Mechanistic understanding of alkyne cyclotrimerization on mononuclear and dinuclear scaffolds: [4 + 2] cycloaddition of the third alkyne onto metallacyclopentadienes and dimetallacyclopentadienes

Cyclotrimerization of alkynes catalyzed by transition metal complexes is a straightforward synthetic method for constructing a benzene skeleton in organic synthesis. Not only mononuclear complexes, but also multinuclear complexes act as catalysts for alkyne cyclotrimerization, and their reaction mechanisms have been intensively investigated toward developing highly efficient and regio- and chemo-selective catalysts. In this review, we summarize stoichiometric and catalytic alkyne coupling reactions on mononuclear and dinuclear scaffolds in relation to the reaction mechanism of alkyne cyclotrimerization, including our recent mechanistic approaches using dinuclear tantalum motifs.