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

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.