Oligomerization alkynes

Reduction of [Mo(CO)(Bu C=CH)2Cp] + BF4 with KBHBu3(s) at — 78°C in an atmosphere of carbon monoxide yields a complex of a vinyl substituted y-lactone linked tj3 t]2 (220). The allylidene ruthenium complex 64, obtained by photochemical addition of one alkyne molecule to a /x-carbene derivative, is transformed into pentadienylidene complexes 65 and 66 on photolysis with more alkyne substrate. These reactions show clearly the stepwise growth of chains in alkyne oligomerizations at dimetal centers [Eq. (31)] (221). Similar reactions are also known for dinuclear iron (222), molybdenum (223), and tungsten (224) complexes. 

Compound 35 does not form above 30°C when an aromatic solvent is used instead, two alkynes are dimerized to give a titanacyclopentadiene having the structure of l,l-bis(r 5-cyclopentadienyl)-2,3,4,5-tetraphenylzirconole (36) (Fig. 15). It is an important intermediate in alkyne oligomerization reactions on transition metal complexes. 

Most of the unsaturated CpCo carbonyls are active catalysts in alkyne oligomerization (Section 5.1.4), as CO can be displaced for aUcynes. In addition to cyclization reactions, CpCo(CO) complexes have been studied as C-H bond activators. Bergman determined that, unlike the corresponding rhodium and iridium complexes, CpCo(CO) is unreactive toward C-H activation. Theoretical studies were used to probe the inability of CpCo(CO) to activate alkanes, and it... 

Many of the complexes discussed in the previous sections are catalysts for alkyne oligomerization. In fact, alkyne dimerization and trimerization (see Cyclodimerization -tri-merization Reactions) at a cobalt center is recognized as one of the most synthetically useful catalytic reactions mediated by a homogeneous transition metal complex. The cobalt complexes most useful and extensively studied are CpCoL2, where L is CO, alkene, diene, or phosphine. The complex types... 

In the case of reactions such as valence isomerization, metathesis reactions of alkenes and alkynes, oligomerization or cyclooligomerization of olefins, metallacycloalkanes are of special importance. Their catalytic efficiency depends on the ease of the M—C bond cleavage, which is the result of reductive elimination of the organic substrate or of /J-hydrogen transfer. Also a- or / -C—C bond rupture has been reported. Heterocycles with an aliphatic carbon skeleton and a donor atom adjacent to the metal are suitable model compounds for the study of individual catalytic steps and structural properties. In connection with the activation of C—H bonds, cyclometa-lation has become a very general reaction and was reviewed in 1977. ... 

Alkynes can also serve as substrates in [3 -I- 2] cycloadditions to MCP, as exemplified by the reaction of but-2-yne under nickel/phosphite catalysis to provide l,2-dimethyl-4-methylene-cyclopent-l-ene (2). 2 However, alkyne oligomerization, specifically cyclotrimerization, cannot be avoided with alkyl-substituted alkynes. When alkynes with electron-withdrawing substituents are employed, cyclotrimerization becomes the exclusive reaction. 

Review W. Reim, A. Behr, M. Roper, Alkene and alkyne oligomerization, cyclooligomerization and telomerization reactions, in Comprehensive Organometallic Chemistry, (Eds. G. Wilkinson, F. G. A. Stone, E. W. Abel), I rgamon, Oxford, 1984. 

Phenylacetylene is trimerized at 371 K by head-to-tail coupling around the mixed-metal cluster [Mo2Ru(//3-S)Cp2(CO)7], It is believed that the dimolybdenum unit serves as the alkyne oligomerization site but the chemistry overall is dependent on the entire cluster functioning as a unitJ - ... 

Pd-catalyzed double coupling of a dihaUde 64 with a terminal acetylene, while effective in the case of (Z)-enediynes, is often problematic with ( )-haloalkenes 65 due to competing alkyne oligomerization reactions (Scheme 25). ° ... 

The mechanism of alkyne oligomerization induced by PdCl2 is therefore seen to involve a series of stepwise c -insertion of coordinated alkynes as shown in Scheme Similarly, phenylacetylene can react with PdCLj " to give the tetramer product as shown in Scheme... 

Ni-alkyne bonding consists of contributions from both the 77, 7t- and cr,diyl tautomers. This bonding picture helps visualize the insertion reactions with alkynes, alkenes, and CO that result in the formation of metallacycles. Thanks to such insertion reactions, Ni-alkyne species are active intermediates in a number of catalytic applications such as alkyne oligomerization, carbonylation, and insertion of heterocumulenes such as CS2 and GO2. For example, a recent example of a C02-fixation reaction involved the stoichiometric, alkylative or arylative carboxylation of alkynes to give a,(3- and / ,/ -unsaturated carboxylic acids. Ni(0)-alkyne complexes have also been used as pre-catalysts in the addition of hydrosilanes to alkynes. In most cases, monoalkynes react to give the products of m-addition, whereas diynes produce enynes (1,2-addition), allenes (1,4-addition), or 1,3-butadienes (1,2,3,4-addition). ... 

Table 3 Athene and alkyne oligomerization and polymerization reactions catalysed by transition-metal compounds 1... 

Circumstantial evidence that -carbene complexes may initiate alkyne oligomerization comes from the observation that diruthenium /x-carbene complexes do cause linking of alkynes as in equation (50) the... 

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