Olefins, metathesis

Olefin Metadiesis, Academic Press, London (1983) 

See also page 463, Section 7 for the synthesis of dienes and polyenes. 

Zn (ZnBr2-K), H20, MeOH, THF (enyne, diyne, propargylic alcohol) 

BrCH2CH2Br, CuBr, LiBr, EtOH (propargylic and homopropargylic alcohol and amine, propargyhc ether, enyne, diyne) 

Cy2BH/HOAc (SiahBH/HOAc K02CN=NC02K, HOAc 

Olefin-metathesis is a useful tool for the formation of unsaturated C-C bonds in organic synthesis. The most widely used catalysts for olefin metathesis include alkoxyl imido molybdenum complex (Schrock catalyst)and benzylidene mthenium complex (Gmbbs catalyst). The former is air- and moisture-sensitive and has some other drawbacks such as intolerance to many functional groups and impurities the latter has increased tolerance to water and many reactions have been used in aqueous solution without any loss of catalytic efficiency. 

The olefin-metathesis in aqueous media has been applied to the synthesis of various polymers. 

cat CoCl2-4PPh3, ROH H2l cat ClRh(PPhj)3 H2, cat [Rh(NBD)(PR3) JPF6 H2, cat nickel boride P-2 

Olefin metathesis has come rightly to occupy a key role in modern organic synthesis, especially in natural product and polymer synthesis, eventually leading 

Olefin metathesis offers a means of shifting olefins to olefins with a different number of carbon atoms. Olefin metathesis is the disproportionation or dismutation of olefins over a catalyst, usually based on molybdenum or tungsten. For example, propylene gives ethylene and 

In this case two molecules of propylene form one molecule each of ethylene and 2-butene. Thus, if a petrochemical complex has an excess of propylene (say) this can be converted into ethylene and butene. Similarly, butenes can be used to produce ethylene, propylene and hexene. 

The reaction is reversible so that ethylene and butene can be converted into propylene. At present the most common use is to produce additional propylene by reacting butene with an excess of ethylene. 

The catalyst has some isomerisation activity so the product linear olefins can have the double bond in any position, similarly any linear isomer can be used as a feedstock. Branched olefins (e.g. isobutene) are not usually converted. 

Ethylene and butenes enter the system and are mixed with recycle streams. A guard bed is present to prevent dienes and acetylenes entering the system. The mixed feed is heated and passed to the metathesis reactor which converts ethylene and 2-butene to propylene  

Olefin metathesis, first discovered in the 1950s, involves the formal exchange of =CR2 fragments (R = H or alkyl) between alkenes. For example, metathesis between molecules of formula H2C—CH2 and HRC=CHR would yield two molecules of H2C=CHR  

New double bonds are formed between the top and bottom two carbons in the diagram, and the original double bonds are severed.  

Predict the possible products of metathesis of the following olefins. Be sure to consider that two molecules of the same structure can also metathesize (undergo self-metathesis). 

In this mechanism, a metal carbene complex first reacts with an alkene to form a metallacyclobutane Intermediate. This intermediate can either revert to reactants or form new products because all steps in the process are equilibria, an equilibrium mixture of alkenes results. 

Grubbs metathesis catalysts in general have less catalytic activity than Schrock catalysts, but are less sensitive to oxygen and water. They are also substantially less expensive than the molybdenum and tungsten catalysts. The catalyst having R = cyclohexyl, X = Cl, and R = phenyl has received particular attention and is marketed as Grubbs s catalyst. One requirement of these catalysts is the presence of 

Olefin metathesis is a catalytic process in which alkenes are converted into new products via the rupture and reformation of carbon-carbon double bonds. The key step in this process is the 2 + 2 reaction between an olefin and a transition metal alkylidene (carbene) complex, generating an unstable metallacyclobutane intermediate. This intermediate can either revert to the starting material, or open productively to regenerate a metal carbene and produce a new olefin (Eq. 1). 

Different types of monoolefin and diolefin undergo metathesis via contact with a suitable catalyst, resulting in a wide variety of possible products (Eqs 2-5) [1]. 

All the reactions are reversible, and when volatile or insoluble products are formed displacement of the equilibrium occurs. Thus when R = H, removal of ethene from the system of Eq. (2) can drive the reaction to completion. The reverse reaction is called cross-metathesis and cross-metathesis with ethene is called ethenolysis . Both linear and branched olefins can undergo metathesis. 

Metathesis is a versatile reaction that forms the basis for several important industrial processes, such as the Phillips triolefin process, which produces propene by cross-metathesis of 2-butene with ethene, and the Shell higher olefins process (SHOP), which involves a combination process that converts ethene to detergent-range olefins. Several interesting polymeric materials are commercially produced via the ROMP of different types of unsaturated cyclic monomers, including nor-bornene, cyclooctene, and dicyclopentadiene [1]. 

Olefin metathesis is being used increasingly in the specialty chemicals market. Olefin interconversion can be used to produce isomerically pure symmetrical internal olefins from a-olefins (Eq. 2 R H), and a-olefins can be produced from internal olefins via ethenolysis. Metathesis of olefins bearing heteroatom functional groups is also a very promising application of the metathesis reaction, which enables the synthesis, in only a few reaction steps, of many products that would otherwise be difficult to obtain. 

Discussions of the history of the metathesis reaction written by two of its discoverers can be found in R. L. Banks, Chemtech, 1986,16, 112 and H. Eleuterio, Chemtech, 1991, 21, 92. 

FiGURE 14.25 Experiment to Test the Alkyl Exchange Mechanism. 

Olefin metathesis represents a reaction during which cleavage of two carbon-carbon double bonds occurs and the formation of two new C = C bonds results. 

Therefore, olefin metathesis leads to the exchange of alkylidene groups of two alkene molecules [equation (13.162)]. This process was first called disproportionation of olefins. However, the term metathesis (from Greek metathesis — transposition), which is used in chemistry to designate the exchange of atoms or groups 

The most important properties of the metathesis reaction are as follows  

essentially first order of the reaction with respect to both the complex and the olefin. 

Metathesis of acyclic olefins is thermoneutral, i.e., the enthalpy of the reaction is approximately equal to zero because double bonds are broken and reformed in the reaction. Therefore, alkene metathesis leads to an equilibrium which is determined by the entropy of the reaction. Metathesis of cyclic olefins proceeds differently it leads to the polymerization of cyclic olefins owing to ring opening (however, such polymerization does not occur for cyclohexene) see equation (13.163). Although the number and type 

6 Olefin Metathesis Ru-catalyzed ring-dosing metathesis (RCM) 

However, they observed slow decomposition of the catalyst in the ionic liquid, behavior that was also observed in organic solvents. This decomposition prevented successful recycling of the ionic catalyst solution and significant loss of catalytic activity was already observed in the third cycle. 

A Grubbs Ru-catalyst with a cationic, imidazolium based tag (Fig. 5.3-19) was prepared and evaluated by Mauduit and coworkers [258]. Again the test reaction was the RCM reaction of diallyltosylamide. 

The same authors later expanded the concept and additionaly provided an imidazolium-tagged Howeyda-Grubbs ruthenium carbene catalyst for the RCM reaction [260]. The resulting system proved to be highly active for the conversion of di-, tri- and tetrasubstituted diene and enyne substrates. In the catalyst solvent system [BMIM][PF6]-CH2Cl2 (volume ratios 1 1 to 1 9) the catalyst could be recycled 17 times with only very slight loss in activity. Also in this work it was demonstrated that the imidazolium tag is essential to obtain a stable and recycleable catalyst. 

Finally, it should be mentioned here that Kiddle and coworkers have successfully carried out several RCM reactions in [BMIM][BF4] under microwave irradiation [262]. 

More generally, the metathesis reaction provides a laboratory analog of an important theorem in topology. Any linked or knotted structure can be converted to simple cycles (unknots) by selective interconversion of overcrossings and under- 

Since the discovery of olefin metathesis by Banks and Bailey in the 1960s using alumina supported Mo(CO)6 [64] this reaction has become key in both petrochemical and fine chemical processes. While the petrochemical industry has relied for more than three decades on the Lummus process, employing WO3 supported 

Early work in this area has involved the investigation of the olefin metathesis activity of [W( Bu)(CH 2Bu)J supported on silica or other oxide supports [65, 66]. While highly active, these systems do not contain a metallocarbene [40, 67]. For instance, the silica-supported system has been characterized as [ iOW( Bu)(CH 2Bu)J, and therefore it is very likely that the propagating metallocarbene is generated in situ even if it is not clear how it is formed. [ iOMo( Bu)(CH 2Bu)J also displays similar reactivity towards olefins, but is more sensitive to functional groups [67, 68]. For tantalum, despite the presence of a weU-defined metallocarbene in [( iO)Ta(=CH Bu)(CH 2Bu)J, this system is 

Tabie 3.5 initial rate of propene and methyl oleate transformation by metathesis. 

Noteworthy is the surface-complex [ iORe( Bu)(=CH Bu)(CH 2Bu)], which displays catalytic activities much higher than both its homogeneous or heterogeneous homologues [68]. In fact, it is possible to achieve the metathesis of functional olefins such as methyl oleate with good activities (TOP and TON) (Table 3.6). 

Because Mo and W are usually more stable and more reactive than Re, the reactivity of the corresponding isoelectronic complexes of Mo, [( iO)Mo( NAr) (=CH Bu)(CH 2Bu)[ [70], and W, [( iO)W(=NAr)(=CH Bu)(CH)Bu)] [71], have been studied. Overall, these systems are more stable than the corresponding silica-supported Re complex, and display reachvities better than those of the well-known corresponding bis-aUcoxide homogeneous derivahves. The better performances of these systems compared to their homogeneous analogues are probably due to the optimized coordination of the metal center in combinahon with a site isolation of the 

Transition Metal-Carbene Complexes in Olefin Metathesis 

The reaction of a metal-carbene complex with an olefin may lead to either cyclopropane or metathesis products, depending on the metal center and its ancillary ligands. In the case of olefin metathesis, the reaction may occur in variations that have enormous numbers of synthetic applications. Although these products are diverse in structure, they are all related by the same basic metal-carbene-mediated mechanism of formation. Because we provide only an overview of applications in this section and do not specify the particular catalyst used in each case, we direct the reader to the extensive reviews that are available for more information [32]. 

Perhaps the most basic form of the olefin metathesis reaction is the cross metathesis (CM) of acyclic olefins to yield new acyclic olefins (Fig. 4.11). The ratio of CM products may be controlled by steric and electronic factors to provide one product preferentially, rather than a statistical mixture, which is key to the synthetic utility of this reaction. For example, various functionalized olefins, dimers with bioactive substituents, and trisubstituted olefins have all been made by CM [33], and one of the industrial applications is the synthesis of insect pheromones [34]. 

Dienes can be cyclized in the ring-closing metathesis (RCM) reaction with concomitant formation of volatile olefin side products (usually ethylene) (Fig. 4.12). RCM has been used to make small- and medium-sized rings, including carbocy- 

The last catalytic scheme we examine is olefin metathesis. This reaction has been widely used to create small and large rings, as well as to open up rings to make polymers. Olefin metathesis involves the pairwise exchange of the alkene carbons in two olefins (Eq. 12.82). These reactions were reported as early as the 1950s, but it was not until the early 1970s that the currently accepted mechanism was proposed by Chauvin. 

The mechanism of olefin metathesis does not involve the classic reactions we have covered—namely, oxidative addition, reductive elimination, (3-hydride elimination, etc. Instead, it simply involves a [2+2] cycloaddition and a [2+2] retrocycloaddition. The [2+2] terminology derives from pericyclic reaction theory, and we will analyze this theory and the orbitals involved in this reaction in Chapter 15. In an organometallic [2+2] cycloaddition, a metal alkylidene (M=CR2) and an olefin react to create a metal lacyclobutane. The metalla-cyclobutane then splits apart in a reverse of the first step, but in a manner that places the alkylidene carbon into the newly formed olefin (Eq. 12.83). Depending upon the organometallic system used, either the alkylidene or the metallacycle can be the resting state of the 

Stereocontrol at Every Step in Asymmetric Allylic Alkylations 

and Van Vranken, D. L. Asymmetric Transition Metal-Catalyzed Allylic Alkylations. Chetti. Rev., 96,395 (1996). 

CHAPTER 12 ORGANOTRANSITION METAL REACTION MECHANISMS AND CATALYSIS 

The formation of carbon-carbon bonds using olefin metathesis methodology is a powerful technique in fine organic synthesis and polymer chemistry. The increasing importance of these reactions is reflected by the numerous publications over the last few years. Many of these pubhcations deal with the design and apphca-tion of polymer-supported olefin metathesis catalysts with the aim to overcome the common drawbacks of the homogeneous catalysts low thermal stability and difficulties associated with their recovery from the reaction mixtures. The modem state of art in this important field is described in chapter 11 of this volume. 

The reuse of the chiral catalyst is highly dependent on workup since the activity of (59) dramatically decreases if hydrolytic quench is not avoided. 

The preparation of polymer-supported iridium catalysts (61) and (62) for the stereoselective isomerization of double bonds using polystyrene based immobilized triphenyl phosphine were recently reported by Ley and coworkers (Fig. 4.5). The immobilized catalyst is potentially useful for deprotection strategies of aUyl ethers [130]. 

In Volume 1 of this Series the development of olefin metathesis from 1964, the time this newly recognized catalytic reaction was reported, through the first half of 1976 was reviewed by Rooney and Stewart. Since their excellent report, interest in olefin metathesis has continued to be high, as evidenced by more than 700 publications, over 30 doctoral theses, and three international symposia (Mainz 1976, Amsterdam 1977, Lyon 1979 ) during the past four years, concerning this intriguing area of catalysis and olefin chemistry. Both overall and specific aspects of olefin metathesis have been reviewed. 

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 

in Catalysis in Organic Syntheses , ed. W. H. Jones, Academic Press, New York, 1980, p. 233. 

This Chapter follows Rooney and Stewart s report in Volume 1 of this Series and is a comprehensive, but not exhaustive, review of recent studies and developments in olefin metathesis. 

The elucidation of the mechanism for olefin metathesis reactions has provided one of the most challenging problems in organometallic chemistry. In Volume 1 Rooney and Stewart concluded that the carbene chain mechanism is now generally accepted for olefin metathesis reactions, but much remains to be learned about the formation and reactivity of metal-carbene intermediates, metallocycles, and especially the mechanistic aspects of chain initiations. Since that report, systems have been designed that begin to reveal the important mechanistic features of olefin metathesis. 

This chapter deals with applying ruthenium catalysed olefin metathesis to the synthesis of piperidine and pyrrolidine containing compounds. 

FIGURE 2 Ring closing metathesis (RCM), ring opening metathesis (ROM) and cross metathesis (CM) 

In classical organic synthesis, the individual bonds in the target molecule are usually formed step by step. Starting from relatively simple starting compounds, the structural complexity in the molecule slowly increases in the course of the synthesis. Domino, or tandem, reactions are 

5 Exchange Reactions of Carbon-Carbon Bonds From the Olefin Metathesis to the Diels-Alder Reaction 

In 2005, Rowan, Nolte, and coworkers described an efficient and templated synthesis of porphyrin boxes using DCC and reversible metathesis reaction [54]. Cyclic tetramers were successfully prepared in good yields (62%) from an olefin-functionalized zinc porphyrin in the presence of first generation Grubb s catalyst and upon addition of a tetrapyridyl porphyrin (TPyP) serving as a template. While a mixture of linear and cyclic oligomers was obtained in the absence of template, addition of TPyP resulted in a reorganization of the DCL to favor the formation of the desired tetrameric box (Fig. 7a). 

A recent report by Miller and coworkers investigated the effects of remote functionality on the efficiency and stereochemical outcome of the olefin metathesis reaction [55]. Using a series of allyl- and homoallylamides, they demonstrated that both the yield of self-metathesis products and the ratio of cis- and trans-olefin isomers formed were strongly dependent on remote functionalities. Although it does not preclude the use of olefin metathesis in DCC experiments, it is an important factor that needs to be considered when designing olefin-based DCLs. Indeed, in an ideal scenario, one would expect the course of the reaction and product distribution in a DCL to be relatively insensitive to functionality remote from the reacting centers, which is unfortunately rarely the case. 

CAACs were explored as ligands in ruthenium-catalysed olefin metathesis. 

The kinetic selectivity of the CAAC-based catalysts was investigated by probing the EjZdiastereoselectivity in the cross-metathesis of cA-l,4-diaceto-2-butene with allylbenzene (Equation (5.4)). Compared to the commercially available Grubbs catalysts, 66-68 afforded lower EjZ ratios (3 1 at 70% 

Catalysts 66-68 also displayed high selectivities for the formation of terminal olefins in the ethenolysis of methyl oleate (Seheme 5.17). Notably, at low catalyst loadings (10 ppm) 68 achieved the highest TONs (35 000) reported to date. 

In die last 10 years or so an exciting new strategy has emerged for the formation of carbon-carbon double bonds, namely olefin metathesis. This work grew out of the development of Ziegler-Natta catalysts for die polymerizarion of cyclic olefins. It was found that when 2-pentene was treated with a catalyst prepared from tungsten hexachloride and ethylaluminum dichloride, a mixture of 2-pentene, 2-butene, and 3-hexene was produced in minutes at room temperature (rt)  

It was shown that the mixture was an equilibrium mixture. Thus it appears that the alkenes are being broken apart at the double bonds and the pieces reassembled randomly. This process was termed olefin metathesis because the ends of the carbon-carbon double bonds are being interchanged. 

Olefin metathesis is a unique reaction and is only possible by transition metal catalysis. In fact only complexes of Mo, W, Re, and Ru are known to catalyze olefin metathesis. Once it was known that metallocarbenes were the actual catalytic species, a variety of metal carbene complexes were prepared and evaluated as catalysts. Two types of catalysts have emerged as the most useful overall. The molybdenum-based catalysts developed by Schrock and ruthenium-based catalysts developed by Grubbs. 

Both are stable metallocarbene complexes, but they have very different reactivity profiles. The molybdenum catalyst is highly reactive and is effective widi sterically demanding olefins. Its drawbacks are diat it is not highly tolerant of diverse functional groups and has high sensitivity to air, moisture, and solvent impurities. The ruthenium system, on die odier hand, is catalytically active in die presence of water or air, and it exhibits a remarkable functional group tolerance. It is not a reactive as the molybdenum catalyst, particularly toward sterically bulky substrates. However, it is readily available and is die reagent of choice for all but die most difficult substrates. 

The rate constant k for die RCM process is greater than the dimerization rate constant k2 because of a distinct entropic advantage. In RCM the two reacting bonds are present in die same molecule and two molecules (die product and ediylene) are formed from one thus RCM proceeds with a gain in entropy. The dimerization process causes loss of translational degrees of freedom because one molecule is formed from two and thus occurs with a loss of entropy. As a result the formation of five- and six-membered rings have larger rate constants and thus proceed at faster rates dian dimerization under normal conditions. 

The 2005 Nobel Prize in Chemistry was jointly awarded to Robert H. Grubbs (Caltech), Yves Chauvin (French Petroleum Institute), and Richard R. Schrock (MIT) for establishing olefin metathesis as a reaction of synthetic versatility and contributing to an understanding of the mechanism of this novel process. Olefin metathesis first surfaced in the late 1950s when industrial researchers found that alkenes underwent a novel reaction when passed over a heated bed of mixed metal oxides. Propene, for example, was converted to a mixture of ethylene and 2-butene (cis -I- trans). 

This same transformation was subsequently duplicated at lower temperatures by homogeneous transition-metal catalysis. An equilibrium is established, and the same mixture is 

When cross-metathesis was first discovered, propene enjoyed only limited use and the reaction was viewed as a potential source of ethylene. Once methods were developed for the preparation of stereoregular polypropylene, however, propene became more valuable and cross-metathesis of ethylene and 2-butene now serves as a source of propene. 

The relationship between reactants and products in cross-metathesis can be analyzed retrosynthetically by joining the double bonds in two reactant molecules by dotted lines, then disconnecting in the other direction. 

Although this representation helps us relate products and reactants, it is not related to the mechanism. Nothing containing a ring of four carbons is an intermediate in olefin cross-metathesis. 

In this case, the ruthenium complex was synthesized with an ionic tag moiety that is fully compatible with the [BMIM]PF6. A minimum amount of this ionic liquid was used for the reaction in a mixture containing 90 vol% CH2CI2. The catalyst (5mol%) was selectively retained in the ionic liquid after 10 repeated uses, without significant loss of activity in tests at 50°C that lasted 3h each (Table VI). 

A Grubbs-type ruthenium complex and a Hoveyda ruthenium complex were compared under similar conditions for recycled activity. Both the reference catalysts showed a large drop in metathesis activity in the subsequent tests. For example, a Grubbs-type ruthenium alkylidene catalyst showed a drop of nearly 50% conversion in the second run. 

Reuse of Catalysts in Ring-Closure Metathesis with an Ionic Tagged Ruthenium Carbene Complex (188) 

RCM is typically performed in chlorinated solvents or aromatic hydrocarbons and requires high dilution conditions for medium and large rings, which are the most attractive target molecules for pharmaceutical or cosmetic applications. 

A large number of enantioselective transition-metal catalysts have been developed, not just for hydrogenation but for other reactions as well. The opportunities for fine-tuning their properties by varying the metal, its oxidation state, and the ligands are almost limitless. 

The word metathesis refers to an interchange, or transposition, of objects. 

This same transformation was subsequently duplicated at lower temperatures by homogeneous transition-metal catalysis. An equilibrium is established, and the same mixture is obtained regardless of whether propene or a 1 1 mixture of ethylene and 2-butene is subjected to the reaction conditions. This type of olefin metathesis is called a crossmetathesis. 

---

Olefin-CO coploymers Olefin p-complexes Olefin Fibers Olefin hydroformylation Olefin hydrogenation Olefimc alcohols Olefin isomerization Olefin metathesis Olefin oligomers Olefin oxides... 

V. Dragutan, A. T. Balaban, and M. Dimonie, Olefin Metathesis andPing-OpeningPoljmericyation of Cjclo-Olefins, 2nd ed., Wiley-Interscience, New York, 1985,/. C Mol, J. Mol Catal 65, 145 (1991). 

Olefin Metathesis. The olefin metathesis (dismutation) reaction (30), discovered by Eleuterio (31), converts olefins to lower and higher molecular weight olefins. For example, propylene is converted into ethylene and butene... 

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). 

To date a number of reactions have been carried out in ionic liquids [for examples, see Dell Anna et al. J Chem Soc, Chem Commun 434 2002 Nara, Harjani and Salunkhe Tetrahedron Lett 43 1127 2002 Semeril et al. J Chem Soc Chem Commun 146 2002 Buijsman, van Vuuren and Sterrenburg Org Lett 3 3785 2007]. These include Diels-Alder reactions, transition-metal mediated catalysis, e.g. Heck and Suzuki coupling reactions, and olefin metathesis reactions. An example of ionic liquid acceleration of reactions carried out on solid phase is given by Revell and Ganesan [Org Lett 4 3071 2002]. 

Synthesis of heterocycles, among them macroheterocycles, using olefin metathesis 98T4413. 

K. J. Ivin, J. C. Mol, Olefin Metathesis and Metathesis Polymerization, Academic Press, London, 1997. 

In addition to the applications reported in detail above, a number of other transition metal-catalyzed reactions in ionic liquids have been carried out with some success in recent years, illustrating the broad versatility of the methodology. Butadiene telomerization [34], olefin metathesis [110], carbonylation [111], allylic alkylation [112] and substitution [113], and Trost-Tsuji-coupling [114] are other examples of high value for synthetic chemists. 

Acyclic diene molecules are capable of undergoing intramolecular and intermolec-ular reactions in the presence of certain transition metal catalysts molybdenum alkylidene and ruthenium carbene complexes, for example [50, 51]. The intramolecular reaction, called ring-closing olefin metathesis (RCM), affords cyclic compounds, while the intermolecular reaction, called acyclic diene metathesis (ADMET) polymerization, provides oligomers and polymers. Alteration of the dilution of the reaction mixture can to some extent control the intrinsic competition between RCM and ADMET. 

Table 8-5 indicates the wide variety of catalysts that can effect this type of disproportionation reaction, and Figure 8-7 is a flow diagram for the Phillips Co. triolefm process for the metathesis of propylene to produce 2-butene and ethylene. Anderson and Brown have discussed in depth this type of reaction and its general utilization. The utility with respect to propylene is to convert excess propylene to olefins of greater economic value. More discussion regarding olefin metathesis is noted in Chapter 9. 

Figure 9-3 shows a simplified flow diagram for the olefin metathesis. 

Grubbs, R., Risse, W. and Novae, B. The Development of Well-defined Catalysts for Ring-Opening Olefin Metathesis. Vol. 102, pp. 47-72. 

Fig. 2 Ruthenium-NHC complexes active in catalytic olefin metathesis...

The possibility of being involved in olefin metathesis is one of the most important properties of Fischer carbene complexes. [2+2] Cycloaddition between the electron-rich alkene 11 and the carbene complex 12 leads to the intermediate metallacyclobutane 13, which undergoes [2+2] cycloreversion to give a new carbene complex 15 and a new alkene 14 [19]. The (methoxy)phenylcar-benetungsten complex is less reactive in this mode than the corresponding chromium and molybdenum analogs (Scheme 3). 

Scheme 3 Preparation of the ethenylcarbene complex 15 by olefin metathesis [19]...

Non-heteroatom-stabilised Fischer carbene complexes also react with alkenes to give mixtures of olefin metathesis products and cyclopropane derivatives which are frequently the minor reaction products [19]. Furthermore, non-heteroatom-stabilised vinylcarbene complexes, generated in situ by reaction of an alkoxy- or aminocarbene complex with an alkyne, are able to react with different types of alkenes in an intramolecular or intermolecular process to produce bicyclic compounds containing a cyclopropane ring [20]. 

Olefin Metathesis Directed to Organic Synthesis Principles and Applications... 

Olefin Metathesis in the Ligand Sphere of Metal Complexes. 258... 

Abstract For many years after its discovery, olefin metathesis was hardly used as a synthetic tool. This situation changed when well-defined and stable carbene complexes of molybdenum and ruthenium were discovered as efficient precatalysts in the early 1990s. In particular, the high activity and selectivity in ring-closure reactions stimulated further research in this area and led to numerous applications in organic synthesis. Today, olefin metathesis is one of the... 

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 ]. 

A mechanism for olefin metathesis reactions, which is now generally accepted, was first proposed in 1970 by Herisson and Chauvin [4]. It is outlined... 

As stated above, olefin metathesis is in principle reversible, because all steps of the catalytic cycle are reversible. In preparatively useful transformations, the equilibrium is shifted to one side. This is most commonly achieved by removal of a volatile alkene, mostly ethene, from the reaction mixture. An obvious and well-established way to classify olefin metathesis reactions is depicted in Scheme 2. Depending on the structure of the olefin, metathesis may occur either inter- or intramolecularly. Intermolecular metathesis of two alkenes is called cross metathesis (CM) (if the two alkenes are identical, as in the case of the Phillips triolefin process, the term self metathesis is sometimes used). The intermolecular metathesis of an a,co-diene leads to polymeric structures and ethene this mode of metathesis is called acyclic diene metathesis (ADMET). Intramolecular metathesis of these substrates gives cycloalkenes and ethene (ring-closing metathesis, RCM) the reverse reaction is the cleavage of a cyclo-... 

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)...

Although olefin metathesis had soon after its discovery attracted considerable interest in industrial chemistry, polymer chemistry and, due to the fact that transition metal carbene species are involved, organometallic chemistry, the reaction was hardly used in organic synthesis for many years. This situation changed when the first structurally defined and stable carbene complexes with high activity in olefin metathesis reactions were described in the late 1980s and early 1990s. A selection of precatalysts discovered in this period and representative applications are summarized in Table 1. 

---