Pflaum Niklas, Pauls Mike, Kumar Ajeet, Kutta Roger Jan, Nuernberger Patrick, Hauer Jürgen, Bannwarth Christoph, Bach Thorsten
Department Chemie and Catalysis Research Center (CRC), School of Natural Sciences Technische Universität München, D-85747 Garching, Germany.
Institut für Physikalische Chemie RWTH Aachen University, D-52074 Aachen, Germany.
J Am Chem Soc. 2025 Apr 23;147(16):13893-13904. doi: 10.1021/jacs.5c02483. Epub 2025 Apr 14.
Chiral spirocyclic oxetanes [2-oxo-spiro(3-indole-3,2'-oxetanes)] were subjected to irradiation in the presence of a chiral thioxanthone catalyst (5 mol %) at λ = 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% ). The minor enantiomer underwent decomposition, and the decomposition products were carefully analyzed. They arise from a photocycloreversion (retro-Paternò-Büchi 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 (S). In the presence of a sensitizer, triplet states are populated, and the first excited triplet state (T) 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.
手性螺环氧杂环丁烷[2-氧代-螺(3-吲哚-3,2'-氧杂环丁烷)]在手性硫杂蒽酮催化剂(5 mol%)存在下于λ = 398 nm处进行辐照。观察到了高效的动力学拆分，这导致一种氧杂环丁烷对映体富集为主对映体(15个实例，产率37 - 50%，ee值93 - 99%)。次要对映体发生分解，并对分解产物进行了仔细分析。它们源于光环化逆转(逆帕特诺-布齐反应)生成羰基组分和烯烃。根据螺环碳原子处发生的是C - O键断裂还是C - C键断裂，环化逆转提供了两条裂解途径。通过一系列机理、光谱和量子化学方法阐明了该反应的过程。在没有催化剂的情况下，裂解仅通过初始的C - O键断裂发生，生成甲醛和四取代烯烃作为裂解产物。飞秒/皮秒时间尺度上的时间分辨光谱、合成实验和计算表明反应从第一激发单重态(S)发生。在敏化剂存在下，三重态被填充，第一激发三重态(T)通过初始的C - C键断裂导致异吲哚酮和1,1 - 二芳基乙烯裂解。动力学拆分的解释是手性催化剂主要募集螺环氧化吲哚中的一种对映体。核磁共振滴定研究和量子化学计算证实，两点氢键相互作用负责对该对映体的识别。纳秒/微秒时间尺度上的瞬态吸收研究允许观察两种氧杂环丁烷对映体中的任何一种对催化剂三重态的猝灭，对次要对映体略有偏好。在两种对映体都存在的竞争情况下，最初，与催化剂结合更好的次要对映体抑制了向主要对映体的能量转移，并且随着反应的进行，氧化吲哚裂解产物会阻塞催化剂的结合位点，从而抑制能量转移。
