Oligomerization of alkynes

Alkyne complexes are essentially similar to the alkenes (p. 932) and those of Pt particularly when the alkyne incorporates the <-butyl group, are the most stable. Ni alkyne complexes are less numerous and generally less stable but are of greater practical importance because of their role as intermediates in the cyclic oligomerization of alkynes, discovered by W. Reppe (see Panel). 

The activation barrier for the oligomerization of alkynes may be overcome thermally or catalytically. Because the classical thermal transformation of ethyne into benzene in a hot tube is rather ineffective (47), more emphasis has been placed on working out catalytic routes for the synthesis of linear oligomers, cyclooligomers, and polymers. Transition metal compounds have proved to act as effective catalysts in homogeneous as well as in heterogeneous processes (48). 

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... 

Some unusual complexes that include pentadienyl or related ligands have been obtained from reactions of alkynes with various transition metal complexes. As the main thrust of this work is on oligomerization of alkynes and not toward the preparation and identification of pentadienyls, only a few key papers and results will be mentioned. 

Oligomerization of alkynes probably occurs by multiple insertions of acetylene into metal-carbon bonds. An insertion reaction of alkynes is shown as follows ... 

Rh(I) complexes such as RhCl(TPPMS)3 4HzO, Rh(cod)Cl(TPPMS) H20, [Rh(cod)(/<2-Ph2P—C6H4-3-S02—0)]2 catalyze oligomerization of alkynes (propynoic acid, arylacetylenes) this has been studied only very recently in terms of biphasic catalysis. The Rh(I) complex Rh(cod)(/<2-Ph2P—C6H4-3-S02—0)]2 already reported by Wilkinson etal. [50] promotes the conversion of phenylacetylene to 1,2,4- and 1,3,5-triphenylbenzene under biphasic conditions (toluene/water) [58],... 

Amer, I., T. Bernstein, M. Eisen, and J. Blum, Oligomerization of Alkynes b the RhCl3-Aliquat 336 Catalyst System 1. Formation of Benzene Derivatives, J. MoL Catal, 60,313 (1990). 

These observations, in addition to those made in the stepwise oligomerization of alkynes by nickel(O) complexes (Scheme IX), help to clarify... 

Sequential insertion of alkynes into the newly formed M-vinyl bond after the first insertion leads to organometalhc products with three alkyne units incorporated (Scheme 6.44) [136]. Although this is formally related to the alkyne cyclotrimerization process, the mechanism of this important catalytic reaction is not fully elucidated. Oligomerization of alkynes to give enynes [137], and polymerization of alkynes are apphcations of this sequential insertion of alkynes into M-vinyl bonds [138]. The insertion of alkynes into the M-C bond of group... 

Oligomerization of alkynes also takes place at dimetal centres bridged by /i-methylene groups. Fig. 7.14 illustrates the types of reactions observed. Similar processes occur at Mo, W. Fe and Co centres. 

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