2H-Imidazol-2-oneO-Oxide: A Criegee Intermediate from a σ⁰π²Singlet Ground-State Carbene

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.

The activated reaction of dichlorocarbene with triplet molecular oxygen

“…The sideways approach in the triplet transition state 3 TS add discussed earlier suggests that dichlorocarbene acts as an electrophile in its reaction with 3 O 2 . While this is in good agreement with the previously observed enhanced reactivity of highly electrophilic singlet carbenes, 25,26 nucleophilic N-heterocyclic carbenes (NHCs) were also reported to yield monoxidation products in their reaction with triplet molecular oxygen. 72 Thus, it seems worthwhile to study the impact of substitution on the 3 O 2 addition reaction in order to develop an understanding which singlet carbenes can undergo facile oxidation reactions.…”

“…16–19 Yet, some singlet carbenes were still observed to react with molecular oxygen at experimentally relevant timescales in laser flash photolysis (LFP) studies, which is often explained by an initial thermal excitation of the carbene to its low-lying triplet state. 20–23 In a similar vein, some highly electrophilic singlet carbenes like difluorovinylidene 24,25 and 2 H -imidazol-2-ylidene 26,27 feature swift reactions with molecular oxygen even at cryogenic temperatures under matrix isolation conditions. However, a rapid oxygen addition reaction would usually not be expected for dichlorocarbene due to its high energy triplet state precluding an initial thermal excitation mechanism.…”

Electronic Absorption Spectroscopy and Photochemistry of Criegee Intermediates

“…This comparative unreactivity arises from the long‐range repulsive interaction imparted by one of the partially occupied π* orbitals of 3 O 2 “attempting” to undergo in‐plane bonding with the more stable σ 2 π 0 configuration of the carbene. The σ 0 π 2 configuration of a carbene is usually of higher energy and thus uncommon, but recent work has identified the prompt reaction of 3 O 2 with the singlet 2H ‐imidazol‐2‐ylidene carbene with a σ 0 π 2 configuration to yield (after a fast ISC process) a stable singlet CI, even in an Ar matrix at 30 K (162).…”

Synthesis of Sterically Encumbered Thiourea S‐Oxides through Direct Thiourea Oxidation

“… [16] So far, 2 H ‐imidazole‐2‐one O ‐oxide is the only known N‐heterocyclic Criegee intermediate which could be prepared from a σ 0 π 2 singlet ground state carbene under matrix isolation conditions (Figure 1b ). [17] However, the nitrogen atoms cannot act as donors in this case, because the respective lone‐pairs are perpendicular to the molecule's π‐system. Due to a lack of accessibility of urea oxides, we reckoned that their mono‐sulfur analogues, namely thiourea S ‐oxides, might serve as more easily available and potentially persistent model compounds for CIs; [18] therefore, we set out to examine the viability of their preparation via direct thiourea monoxidation (Figure 1b ).…”