PyBroP-mediated nucleophilic addition of oxindoles with pyridine N-oxides to access 3-pyridyl-3,3-disubstituted oxindoles

A sustainable and efficient strategy for the synthesis of 3-pyridyl-3,3-disubstituted oxindole derivatives has been achieved via PyBroP (bromotripyrrolidinophosphonium hexafluorophosphate) promoted nucleophilic addition reaction of oxindoles with pyridine N-oxides. The approach worked well under relatively mild conditions. The protocol features a broad substrate scope and good functional group tolerance and gives products in moderate to excellent (52-95%) yields. An attractive topic of metal-free methods is to achieve oxindoles that bear an N-heteroaromatic group-substituted quaternary center at the 3-position.