The Elusive 1,4‐Diazabutatrienes: Lurking in the Shadows

Isocyanides are isomers of cyanides in which the terminal carbon atom can exist in a divalent state. Their ambivalent electronic nature as nucleophile or electrophile and wide reactivity make the isocyanide scaffold a valuable synthon for the synthesis of structurally complex molecules, in particular azacycles. Although they are known to polymerize easily, the dimerization of isocyanides affording head‐to‐head dimers, namely 1,4‐diazabutatrienes R 1 ‐N=C=C=N–R 1 , has been scarcely explored. Nowadays the experimental isolation of this kind of heterocumulenes is still a challenge. In this minireview, covering a period of sixty years, we highlight the role of 1,4‐diazabutatienes as putative reaction intermediates in the homo‐ and heterodimerization of different types of isocyanides. Furthermore, we revise a set of reported reactions in which the formation of a transient 1,4‐diazabutatriene intermediate has not been proposed but could be considered as a plausible alternative.