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

ABSTRACT: 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 ?=350?nm and upon irradiation at ?=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 (?=22??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.

SUBMITTER: Li X 

PROVIDER: S-EPMC7756555 | biostudies-literature | 2020 Nov

REPOSITORIES: biostudies-literature