Methyleneketenimines, due to their highly unsaturated and CN-cumulenic nature, represent a rare class of heterocumulenes with potential as building blocks for pharmaceuticals and molecular electronics. However, their systematic chemistry has been limited. Herein, we report a concise C═C═N homologation reaction, driven by the strain release of the three-membered-ring carbene, bis(diisopropylamino)cyclopropenylidene (BAC), which unlocks rapid and scalable access to a series of isolable methyleneketenimines, including the first example of a bis(methyleneketenimine), featuring terminal azetidine moieties (azetidinylideneketenimines). Mechanistic studies, supported by control experiments and computational analysis, elucidate the important role of a methyleneketeniminyl carbene intermediate in forming the target heterocumulene. Spectroscopic and electrochemical investigations show that the visible-light absorption and redox potentials correlate with the degree of N-substituent π-conjugation. Most importantly, the highly polarized nature of the C═C═C═N framework enables a multimodal reactivity platform, spanning from unprecedented coordination to transition metals (Au(I), Rh(I)), CO2 splitting, alkyne C≡C bond insertion, to divergent skeletal rearrangements via (2 + 2) or (3 + 2) cycloaddition. This work establishes methyleneketenimines as a versatile toolkit for accessing complex heterocycles and novel organometallic complexes.
Koike et al. (Mon,) studied this question.