AbstractA metal-free synthesis of 2-benzylideneindolin-3-ones through a formal [4+1] annulation from o-azidobenzaldehydes and terminal alkynes has been developed. The method features operational simplicity, mild reaction conditions, and ready availability of starting materials. A broad range of 2-benzylideneindolin-3-ones were prepared in moderate to excellent yields. Mechanistic studies indicated that the reaction might proceed through a nucleophilic addition/rearrangement/azide–alkene cycloaddition pathway to produce 2-benzylideneindolin-3-ones.