Could π-aromaticity cross an unsaturated system to a fully saturated one?

The classification of π-/σ-aromaticity depends on the electrons with the dominating contributions.Traditionally,π-andσ-aromaticity are used to describe the unsaturated and saturated systems,respectively.Thus,it is rarely reported that π-aromaticity is dominated in a saturated system.Here we demonstrate that π-aromaticity could be dominating in several fully saturated four-membered rings(4MRs),supported by various aromaticity indices including ΔBL,NICS,EDDB,MCI,and Ad NDP.The origin of suchπ-aromaticity in saturated rings could be attributed to an introduction of two additional electrons into the π-type LUMO of the parent neutral species.Our findings represent a novel approach to achieve π-aromaticity into a fully saturated system which has traditionally been dominated by σ-aromaticity.