Metallaphosphirenes with σ-Aromaticity: Capture of Phosphorus Atom by Metalla-Aromatics

Unsaturated phosphorus heterocycles typically exhibit diminished π-aromaticity. A unique entry into metallaphosphirenes featuring σ-aromaticity within an unsaturated framework is reported. Reaction of sodium phosphaethynolate (NaOCP) with cyclic metal carbyne 1 affords phosphorus atom transfer through a formal [1 + 2] cycloaddition pathway. The formation of metallaphosphirene ring was computationally modelled using density functional theory, revealing that [1 + 2] cycloaddition involves direct nucleophilic attack step and, thus, depends on the amphiphilicity of carbyne atom. Comprehensive DFT calculations, including NICS, ACID, GIMIC, EDDB, and NOBD analysis, confirm the predominantly σ-aromatic nature of the metal-based phosphirene system. The reactivity of the metallaphosphirene in cycloaddition reactions is in accord with its σ-aromaticity, which tends to retain the three-membered heterocycle with λ³-P atom.