Department Chemie and Catalysis Research Center (CRC), Technische Universität München, Lichtenbergstraße 4, 85747, Garching, Germany.
Institut für Physikalische und Theoretische Chemie, Universität Regensburg, Universitätsstr. 31, 93053, Regensburg, Germany.
Angew Chem Int Ed Engl. 2020 Nov 23;59(48):21640-21647. doi: 10.1002/anie.202008384. Epub 2020 Sep 17.
The photochemical deracemization of spiro[cyclopropane-1,3'-indolin]-2'-ones (spirocyclopropyl oxindoles) was studied. The corresponding 2,2-dichloro compound is configurationally labile upon direct irradiation at λ=350 nm and upon irradiation at λ=405 nm in the presence of achiral thioxanthen-9-one as the sensitizer. The triplet 1,3-diradical intermediate generated in the latter reaction was detected by transient absorption spectroscopy and its lifetime determined (τ=22 μs). Using a chiral thioxanthone or xanthone, with a lactam hydrogen bonding site as a photosensitizer, allowed the deracemization of differently substituted chiral spirocyclopropyl oxindoles with yields of 65-98 % and in 50-85 % ee (17 examples). Three mechanistic contributions were identified to co-act favorably for high enantioselectivity: the difference in binding constants to the chiral thioxanthone, the smaller molecular distance in the complex of the minor enantiomer, and the lifetime of the intermediate 1,3-diradical.
我们研究了螺[环丙烷-1,3'-吲哚啉]-2'-酮（螺环丙基氧化吲哚）的光化学外消旋化。相应的 2,2-二氯化合物在直接照射λ=350nm 时或在存在非手性硫代吖啶酮-9-酮作为敏化剂时在λ=405nm 照射时，其构型不稳定。在后一种反应中生成的三重态 1,3-二自由基中间体通过瞬态吸收光谱检测，并确定其寿命（τ=22μs）。使用手性噻吨酮或呫吨酮，带有内酰胺氢键结合位点作为光引发剂，允许不同取代的手性螺环丙基氧化吲哚以 65-98%的产率和 50-85%ee（17 个实例）进行外消旋化。确定了三种有利于高对映选择性的机制贡献：与手性噻吨酮的结合常数差异、较小的非对映异构体复合物中的分子距离和中间 1,3-二自由基的寿命。
