Transition metal-catalysed (4 + 3) cycloaddition reactions involving allyl cations

In this emerging area article, we focus on novel intramolecular transition metal catalysed (4 + 3)-cycloaddition reactions of allenedienes in which the allene acts as an allylic-cation surrogate. This process has emerged as a powerful tool for the construction not only of complex seven-membered rings containing compounds but also different types of useful molecular skeletons by the proper selection of the catalyst. The transformation proceeds with high chemo- and stereoselectivity mainly because it occurs through an exo-like concerted transition state which exhibits a clear in-plane aromatic character. Despite that, different reaction mechanisms (i.e. stepwise processes) are also possible depending on the nucleophilicity of the diene moiety.