Recently, we found that the σ-aromaticity wass first determined to be dominating in an unsaturated system.. This research has been published on Angew. Chem. Int. Ed. (2015, 54, 3102) entitled “sigma-Aromaticity in an unsaturated ring: Osmapentalene derivatives containing a metallacyclopropene unit”.
In general, aromaticity can be clarified as π- and σ-aromaticity according to the type of electrons with major contributions. The traditional π-aromaticity generally describes the π-conjugation in fully unsaturated rings whereas σ-aromaticity may stabilize fully saturated rings with delocalization caused by σ-electron conjugation. We now report an example of σ-aromaticity in an unsaturated three-membered ring (3MR), which is supported by experimental observations and theoretical calculations. Specifically, when the 3MR in cyclopropaosmapentalene is cleaved by ethane via two isodesmic reactions, both of them are highly endothermic (+29.7 and +35.0 kcal mol-1). These positive values are sharply contrary to the expected exothermicity, indicating aromaticity in the 3MR. Further nucleus-independent chemical shift and anisotropy of the current-induced density calculations reveal the nature of σ-aromaticity in the unsaturated 3MR.
This research was supported by the National Basic Research Program of China, the National Science Foundation of China and the Program for New Century Excellent Talents in University.
URL link for article: http://onlinelibrary.wiley.com/doi/10.1002/anie.201411220/abstract
