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 l=350 nm
and upon irradiation at l=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 (t=22 ms). 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.