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.

中文翻译：

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螺环丙基氧吲哚的光化学诱导开环：三重态1，3-二基中间体和手性增敏剂脱硫的证据。

研究了螺[环丙烷-1,3'-吲哚啉] -2'-酮（螺环丙基羟吲哚）的光化学脱硝作用。相应的2,2-二氯化合物在波长为= 350 nm的直接照射下以及在以非手性噻吨酮-9-one为敏化剂的情况下在波长为λ = 405 nm照射下的结构不稳定。通过瞬态吸收光谱法检测了后者反应中生成的三重态1,3-双自由基中间体，并确定了其寿命（τ = 22μs）。使用具有内酰胺氢键位的手性噻吨酮或x吨酮作为光敏剂，可以脱除不同取代的手性螺环丙基氧杂吲哚，产率为65-98％，ee为50-85％（17个示例）。鉴定出三种机制有助于高对映选择性的协同作用：与手性噻吨酮的结合常数的差异，次要对映异构体的复合物中较小的分子距离以及中间1,3-双自由基的寿命。