Insertion reactions into metal-acetylene bonds

Little success has been achieved in the synthesis of alkyl metal complexes by insertion reactions into metal-carbon bonds. This is due undoubtedly to further reactions of the immediate insertion products to give polyinsertion or decomposition products. The insertion of olefins and acetylenes into metal-carbon bonds to give new alkyl or vinyl derivatives is discussed in Section IV,B. 

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

Generally, insertion of the alkyne into a metal-P bond is observed (Scheme 10).188,190 When aminoalkynes are used, the formation of a C=N double bond inhibits the interaction of that carbon with the metal centers of the cluster.186 187 When two PR groups are present, the alkyne has been observed to bridge between them as seen in Scheme 10.195,285 A second equivalent of diphenylacetylene can substitute for two carbonyl groups on the iron triangle.195 The hetero-main group element species Fe3(CO)9(NPh) (P Bu) and Fe3(CO)9(NPh)2 have been reacted with diphenylacetylene.273 Some of the products involved in the acetylene addition reaction are shown here (241-243). 

The five-coordinate Ru-boryl complex, Ru(Bcat)Cl(CO)(PPh3)2, inserts ethyne into the Ru-B bond forming the borylvinyl-compound (13) (Scheme 8). This reaction and product models the acetylene insertion step to give intermediate C in the metal-catalysed diboryl-ation reaction shown in L... 

For the Reppe carbonylation, it is proposed that the reaction involves the initial oxidative addition of a nucleophile to the transition metal complex, followed by the complexation of an unsaturated hydrocarbon to the metal and insertion into a metal-H bond. Subsequently, migration of hydrocarbon species to CO followed by a reductive elimination afford the corresponding product." Scheme 1 illustrates the formation of ester from acetylene, CO, and methanol in the presence of a catalytic amount of Pd[CO]4. In addition, a mechanism analogous to that of Hydroformylation is proposed and displayed in Scheme 2 for the Reppe formylation. 

As has been observed previously for Os3(CO)9(/ 3-77 77 t) -C6H6), the coordination mode of the [2.2]-paracyclo-phane ligand in Ru3(CO)9(jU3- 7 77 77 -Ci6Hi6) can be adjusted to a terminal mode by reaction with diphenyl-acetylene in the presence of trimethylamine-A -oxide the alkyne caps the triruthenium face in the place of the arene. When the reaction is conducted thermally in the absence of trimethylamine-W-oxide, additional products are also observed, including a related compound formed by insertion of a CO ligand into the alkyne-metal bond as shown in Scheme 3. 

The ligand migration or insertion reaction is a fundamental mechanistic process of organometallic chemistry. As relating to acetylene complexes, it may be represented as shown in [Eq. (58)]. The reaction thus may be viewed either as an addition of M—L to the acetylene or, alternatively, as an insertion of the acetylene into the M—L bond. In cases where L = carbon monoxide, alkene, acetylene, or other potentially bidentate ligands, L may remain bonded to the metal, resulting in metallocycle formation [Eq. (57)]. 

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