General Remarks on Transition Metal-Catalyzed Reactions of Alkynes

General Remarks on Transition Metal-Catalyzed Reactions of Alkynes 

Reactions between alkynes and transition metal compounds yield a surprising variety of products (76, 77), indicating nonspecific mechanisms of formation. At least for the reaction of alkynes with metal carbonyls any simple polar mechanism must be excluded, in view of the insensitivity of the reactions to the degree of polarity of the solvents. A radical mechanism would perhaps be better suited for a general description but this has so far been rejected, since inhibition of the reactions with f-butylphenol or hydroquinone proved unsuccessful (78). Likewise, iron carbonyls react with diphenylacetylene, using ethyl acrylate, vinyl methyl ketone or vinyl acetate as the solvent, without polymerization of the vinyl compounds (79). These experiments, however, do not fully eliminate the possibility of a radical mechanism. 

We have recently found (79a) that during pol)rmerization of both mono-and disubstituted acetylenes with a large variety of catalysts [e.g., phosphine substituted nickel carbonyls, nickelous cyanide, bis(acrylonitrile)-nickel. 

Acetylenes are a priori electron-donating ligands with only weak acceptor properties and their n complexes should be relatively labile. The initial step of at least some of the reactions could be a conversion of the complexed acetylene in its ground state to a biradical with sp hybridized carbon atoms. This activated acetylene complex may then stabilize itself in many ways, depending largely on the type of the additional coordinated ligands in the transition state. 

Longuet-Higgins and Orgel (SO) predicted that two ir-bonded acetylene molecules should combine to form a cyclobutadiene complex, a proposal which was brilliantly confirmed (81). The formation of four-membered rings, however, occurs only relatively rarely, and many different ways in which TT-complexed acetylenes react have become known. 

---