Alkene and alkyne metathesis reactions

The organic reactivity of the alkylidyne complexes has been extensively studied especially with regard to alkene and alkyne metathesis reactions.353,356... 

SCHEME 1. Types of alkene and alkyne metathesis reactions. DBC, double bond cleavage TBC, triple bond cleavage ADMET, acyclic diene metathesis RCM ring-closing metathesis ROMP ring-opening metathesis polymerization... 

Grubbs, R. H., Alkene and Alkyne Metathesis Reactions , in Comprehensive Organometallic Chemistry, Pergamon Press, Oxford, 1982, Vol. 8, pp. 499-551. 

From the above, a parallel appears to exist between the metathesis of alkenes and alkynes. Both reactions result in a redistribution of, respectively, alkylidene and alkylidyne groups. Moreover, the results obtained so far suggest that in both cases the reaction might proceed via a metallocyclic intermediate. 

Aside from the Ziegler-Natta polymerization, alkene and alkyne metathesis, and other reactions of Ti-methylene complexes, carbometallation reactions induced by alkyltitanium compounds have been dominated by those involving... 

The application of alkene [1] - and, more recently, enyne [2] and alkyne - metathesis to the synthesis of natural products has been triggered by the development of powerful catalysts that allow metathesis reactions to be carried out under mild conditions. Scheme 1 outlines two important cases of alkene and alkyne metathesis of particular interest to the synthesis of natural products (together with the general scheme of enyne metathesis, not discussed in this review). The metathesis products can be obtained in high yields, since ethene/2-butyne are formed as volatile products. After the alkene/alkyne metathesis, the substituents (R) of the alkenes/alkynes are located on the same multiple bond. Enyne metathesis can be considered as the more general case of alkene metathesis, because two new double bonds are again formed, albeit now connected by a single bond. 

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. 

C. (2008) Reactions of strained hydrocarbons with alkene and alkyne metathesis catalysts. J. Am. Chem. Soc., 130, 14078-14079. 

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. 

Insights from Computational Studies on d° Metal-Catalyzed Alkene and Alkyne Metathesis and Related Reactions... 

Thus far, chemists have been able to influence the stereoselectivity of macro-cyclic RCM through steric and electronic substrate features or by the choice of a catalyst with appropriate activity, but there still exists a lack of prediction over the stereochemistry of macrocyclic RCM. One of the most important extensions of the original metathesis reaction for the synthesis of stereochemi-cally defined (cyclo)alkenes is alkyne metathesis, followed by selective partial hydrogenation. 

The gas-phase reaction of cationic zirconocene species, ZrMeCp2, with alkenes and alkynes was reported to involve two major reaction sequences, which are the migratory insertion of these unsaturated hydrocarbons into the Zr-Me bond (Eq. 3) and the activation of the C-H bond via er-bonds metathesis rather than /J-hydrogen shift/alkene elimination (Eq. 4) [130,131]. The insertion in the gas-phase closely parallels the solution chemistry of Zr(R)Cp2 and other isoelec-tronic complexes. Thus, the results derived from calculations based on this gas-phase reactivity should be correlated directly to the solution reactivity (vide infra). 

The currently known carbometallation chemistry of the group 6 metals is dominated by the reactions of metal-carbene and metal-carbyne complexes with alkenes and alkynes leading to the formation of four-membered metallacycles, shown in Scheme 1. Many different fates of such species have been reported, and the readers are referred to reviews discussing these reactions.253 An especially noteworthy reaction of this class is the Dotz reaction,254 which is stoichiometric in Cr in essentially all cases. Beyond the formation of the four-membered metallacycles via carbometallation, metathesis and other processes that may not involve carbometallation appear to dominate. It is, however, of interest to note that metallacyclobutadienes containing group 6 metals can undergo the second carbometallation with alkynes to produce metallabenzenes, as shown in Scheme 53.255 As the observed conversion of metallacyclobutadienes to metallabenzenes can also proceed via a Diels-Alder-like... 

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. 

Alkyne metathesis, a mechanistic cousin of alkene metathesis, has thus far found only limited exploration. In 2004, Zhang and Moore reported that precipitation-driven alkyne metathesis reactions could efficiently produce arylene ethynylene macrocycles [50]. This was explored further in a 2005 report, verifying that the products obtained were indeed the result of thermodynamic self-selection [51]. 

Metathesis of enynes, which have alkene and alkyne moieties in the molecule, is also a very interesting reaction. In this reaction, the double bond is cleaved and carbon-carbon bond formation occurs between the double and triple bonds, and the cleaved alkylidene part migrates onto the alkyne carbon to produce a cyclic compound having a diene... 

A concise total synthesis of dehydrohomoancepsenolide is achieved in an optically active form. The key steps are alkene metathesis and alkyne metathesis. A three-component coupling reaction affords dienyne 137, which undergoes ring-closing alkene metathesis in the presence of the first-generation ruthenium carbene complex to give 138,... 

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). 

Allosedamine, via ring-closing diene metathesis, 11, 224-225 Allyamines, isomerization, 10, 71 Allyindation reactions, alkenes and alkynes, 9, 693 Allyl acetals... 

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