Wagner JP. 2
H-Imidazol-2-one
O-Oxide: A Criegee Intermediate from a σ
0π
2 Singlet Ground-State Carbene.
J Am Chem Soc 2022;
144:5937-5944. [PMID:
35325537 DOI:
10.1021/jacs.1c13705]
[Citation(s) in RCA: 5] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/30/2022]
Abstract
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.
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