The cyclotrimerization of acetylenic compounds is a useful method for the construction of three new bonds in a one-step process. Although symmetry-allowed, there is a paucity of examples of purely thermal [2+2+2] cycloadditions. The first example reported was in 1866, which was the cyclization of acetylene to benzene. High temperatures are required for this reaction and a mixture of products is formed. In 1948, Reppe discovered the first transition-metal catalyzed version of this reaction in which nickel was used and substituted benzenes were formed. Since then catalysts based on transition metals, lanthanides, actinide, and aluminum have been developed for the cycloaddition of substituted alkynes to benzene derivatives. Many compounds can take part in the cyclization reactions to give four-, five-,six, or eight membered rings. Many of these reactions have found many applications in organic synthesis
This chapter is devoted to early to late transition metal-mediated cotrimerization of acetylenic compounds directed towards organic synthesis. It addresses the scope and generality of the reaction, as well as the current state of the art, with regard to region-and stereoselectivities. In the context of selectivity, a mechanistic overview is provided. Inter-and intramolecular versions of the cyclizations are presented with an emphasis on applications in synthesis. The literature up to mid-2004 is covered.