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

Abstract

Isocyanides are isomers of cyanides in which its terminal carbon atom can exist in a divalent state. Their ambivalent electronic nature as nucleophile or electrophile and wide reactivity turn the isocyanide scaffold in 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 R1-N=C=C=N-R1, 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. Added to this, 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.