Acetylene complexes linear oligomerization

Acetylenes are also oligomerized to mono- or divinylacetylenes, or dienyl-acetylenes by Ni(0) (112), Rh(I) (118), or Pd(II) (114) complexes (Scheme 5). Meriwether et al. (112) proposed hydrido-a-alkynylnickel complexes as active intermediates in the catalytic linear oligomerization. Subsequent insertion of acetylene into an M-o--alkynyl bond has been assumed. 

Conversion of an j2-acetylene complex to the hydridoalkynyl complex will lead to linear oligomerization or polymerization. The tendency of some Ilh or Pd complexes to form hydridooalkynyl complexes explains their catalytic activity toward linear oligomerization. Recently, Hagihara et al. 115) examined the reaction of preformed hydrido-alkynyl complexes, MH(insertion into the M—H bond. 

Nickel complexes of alkynes are involved in many important catal)rtic transformations. A fundamental transformation of nickel(0)-alkyne complexes that forms the basis of a number of usefiil stoichiometric and catal)rtic reactions is the oxidative cyclization of one alkyne and a second unsaturated imit to form a five-membered metallacyclopentene 47 (Scheme 29). If the second unsaturated unit is also an aUqme, linear oligomerizations or cydooligomerizations result The catal5rtic tetramerization of acetylene to octatetra-ene was discovered more than 50 years ago by Reppe,l l and an excellent review on the historical development of this area has appeared.Pl The complexities of the mechanistic... 

Oligomerization of alkynes can lead to linear enyne dimers or afford cyclic oligomers. Dimerization of terminal acetylenes induced by Ru complexes yields stereoisomeric conjugated head-to-head enynes depending on the substituents 532... 

The tendency of acetylene to undergo thermal oligo- and polymerization to give (actually in very low yields ) linear polyenes or benzene is a reaction known for almost a century. Formally, the same process is implied in the formation of 137 from four acetylene molecules. Yet this deceptively simple cyclotetramer-ization scheme turned to be a viable reaction only after Reppe s discovery that a simple catalyst, nickel(n) cyanide, could serve as a highly efficient device to control the course of acetylene oligomerization. It is the ability of this catalyst to form a complex, 401, with four molecules of acetylene that ensured the required selectivity of cyclotetramer formation (Scheme 2.135). 

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