Transition metal-free formal [2+2+2] cycloaddition reactions of alkynes

Cyclotrimerization reactions of alkynes and their oligomers with themselves or other heterocumulenes are usually performed by stoichiometric or catalytic mediacy of transition metal complexes. However, a number of transition metal-free variations have appeared, ranging from stepwise cyclizations of alkynes or oligoalkynes under rather drastical reaction conditions to trimeric condensation reactions of ketoalkynes under amine catalysis and finally uncatalyzed cyclizations of oligoalkynes under partially rather mild conditions. Consequentially a range of different reaction conditions and mechanism are involved in these transformations. The presented digest accounts selected examples of such transition metal-free formal [2+2+2] cycloadditions and exemplarily discusses their mechanistic course of action.