Journal
ANGEWANDTE CHEMIE-INTERNATIONAL EDITION
Volume 59, Issue 48, Pages 21640-21647Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/anie.202008384
Keywords
enantioselectivity; hydrogen bonds; photochemistry; time-resolved spectroscopy; xanthones
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Funding
- Deutsche Forschungsgemeinschaft [Ba 1372/24-1]
- Alexander von Humboldt foundation
- Projekt DEAL
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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 lambda=350 nm and upon irradiation at lambda=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 (tau=22 mu s). 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.
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