Visible-Light-Mediated Intermolecular [2 + 2]-Cycloaddition Reaction of 3-Alkylideneindolin-2-one with Alkenes via Triplet Energy Transfer for the Synthesis of 3-Spirocyclobutyl Oxindoles

[2 + 2]-Cycloaddition is the most straightforward approach to the construction of cyclobutanes. In this paper, the intermolecular [2 + 2]-cycloaddition reaction of 3-alkylideneindolin-2-ones with alkenes was achieved. This reaction can be used in the synthesis of 3-spirocyclobutyl oxindoles, polycyclic oxindoles, and late stage modification of some drug molecules.