Photoredox-Catalyzed Reductive Dimerization of Isatins and Isatin-Derived Ketimines: Diastereoselective Construction of 3,3′-Disubstituted Bisoxindoles

Reductive dimerization of isatin
and its derivatives can be regarded
as a step-economical pathway to construct 3,3′-disubstituted
bisoxindoles, which was unfortunately accompanied by severe direct
reduction as well as low efficiency. A visible-light driven, photoredox-catalytic
protocol was developed to readily furnish 3,3′-dihydroxy- (dl-, > 20:1 dr) and 3,3′-diamino-bisoxindoles
(meso-, 3.5:1 to 5:1 dr) in moderate to good yields,
successfully
circumventing the common problem. Two vicinal quaternary carbon centers
were effectively assembled under the irradiation of visible light.