Pflaum, Niklas and Pauls, Mike and Kumar, Ajeet and Kutta, Roger Jan and Nuernberger, Patrick and Hauer, Juergen and Bannwarth, Christoph and Bach, Thorsten (2025) Oxetane Cleavage Pathways in the Excited State: Photochemical Kinetic Resolution as an Approach to Enantiopure Oxetanes. JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 147 (16). pp. 13893-13904. ISSN 0002-7863, 1520-5126
Full text not available from this repository. (Request a copy)Abstract
Chiral spirocyclic oxetanes [2-oxo-spiro(3H-indole-3,2 '-oxetanes)] were subjected to irradiation in the presence of a chiral thioxanthone catalyst (5 mol %) at lambda = 398 nm. An efficient kinetic resolution was observed, which led to an enrichment of one oxetane enantiomer as the major enantiomer (15 examples, 37-50% yield, 93-99% ee). The minor enantiomer underwent decomposition, and the decomposition products were carefully analyzed. They arise from a photocycloreversion (retro-Paterno-Buchi reaction) into a carbonyl component and an olefin. The cycloreversion offers two cleavage pathways depending on whether a C-O bond scission or a C-C bond scission occurs at the spirocyclic carbon atom. The course of this reaction was elucidated by a suite of mechanistic, spectroscopic, and quantum chemical methods. In the absence of a catalyst, cleavage occurs exclusively by initial C-O bond scission, leading to formaldehyde and a tetrasubstituted olefin as cleavage products. Time-resolved spectroscopy on the femtosecond/picosecond time scale, synthetic experiments, and calculations suggest the reaction to occur from the first excited singlet state (S1). In the presence of a sensitizer, triplet states are populated, and the first excited triplet state (T1) is responsible for cleavage into an isatin and a 1,1-diarylethene by an initial C-C bond scission. The kinetic resolution is explained by the chiral catalyst recruiting predominantly one enantiomer of the spirocyclic oxindole. A two-point hydrogen-bonding interaction is responsible for the recognition of this enantiomer, as corroborated by NMR titration studies and quantum chemical calculations. Transient absorption studies on the nanosecond/microsecond time scale allowed for observing the quenching of the catalyst triplet by either one of the two oxetane enantiomers with a slight preference for the minor enantiomer. In a competing situation with both enantiomers present, energy transfer to the major enantiomer is suppressed initially by the better-binding minor enantiomer and-as the reaction progresses-by oxindole fragmentation products blocking the binding site of the catalyst.
| Item Type: | Article |
|---|---|
| Uncontrolled Keywords: | ARENECARBOXYLIC ACID-ESTERS; 2+2 CYCLO-ADDITION; ENERGY-TRANSFER; CONFIGURATION; THIOXANTHONE; 2,2-DIARYLOXETANES; POLYMERIZATION; DERACEMIZATION; LUMINESCENCE; METATHESIS |
| Subjects: | 500 Science > 540 Chemistry & allied sciences |
| Divisions: | Chemistry and Pharmacy > Institut für Physikalische und Theoretische Chemie > Chair of Physical Chemistry I > Prof. Dr. Patrick Nürnberger |
| Depositing User: | Dr. Gernot Deinzer |
| Date Deposited: | 23 Jun 2026 06:18 |
| Last Modified: | 23 Jun 2026 06:18 |
| URI: | https://pred.uni-regensburg.de/id/eprint/67318 |
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