Metal—carbon triple bonds metathesis reactions

Alkynes can also undergo metathesis reactions catalyzed by transition metal car-byne complexes. The intermediates in these reactions are believed to be metallacy-clobutadiene species, formed from the addition of an alkyne across a metal-carbon triple bond of the carbyne (Figure 14-26). The structures of a variety of metallacyclobu-tadiene complexes have been determined, and some have been shown to catalyze alkyne metathesis. 

Freudenberger and Schrock reported the metathesis of the metal-carbon triple bond in 285 with the carbon-nitrogen triple bond in acetonitrile [Eq. (210)] (207). Metathesis-like reactions of 285 also occur with organic carbonyl groups to generate 0x0 vinyl metal complexes [Eq. (211)]... 

The chemistry of metal-carbon triple bonds has developed considerably during the late 1980s. The synthetic basis was broadened, the utility of high-valent metal alkylidynes in metathesis reactions was further developed and refined, and the potential of low-valent carbyne complexes for applications in organic synthesis has become more apparent. The discovery of novel iridium alkylidyne complexes indicates that the full range of metal-carbon triple bonds is not yet known. We can therefore expect that future work in this area of organometallic chemistry will lead to new discoveries with fundamental implications and practical applications. 

The metathesis of metal-metal triple bonds with carbon-carbon triple bonds directly connects the chemistry of inorganic, organometallic, and organic multiple bonds. This reaction is synthetically very useful. Krouse and Schrock prepared the series of /t-a,[Pg.256]

Perhaps the most remarkable illustration of the ability of metals to activate alkynes comes from reactions in which complete scission of the carbon-carbon triple bond occurs. On the stoichiometric level these include examples in which carbyne complexes are produced from alkyne completes as in the melt-thermolysis of CpCo(PPh3)(RCsCR) [112] or from reactions of alkynes with unsaturated metal species (Scheme 4-34) [113]. The remarkable alkyne metathesis reaction (Scheme 4-35), which involves overall cleavage and regeneration of two o-and four rt-bonds, is conceptually related. A variety of functionalized alkynes can be tolerated as metathesis substrates [114] and especially effective catalysts for these reactions are Mo(VI)-and W(VI)-carbyne complexes. Metallacyclobutadienes 64, formed by the reaction of the alkyne with a metal-carbyne complex, appear to be central intermediates in these reactions and the equilibrium between metallacycle and alkyne/metal-carbyne is observable in some cases [115]. 

Olefin metathesis reactions cleave carbon-carbon double bonds and reassemble tiiem to generate products containing new carbon-carbon double bonds. This process requires a catalyst and is largely controlled by thermodynamics (Equation 21.1). Alkyne metathesis reactions cleave carbon-carbon triple bonds and reassemble them to form products containing new carbon-carbon triple bonds (Equation 21.2). The observation of complete cleavage of strong carbon-carbon multiple bonds by a catalytic process was remarkable when first discovered, but many transition metal complexes are now known that catalyze these reactions with fast rates. One might expect that the equilibrium control of this reaction would limit its use, but olefin metathesis has become one of the most useful reactions catalyzed by transition metal complexes. 

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 ... 

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