The versatile world of cumulene chemistry

Despite a history spanning over a century, cumulenes are often relegated to the realm of perceived curiosities rather than practical synthetic intermediates as a result of their high intrinsic reactivity. In this review, we bridge the gap between the known synthetic strategies for these "exotic" molecules and the potential of their reactivity. By surveying both even and odd [n]cumulenes, paying particular attention to the most accessible [3]cumulenes, alongside more limited but instructive examples of [4]- and [5]cumulenes, we demonstrate that these sp-hybridized frameworks offer exceptional synthetic versatility. Indeed, cumulenes can function effectively as nucleophiles, electrophiles, and dienophiles, enabling cyclooligomerization, cycloaddition, organometallic coupling, and other transformations. We describe how these reactions utilize the "naked" sp-hybridized carbon atoms of the cumulene and their substantial internal energy to access structurally diverse products that would otherwise be challenging or even impossible to obtain using more traditional routes. In doing so, we aim to showcase their potential in organic synthesis and highlight the opportunities they present for constructing novel molecular architectures. By reframing cumulenes as valuable synthetic building blocks, rather than mere curiosities, this review hopes to persuade chemists to incorporate these intriguing scaffolds more broadly into modern organic synthesis.