cyclotrimerization

Cyclotrimerization is a chemical reaction in which three unsaturated molecules unite to form a cyclic product. The most studied form is the [2+2+2] cyclotrimerization of alkynes, in which three alkyne units assemble to give a substituted benzene ring, typically a 1,3,5-triarylbenzene framework. The reaction is typically catalyzed by transition-metal complexes such as nickel, cobalt, iron, ruthenium, or molybdenum compounds. In these processes, the metal center binds the three alkynes and promotes their union through metallacyclic intermediates, with eventual aromatization of a cyclohexatriene to the benzene core. The substituents on the starting alkynes are delivered to the corresponding positions on the benzene ring, allowing rapid construction of multi-substituted arenes.

Substrates can include terminal and internal alkynes; three different alkynes can combine to yield asymmetrical triarylated benzenes, while three identical alkynes give symmetrical products. Reaction scope is influenced by electronic and steric properties of substituents and by the choice of catalyst and reaction conditions, including solvent, temperature, and pressure.

Applications and significance: Cyclotrimerization provides a method to assemble complex arenes efficiently from simple alkynes, and is used in the synthesis of functional materials, dyes, and ligands for catalysis. Limitations include regioselectivity control, competing oligomerization or polymerization, and the need for specific catalysts to achieve high yields.