Metal-alkyne cleavage reactions

Other synthetic routes to alkene-metal complexes reported in the literature involve bridge cleavage reactions of halogeno complexes (method F), facile intermolecular scrambling reactions involving metal-alkene and metal-PF3 complexes (18, 81) (method G) (Scheme 4), and the coupling together of two alkyne units to afford a metallo-cyclopentadiene derivative (method H). 

There have been far fewer studies on the reactions between tetranuclear clusters and alkynes or alkenes than have been reported for trinuclear systems. The reactions that have been investigated have largely been with alkenes. Both R H CO) 2 and Os4H4(CO)12 react with mono- and polyalkenes, and substitution of hydrides and carbon monoxide ligands is observed in most cases (180-187), as shown by Eq. (10). There are a few examples, however, where metal-metal bond cleavage does occur and trinuclear clusters are isolated (181-183,188). In contrast, when Rh4(CO)12 and Ir4(CO)12 are treated with dienes, cluster growth occurs (189-193). 

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 reaction of 1,1-dibromoalk-l-enes with Smia in benzene containing 10% HMPA affords alkynes in reactions probably involving samarium vinyl intermediates. The reactions of diaryl ketones with Sm or Yb metals results in cleavage of the C=0 bonds in reactions possibly proceeding via a l-oxo j,-alkylidene intermediates. ... 

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