Rational Design and Synthesis of Unsaturated Se-Containing Osmacycles with σ-Aromaticity.

Isolation of the simplest 4π three-membered heterocycles (1H-azirine, oxirene, thiirene, and selenirene) remains a big challenge due to their π-antiaromaticity and significant ring strain. Here we demonstrate that the incorporation of a transition-metal fragment could stabilize the antiaromatic selenirene and pentalene frameworks simultaneously by density functional theory (DFT) calculations. Experimental verification leads to the Se-containing metallapolycycles, osmapentaloselenirenes, with remarkable thermal stability. The osmaselenirene unit in the metallapolycycle is determined to be the first example of σ-aromaticity dominating in an unsaturated Se-containing ring. Our results not only highlight a remarkable stabilization by the transition-metal but also widen the scope of σ-aromaticity in unsaturated rings, which is traditionally reserved for the domain of π-aromaticity.