Enantioselective Oxidative Enolate Coupling of Oxindoles Catalyzed by Chiral Guanidinium Hypoiodite

Hypoiodite is an environmentally
friendly oxidant that has been
developed to drive a variety of bond-forming reactions. Recently,
asymmetric oxidative bond-forming reactions using hypoiodite in the
presence of chiral onium organocatalysts have been used to synthesize
carbon–heteroatom bonds, but asymmetric carbon–carbon
bond-forming reactions have not been reported. Herein, we present
an oxidative enolate coupling reaction of oxindoles to provide optically
active oxindole dimers bearing consecutive all-carbon quaternary stereogenic
centers, using a chiral guanidinium hypoiodite catalyst. This reaction
affords the corresponding homocoupling products with high enantioselectivity.
We also describe oxidative enolate cross-coupling reactions of two
oxindoles bearing electronically distinct substituents.