Allenes, versatile unsaturated motifs in transitionmetal-catalysed [2+2+2] cycloaddition reactions

The development of efficient reactions that enable the construction of multiple bonds and/or stereogenic centres in a single synthetic operation is of great interest for greener and step-economical syntheses of complex organic targets. Transition-metal-catalysed [2+2+2] cycloadditions excell in this regard: no fewer than three new sigma bonds and a new ring system are formed from simple unsaturated components. Allenes constitute an important subclass among these. They are more reactive than simple alkenes and generate sp3 centres, with important stereochemical implications. In addition, further manipulation of the resulting cycloadduct through the remaining double bond is possible, increasing molecular complexity. This tutorial review highlights the value that allenes have as reagents in [2+2+2] cycloaddition and their great versatility for the development of methods to generate complex architecturesWe gratefully acknowledge the financial support of our own research in this area by the Spanish Ministry of Education and Science (MINECO) (Project CTQ2014-54306-P and a RyC contract to A. L.) and the DIUE of the Generalitat de Catalunya (Project 2014-SGR-931