σ-Aromaticity in a Fully Unsaturated Ring

Aromaticity is one of the most fundamental and fascinating chemical topics, attracting both experimental and theoretical chemists owing to its many manifestations. Both sigma- and pi-aromaticity can be classified depending on the character of the cyclic electron delocalization. In general, sigma-aromaticity stabilizes fully saturated rings with sigma-electron delocalization whereas the traditional pi-aromaticity describes the pi-conjugation in fully unsaturated rings. Here, we demonstrate a strong correlation between nucleus-independent chemical shift (NICS) values and extra cyclic resonance energies (ECREs), which are used to evaluate the sigma-aromaticity in an unsaturated three-membered ring (3MR) of cyclopropene, which were computed by molecular orbital (MO) theory and valence bond (VB) theory, respectively. Further study shows that the fully unsaturated ring in methylenecyclopropene and its metallic analogy is sigma-aromatic. Our findings revolutionize the fundamental knowledge of the concept of sigma-aromaticity, thus opening an avenue to design sigma-aromaticity in other fully unsaturated systems, which are traditionally reserved as the domain of pi-aromaticity.