Tetramerization, of alkynes

All-carbon 2 + 3 cycloadditions 5-49 Dimerization of olefins 5-50 Addition of carbenes or carbenoids to olefins or alkynes 5-51 Tetramerization of alkynes... 

Tetramerization of alkynes 5-52 Dimerization of dienes 5-53 Addition of two alkyl groups to an alkyne... 

Transition metals induce the stoichiometric or catalytic dimerization, trimerization or tetramerization of alkynes. When they are carried out with metal carbonyls, these reactions lead to various cyclic ketone derivatives. Catalytic versions of such reactions have been developed (see Chap. 21.4) ... 

Polymerization of alkynes by Ni" complexes produces a variety of products which depend on conditions and especially on the particular nickel complex used. If, for instance, O-donor ligands such as acetylacetone or salicaldehyde are employed in a solvent such as tetrahydrofuran or dioxan, 4 coordination sites are available and cyclotetramerization occurs to give mainly cyclo-octatetraene (cot). If a less-labile ligand such as PPhj is incorporated, the coordination sites required for tetramerization are not available and cyclic trimerization to benzene predominates (Fig. A). These syntheses are amenable to extensive variation and adaptation. Substituted ring systems can be obtained from the appropriately substituted alkynes while linear polymers can also be produced. 

The use of alkynes in transition metal catalyzed reactions is often complicated by their tendency to undergo cyclo-tiimerization and -tetramerization. Thus, it is useful to note that a phosphite-modified catalyst, Ni(COD)2Aris(o-phenylphenyl) phosphite (TOPP), promotes codimerization of alkynes with methylenecyclopropane and its a ylidene analogs. Both electron-rich and electron-poor alkynes participate in cycloaddition with moderate regioselectivity. Opposite regiochemistiy is sometimes observed widi disubstituted alkylidene systems (equations 97-99). 

The reason why the tetramerization reaction takes place catalytically in these cases without any separation of metallic nickel is not yet understood. This-could be ascribed to the nature of the ligands, however, which can react with the nickel m statu nascendi These results are interesting because they show that the tetramerization of the alkynes can be induced catalytically not only with nickel (II) compounds but sometimes also with tetrahedral nickel (0) derivatives (170). 

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... 

Cyclooctatetraenes (COTTs) have been the targets of intensive experimental and theoretical studies. Since the discovery of the Ni-catalyzed tetramerization of ethyne affording cyclooctatetraene by Reppe in 1948, transition-metal-mediated methodologies to this type of compounds still remain privileged [62]. To remedy the regioselectivity issue when simple alkynes are employed, Wender and Christy have developed [2-I-2-I-2-I-2] cycloadditions of 1,6- and 1,7-diynes under conditions that favor COTTs over competing [2-1-2-1-2] cycloadditions leading to benzene derivatives (Scheme 8.40) [63]. 

Scheme 6.25 Alkyne metathesis leads in high yields to the formation of tetrameric carbazole-acetylene macrocycles.

The structure of CuCl(hepta-l,6-diyne) consists of Cl-Cu-Cl chains cross-linked by the diyne. Compound CuCI(but-2-yne) is tetrameric, and has a puckered CU4CI4 ring with the copper atoms on the perpendiular bisectors of the triple bonds. Unlike the C=C bond in Zeise s salt, the C=C bond in the alkyne complex is coplanar with the copper and two adjacent chlorine atoms (58). 

The tetrameric iron cluster [Fe(77 -C5H5)(/i3-CO)]4 catalyses the selective hydrogenation of terminal aUcynes to alkenes in the presence of internal alkynes and alkenes, Internal alkynes are themselves slowly reduced to... 

Scheme 13 A computed mechanistic pathway for the formation of a copper triazolide, starting from a tetrameric alkyne-copper complex using phenanthroline as a disaggregating ligand...

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