Metathesis chemistry alkynes

The tricyclic compound 20-C, a potential intermediate for alkaloid synthesis, has been prepared by an intramolecular Diels-Alder reaction of the ketone obtained by deprotection and oxidation of 20-B. Compound 20-B was prepared from 20-A using alkyne-ethene metathesis chemistry. Show the mechanistic steps involved in conversion of 20-A to 20-B. 

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. 

The chemistry of alkylidene and alkylidyne complexes of early transition metals was developed by Schrock and co-workers and these complexes turned out to be of crucial importance to alkene and alkyne metathesis. Initially their research focused on tantalum complexes of the type CpTaCEIE, which after a-elimination (Figure 16.6) led to alkylidene complexes Cp(R)Cl2Ta=CHR [11]. 

Alkyne metathesis is now shown to be extremely useful in synthetic organic chemistry and growing to be applied extensively. 

The chemistry of CR fragments ligating metal centers has been a topic of considerable interest since the discovery of the first alkylidyne-metal complexes in E.O. Fischer s Laboratory in 1973 (1). These ligands have been implicated in Fischer-Tropsch reactions (2) and in alkyne metathesis (5). Moreover, the isolation of stable compounds containing carbon-metal triple bonds completed the matrix of bond types represented here ... 

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

The authors of this work did not consider I-IV as cluster compounds nor did they view II and IV as cluster isomers. Let us see if we can relate the observed chemistry to cluster isomerization albeit one in which the forward (C apart to C adjacent) and reverse (C adjacent to C apart) have similar barriers. That is, an effective alkyne metathesis catalyst cannot have any species in the catalytic cycle of high stability (Bunz, 2005). 

Addition of pyridine and acetylene results in generation and trapping of the complex shown in equation (87), which has a reduced bond order. In this complex, the acetylene bridges the two metals. This type of chemistry was initially developed by Schrock and coworkers for alkyne metathesis (see Alkyne Metathesis) reactions but has been subsequently developed by a number of other researchers. 

This chapter will cover both the synthesis and basic principles of modern, well-defined metal alkyhdenes related to their use in ROMP, ADMET-polymerization and alkyne metathesis polymerization. The term well-defined is limited to catalytic systems, usually metal alkyhdenes, that are characterized by a uniform and stoichiometric composition and for which the actual propagating species is well known and characterized. Consequently, the entire chemistry of standard binary or ternary systems such as WCl6/AlEt2Cl/ethanol has been neglected. Where ap-phcable, general aspects of the preparation of advanced materials via ROMP and alkyne-polymerization will be mentioned briefly. 

An approach to inducing an ot-helical conformation in peptides that is more definitive than relying on salt-bridging interactions involves chemical ligation of side-chain residues using either ring-closing olefin metathesis or Cu-catalyzed azide cycloaddition to an alkyne (click chemistry) [147-153]. This approach,... 

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. 

Alkynes can also participate in alkene metathesis chemistry. The reaction between an alkene and an alkyne is known as ene-yne metathesis. ° The mechanism is slightly more complicated then for alkene metathesis and it is unclear whether it starts with the alkene or the alkyne (Scheme 8.109). In any event, the same product, a conjugated diene 8394, is obtained. If the catalytically active carbene species ([Ru] =) reacts first with the alkyne. 

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