathesis. Very recently, intermolecular enyne metathesis has been developed between alkyne and ethylene as novel diene synthesis. 

This review focuses on the cross-metathesis reactions of functionalised alkenes catalysed by well-defined metal carbene complexes. The cross- and self-metath-esis reactions of unfunctionalised alkenes are of limited use to the synthetic organic chemist and therefore outside the scope of this review. Similarly, ill-defined multicomponent catalyst systems, which generally have very limited functional group tolerance, will only be included as a brief introduction to the subject area. 

When alkenes are allowed to react with certain catalysts (mostly tungsten and molybdenum complexes), they are converted to other alkenes in a reaction in which the substituents on the alkenes formally interchange. This interconversion is called metathesis 126>. For some time its mechanism was believed to involve a cyclobutane intermediate (Eq. (16)). Although this has since been proven wrong and found that the catalytic metathesis rather proceeds via metal carbene complexes and metallo-cyclobutanes as discrete intermediates, reactions of olefins forming cyclobutanes,... 

Although the transformation of a primary alkyne into a vinylidene complex, 2, in presence of a number of transition metal systems is well reported [2, 3], only rare examples are known for the transformation of an alkene into a carbene complex [4, 5]. Given the increased role played by vinylidene and carbene complexes as key partners in metathesis reactions and related catalytic processes [6, 7], opening up new efficient and easy synthetic routes to such complexes is an important challenge. 

Metal allenylidene complexes (M=C=C=CR2) are organometallic species having a double bond betv een a metal and a carbon, such as metal carbenes (M=CR2), metal vinylidenes (M=C=CR2), and other metal cumulenylidenes like M=C=C= C=CR2 [1]. These metal-carbon double bonds are reactive enough to be employed for many organic transformations, both catalytically and stoichiometrically [1, 2]. Especially, the metathesis of alkenes via metal carbenes may be one ofthe most useful reactions in the field of recent organic synthesis [3], vhile metal vinylidenes are also revealed to be the important species in many organic syntheses such as alkyne polymerization and cycloaromatization [4, 5]. 

C. P. Casey, and T. J. Burkhardt, Reactions of (Diphenylcarbene)pentacarbonyl-tung-sten(0) with Alkenes. Role of Metal-Carbene Complexes in Cyclopropanation and Olefin Metathesis Reactions, J. Am. Chem. Soc. 96, 7808-7809 (1974). 

Grubbs [3] prepared high activity metathesis ruthenium metal carbene complexes, (IV), that were effective as depolymerization catalysts of unsaturated polymers and synthetic agents in preparing telechelic and alkene polymers. Other high activity metathesis ruthenium carbene metal complexes, (V), were prepared by Fogg [4]. 

Evidence for the carbene mechanism is now so overwhelming (as discussed below) that the pairwise mechanism is only mentioned in this review for historical reasons. All alkene metathesis reactions are catalyzed by a metal carbene complex of some description, and the widely variable compositions used as catalysts are necessary to generate an active metal carbene group. 

More recent developments in the mechanistic aspects of the alkene metathesis reaction include the observation that the alkene coordinates to the metal carbene complex prior to the formation of the metallacyclobutane complex. Thns a 2 - - 2 addition reaction of the alkene to the carbene is very unlikely, and a vacant coordination site appears to be necessary for catalytic activity. It has also been shown that the metal carbene complex can exist in different rotameric forms (equation 11) and that the two rotamers can have different reactivities toward alkenes. " The latter observation may explain why similar ROMP catalysts can produce polymers that have very different stereochemistries. Finally, the synthesis of a well-defined Ru carbene complex (equation 12) that is a good initiator for ROMP reactions suggests that carbenes are probably the active species in catalysts derived from the later transition elements. ... 

Alkene metathesis is a reaction catalysed by transition metals in which the carbon atoms that constitute the double bond of the alkene are exchanged with those of another alkene via a metal-carbene complex (Scheme 3.21). 

Metal-carbene complex B, also formed in Step [2], adds to alkene RCH=CH2 to form another metallocyclobutane, which decomposes to RCH=CHR, the second metathesis product, in Step [4]. Ru can bond to either the more or less substituted end of the alkene, but product is formed only when Ru bonds to the less substituted C, as shown. 

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. 

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. 

Metal carbene complexes are also involved in metathesis (described in chapter 15). Exchange of carbene complexes with alkenes via a metallacyclobutane releases volatile alkenes such as ethylene with the formation of new alkenes. Ring closing metathesis is particularly favoured but normally leads to no new chiral centres. The simple Mo and Ru carbene catalysts described in chapter 15 cannot of course be used to induce asymmetry but a new generation of asymmetric Schrock 152 and Grubbs 153 catalysts can create asymmetry if a choice between two enantiotopic alkenes is offered.36... 

Herisson and Chanvin proposed that metathesis reactions are catalyzed by carbene (alkyl-idene) complexes that react with alkenes via the formation of a cyclic intermediate, a metallacyclobutane, as shown in Figure 14.24c. In this mechanism, a metal carbene complex first reacts with an alkene to form the metallacyclobutane. This intermediate can either revert to reactants or form new products because all steps in the process are in equilibria, an equilibrium mixture of alkenes results. This non-pairwise mechanism would enable the statistical mixture of products to form from the start by the action of catalytic amounts of the necessary carbene complexes, with both R and R groups, as shown in Figure 14.24c. 

What happens is a metathesis—an exchange of groups between the two arms of the molecule. But how The mechanism is not difficult, but is unlike any other you have met before, except, perhaps, the Wittig reaction, which also forms alkenes. First, the carbene complex adds to one of the alkenes in what can be drawn as a [2 + 2] cycloaddition (Chapter 34) to give a four-membered ring with the metal atom in the ring (a metallacyclobutane )-... 

The advent of stable carbene complex catalysts has had a profound effect on the general utility of alkene metathesis. Early metathesis catalysts displayed a low tolerance for functional groups, and successful metathesis reactions were primarily restricted to relatively unfunctionalized substrates. Early demonstrations that discreet carbene complex catalysts could initiate alkene metathesis inspired the development new transition metal carbene complex catalysts. Some of the more commonly employed alkene metathesis initiators are depicted in... 

Many of the leading workers in this field participated in the Lyon symposium on the subject.The mechanism of alkene metathesis proceeds through a metallocyclobutane derivative formed by reaction of an alkene with a metal-carbene complex. Retro-carbene addition yields a product molecule and a new carbene... 

Unsaturated metallocycles also exist. They are formed according to the same mode as those described above by replacing the alkenes by the alkynes (see Chap. 15) metallacyclopropenes, metallacyclobutenes and metallacyclobutadienes (intermediates in alkyne metathesis and alkyne polymerization), metallacyclopentadienes (intermediates of alkyne trimerization, see Chap. 21) and metallabenzenes (see Chap. 15) are obtained in this way. Finally, heterometallocycles are formed analogously by reactions of metal-oxo and metal-imido complexes that are isoelectronic to metal-carbene complexes. Their role is crucial as intermediates in metathesis (Chap. 15) and oxidation catalysis (Chap. 17). 

Grubbs, contains a ruthenium atom linked by a double bond to carbon, an example of a so-caUed metal carbene complex. Metal carbenes wctc proposed (by Chauvin) and confirmed as intermediates in alkene metathesis decades ago. Subsequently,... 

Casey CP, Burkhardt TJ. Reactions of (diphenylcarbene)pen-tacarbonyltungsten(O) with alkenes. Role of metal-carbene complexes in cyclopropanation and olefin metathesis reactions. J. Am. Chem. Soc. 1974 96 7808-7809. 

These carbene (or alkylidene) complexes are used for various transformations. Known reactions of these complexes are (a) alkene metathesis, (b) alkene cyclopropanation, (c) carbonyl alkenation, (d) insertion into C-H, N-H and O-H bonds, (e) ylide formation and (f) dimerization. The reactivity of these complexes can be tuned by varying the metal, oxidation state or ligands. Nowadays carbene complexes with cumulated double bonds have also been synthesized and investigated [45-49] as well as carbene cluster compounds, which will not be discussed here [50]. 

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