Wagner J Philipp
Institut für Organische Chemie, Eberhard Karls Universität Tübingen, Auf der Morgenstelle 18, 72076 Tübingen, Germany.
J Am Chem Soc. 2022 Apr 6;144(13):5937-5944. doi: 10.1021/jacs.1c13705. Epub 2022 Mar 24.
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.
三线态卡宾和三线态分子氧可以结合，通过一个整体自旋允许的转变形成单线态羰基氧化物——克里奇中间体。虽然在三线态卡宾的情况下，该反应在扩散极限下进行，但单线态卡宾通常更不愿意与氧结合。与这种通常遇到的自旋选择性相反，σπ单线态基态卡宾2-咪唑-2-亚基在低至30 K的温度下与氧反应极快。这种低温反应的产物是单线态2-咪唑-2-酮氧化物，一种N-杂环克里奇中间体。通过三线态二氧的亲电活化，加成反应成为可能，其中氧的结合最初可以无障碍地进行。克里奇中间体在1105 cm处表现出前所未有的高O-O伸缩振动，这可以用具有O双键的共振结构来解释。此外，2-咪唑-2-酮和螺二氧杂环丙烷可以被鉴定为本报告的羰基氧化物的光分解产物。
