2H-Imidazol-2-one O-Oxide: A Criegee Intermediate from a σ⁰π² Singlet Ground-State Carbene

Triplet carbenes and triplet molecular oxygen can combine to form singlet carbonyl O-oxide Criegee intermediates in an overall spin-allowed transformation. While this reaction runs at the diffusion limit in the case of triplet carbenes, singlet carbenes are commonly more reluctant to bind ³O₂. In contradiction to this customarily encountered spin selectivity, the σ⁰π² singlet ground-state carbene 2H-imidazol-2-ylidene 1 reacts extremely rapidly with ³O₂ at temperatures as low as 30 K. The product of this cryogenic reaction is singlet 2H-imidazol-2-one O-oxide 7, an N-heterocyclic Criegee intermediate. The addition reaction becomes possible through the electrophilic activation of dioxygen in the triplet state, in which O₂ binding can initially proceed without a barrier. Criegee intermediate 7 exhibits an unprecedented high O-O stretching vibration at 1105 cm^-1, which can be explained by a resonance structure with an O₂ double bond. Furthermore, 2H-imidazol-2-one 5 and spiro-dioxirane 6 could be identified as the photodecomposition products of the herein-reported carbonyl oxide.