Metathesis, alkene olefin alkynes

Intermolecular-enyne metathesis, if it is possible, is very unique because the double bond of the alkene is cleaved and each alkylidene part is then introduced onto each alkyne carbon, respectively, as shown in Scheme 9. If metathesis is carried out between alkene and alkyne, many olefins, dienes and polymers would be produced, because intermolecular enyne metathesis includes alkene metathesis, alkyne metathesis and enyne metathesis. The reaction course for intermolecular enyne metathesis between a symmetrical alkyne and an unsym-metrical alkene is shown in Scheme 9. The reaction course is very complicated, and it seems impossible to develop this reaction in synthetic organic chemistry. 

We note that there are NMR-based kinetic studies on zirconocene-catalyzed pro-pene polymerization [32], Rh-catalyzed asymmetric hydrogenation of olefins [33], titanocene-catalyzed hydroboration of alkenes and alkynes [34], Pd-catalyzed olefin polymerizations [35], ethylene and CO copolymerization [36] and phosphine dissociation from a Ru-carbene metathesis catalyst [37], just to mention a few. 

With the discovery by Grubbs of ruthenium carbene complexes such as Cl2(PCy3)2Ru=CHR, which mediate olefin metathesis under mild reaction conditions and which are compatible with a broad range of functional groups [111], the application of olefin metathesis to solid-phase synthesis became a realistic approach for the preparation of alkenes. Both ring-closing metathesis and cross-metathesis of alkenes and alkynes bound to insoluble supports have been realized (Figure 5.12). 

Several reactions in organometaUic chemistry also appear to contravene the rule, but which can be explained in a somewhat similar way. Hydrometallation [5.45, see (Section 5.1.3.4) page 162], carbometallation, metallo-metallation, and olefin metathesis reactions are all stereospecifically suprafacial [2 + 2] additions to an alkene or alkyne, for which the all-suprafacial pathway is forbidden. Hydroboration, for example, begins with electrophilic attack by the boron atom, but it is not fully stepwise, because electron-donating substituents on the alkene do not speed up the reaction as much as they do when alkenes are attacked by electrophiles. Nevertheless, the reaction is stereospecifically syn—there must be some hydride delivery more or less concerted with the electrophilic attack. The empty p orbital on the boron is the electrophilic site and the s orbital of the hydrogen atom is the nucleophilic site. These orbitals are orthogonal, and so the addition 6.126 is not pericyclic. 

In the case of reactions such as valence isomerization, metathesis reactions of alkenes and alkynes, oligomerization or cyclooligomerization of olefins, metallacycloalkanes are of special importance. Their catalytic efficiency depends on the ease of the M—C bond cleavage, which is the result of reductive elimination of the organic substrate or of /J-hydrogen transfer. Also a- or / -C—C bond rupture has been reported. Heterocycles with an aliphatic carbon skeleton and a donor atom adjacent to the metal are suitable model compounds for the study of individual catalytic steps and structural properties. In connection with the activation of C—H bonds, cyclometa-lation has become a very general reaction and was reviewed in 1977. ... 

Alkylidenes (and alkylidynes) can react with alkenes (or alkynes) to produce metallacyclobutanes (or metallacy-clobutenes), as shown in Equation (12). This is the key first step in alkene (olefin) metathesis. 

Homogeneous catalysis alkene (olefin) and alkyne metathesis... 

In Section 24.12, we introduced alkene (olefin) metathesis, i.e. metal-catalysed reactions in which C=C bonds are redistributed. The importance of alkene and alkyne metathesis was recognized by the award of the 2005 Nobel Prize in Chemistry to Yves Chauvin, Robert H. Grubbs and Richard R. Schrock for the development of the metathesis method in organic synthesis . Examples of alkene metathesis are shown in Figure 27.3. The Chauvin mechanism for metal-catalysed alkene metathesis involves a metal alkyli-dene species and a series of [2 + 2]-cycloadditions and cycloreversions (Figure 27.4). Scheme 27.6 shows the mechanism for alkyne metathesis which involves a high oxidation state metal alkylidyne complex, L M=CR. 

In addition to the metathesis of olefins, metathesis between an olefin and an aUcyne and metathesis between two alkynes are known and can be synthetically valuable. The metathesis between an olefin and an alkyne is called en)me metathesis, and enyne methathesis is the final class of reaction involving an alkene shown in Scheme 21.1. This process combines an olefin with an alkyne to generate a diene. The thermodynamic driving force for this process is created by tire generation of a new carbon-carbon single bond from the cleavage of one of the ir-bonds in an alkyne. 

Olefin metathesis is a reaction that is over fifty years old and has been developed over this time period from a process nm at high temperatures with ill-defined catalysts by unknown mechanisms to a process that can be conducted imder nuld conditions with designed catalysts by mechanisms that occur by established steps. Olefin metathesis, and the related alk3me metathesis, fully cleaves carbon-carbon double and triple bonds and reforms these bonds to generate new alkenes and alkynes. The reaction is often under equilibrium control, but certain classes of reactions can be conducted in a selective fashion that is controlled by relative rates or thermod)mamic preferences. This reaction can open strained rings to form polymers or small dienes. It can close small rings and macrocycles by a reaction that is driven by the expulsion of ethylene that makes the reaction favored entropically or by running in an open system under non-equilibrium conditions. It can also be run as a "cross metathesis" to form imsymmetrical alkenes when the steric or electronic properties of the two alkenes properly match. 

The metathesis and polymerization of alkenes (olefins) and alkynes (acetylenes) (Eqs 7.3 and 7.4) is of considerable industrial importance, and much research has been devoted to Ihe synthesis of transition metal catalysts. 

Alkene metathesis (or olefin metathesis) breaks the double bond of an alkene and then rejoins the fragments. When the fragments are joined, the new double bond is formed between two sp carbons that were not previously bonded. Alkynes also undergo metathesis. Terminal alkenes give the best yield of a single alkene product in metathesis because one of the products is ethene, which can be easily removed from the reaction mixture, thus shifting the equilibrium in favor of the other new alkene product. 

Very reactive commercial prototype of the family of Schrock s metathesis catalysts, 1990 (can achieve RCM of tri- and tetrasubstituted olefins) - (2) Prototype of Schrock s alkyne metathesis catalysts, 1982 - (3) Basset s well-defined active catalyst for the metathesis of alkenes and alkynes, 2001. 

Besides alkene or alkyne metathesis, a broad range of other non-metathetical reactions promoted by NHC Ru complexes was reported in the literature. " Some of them were discovered serendipitously, as they constituted side reactions in metathesis catalysis. Other were deliberately investigated and optimised. Despite the usefiilness of several of these processes, their significance has remained undervalued due to the huge appeal of olefin metathesis and related reactions. 

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]. 

Only recently a selective crossed metathesis between terminal alkenes and terminal alkynes has been described using the same catalyst.6 Allyltrimethylsilane proved to be a suitable alkene component for this reaction. Therefore, the concept of immobilizing terminal olefins onto polymer-supported allylsilane was extended to the binding of terminal alkynes. A series of structurally diverse terminal alkynes was reacted with 1 in the presence of catalytic amounts of Ru.7 The resulting polymer-bound dienes 3 are subject to protodesilylation (1.5% TFA) via a conjugate mechanism resulting in the formation of products of type 6 (Table 13.3). Mixtures of E- and Z-isomers (E/Z = 8 1 -1 1) are formed. The identity of the dominating E-isomer was established by NOE analysis. 

Metallacyclobutanes or other four-membered metallacycles can serve as precursors of certain types of carbene complex. [2 + 2] Cycloreversion can be induced thermally, chemically, or photochemically [49,591-595]. The most important application of this process is carbene-complex-catalyzed olefin metathesis. This reaction consists in reversible [2 + 2] cycloadditions of an alkene or an alkyne to a carbene complex, forming an intermediate metallacyclobutane. This process is discussed more thoroughly in Section 3.2.5. 

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