Alkenes in [2+2+2] Cycloadditions.

Participation of alkenes and allenes in [2+2+2] cycloaddition reactions has attracted much attention recently. This version of the well-established alkyne cyclotrimerization renders interesting products, such as cyclohexadienes and other polycycles, through cascade processes. Many mechanistic variations are observed when using certain metal complexes as catalysts. The frequent generation of stereogenic centers has prompted the development of efficient asymmetric versions. This Minireview summarizes the efforts reported to date on the use of double bonds as partners in [2+2+2] cyclotrimerizations.