2-Imidazol-2-one -Oxide: A Criegee Intermediate from a σπ Singlet Ground-State Carbene.

Triplet carbenes and triplet molecular oxygen can combine to form singlet carbonyl -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 O. In contradiction to this customarily encountered spin selectivity, the σπ singlet ground-state carbene 2-imidazol-2-ylidene reacts extremely rapidly with O at temperatures as low as 30 K. The product of this cryogenic reaction is singlet 2-imidazol-2-one -oxide , an N-heterocyclic Criegee intermediate. The addition reaction becomes possible through the electrophilic activation of dioxygen in the triplet state, in which O binding can initially proceed without a barrier. Criegee intermediate exhibits an unprecedented high O-O stretching vibration at 1105 cm, which can be explained by a resonance structure with an O double bond. Furthermore, 2-imidazol-2-one and spiro-dioxirane could be identified as the photodecomposition products of the herein-reported carbonyl oxide.