Exchange alkyne metathesis

Metathesis is a versatile reaction applicable to almost any olefinic substrate internal, terminal or cyclic alkenes, as well as dienes or polyenes. (Alkyne metathesis is a growing area, but will not be dealt with here.) The reaction is also known as olefin disproportionation or olefin transmutation, and involves the exchange of fragments between two double bonds. Cross metathesis (CM, Figure 1) is defined as the reaction of two discrete alkene molecules to form two new alkenes. Where the two starting alkene molecules are the same it is called self-metathesis. Ethenolysis is a specific type of cross metathesis where ethylene... 

The alkyne metathesis reaction is a process that allows for the exchange of alkyli-dyne units between two alkynes (Scheme 6.18a) [69, 70]. This reaction has not been studied as much as alkene metathesis, despite a growing interest in this topic (Scheme 6.19). 

Synthesis of ferrocene-fttnctionaHzed polymer brushes via ROMP and subsequent surface grafting. (Reprinted with permission from Eder et al. 2001. Alkyne metathesis graft polymerization Synthesis of poly(ferricinium)-based siUca supports for anion-exchange chromatography of oligonucleotides. Macromolecuks 34 (13) 4334 4341, copyright (2001) American Chemical Society.)... 

With R H, alkyne metathesis can lead either to polymerization or carbyne exchange depending on the catalyst and solvent, cat. Mo O 1 H 1 in hO H H O hats or one of the 3 unimolecular catalyst... 

Since its discovery, alkyne metathesis has become a potent catalytic tool apphed in the preparation of materials and organometalHc complexes as well as bioactive organic molecules including natural products. In an analogous fashion to alkene metathesis, the process involves exchange of methine units between two carbon-carbon triple bond systems (Scheme 7.1). 

Homogeneous, single-component catalysts such as, e.g., W(=CCMe3)(OCMe3)3 or W(=CMe)(OCMe2CF3)3, cannot only be used for exchange metathesis of alkynes but also for ROMP of cycloalkynes, ADMET of a,to-diynes, and RCM of a,co-diynes [751]. 

A route to alkylidynes containing metals with high oxidation states involves a metathesis exchange reaction that we have referred to briefly in earlier sections of Chapter 10. Scheme 10.10 shows the triply-bonded ditungsten complex 79 reacting with either alkynes or nitriles to give the corresponding metal carbyne (path a)90 or metal-carbyne plus nitride complex (path b).91 Scheme 10.10 also shows results of molecular orbital calculations at the DFT level.92 In the case... 

Katz has proposed a carbenic Chauvin-type metathesis mechanism for the polymerization of alkynes. This mechanism involves two alkynes coordinated to in the catalytically active species. The ETC catalyzed ligand exchange process is adequate to rapidly introduce two or three alkyne ligands on in such a way that the Katz mechanism is then operating as shown on the following scheme. 

Metal vinylidenes easily form by isomerization of metal-alkynecomplexes, which is important for instance in alkyne polymerization by the metathesis mechanism. Synthesis of Grubbs ruthenium vinylidene complexes [Ru(PR3)2Cl2(=C=CHPh)] originally proceeded by reaction of the ruthenium precursor with diphenyl-cyclopropene, but a more modern method leading to the parent vinylidene complex involves benzylidene exchange for the vinylidene in the metathesis with 1,3-butadiene ... 

Although the metathesis of terminal alkynes also proceeds rapidly, polymerization of the terminal alkynes displaces the carbyne exchange reactions towards the wrong side towards the starting terminal alkyne that polymerizes except if suitable parameters are ad usted such as temperature, solvent diethyl ether and especially the presence of the potential ligand quinuclidine together with the... 

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