Cascade reaction involving intramolecular oxygen-migration: Efficient synthesis of 3-allylidene-indolin-2-one compounds under metal-free conditions

An efficient synthesis of 3-(diarylallylidene)oxindoles from 3-(1H-indol-3-yl)-1,1-diarylpropan-1-ol under metal-free conditions is described. NBS serves as a critical medium leading to a facile oxygen-migration in the transformation. The protocol has advantages of high efficiency, simple opreation, mild reaction conditions, good atom-economy, wide substrate scope and good yields of products. A detailed mechanism is proposed after careful investigation.