singlet oxygen

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photoredox catalysis

synthesis spiro

Angewandte Chemie

The photochemical deracemization of spiro[cyclopropane-1,3'-indolin]-2'-ones (spirocyclopropyl oxindoles) was studied. For the corresponding 2,2-dichloro compound it was found that it 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 was determined (τ = 22 ms). Using a chiral thio-xanthone or xanthone both 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 favourably for high enantioselectivity. First, the binding constants to the chiral thioxanthone differ by a factor of 3 for the minor and the major enantiomers. Second, the molecular distance is smaller by ≈ 100 pm in the complex of the minor enantiomer, presumably promoting a more efficient energy transfer. Third, the lifetime of the intermediate 1,3-diradical exceeds the one of the complex, facilitating a racemic deactivation back to the ground state.

Photochemically Induced Ring Opening of Spirocyclopropyl Oxindoles: Evidence for a Triplet 1,3‐Diradical Intermediate and Deracemization by a Chiral Sensitizer