Selective Synthesis and Functionalization of an Acyclic Methylene‐Bridged‐Arene Trimer in a Cage

Abstract Arene‐formaldehyde condensation is a versatile reaction for producing various oligomeric/polymeric materials. However, the precise control of oligomerization degree is still challenging because the starting materials and intermediates have similar reactivities. Here, we demonstrate the selective synthesis of a methylene‐bridged arene trimer using the confined cavity of a coordination cage. The limited space of the cavity prevents unregulated polymerization. The confinement effect for the kinetic protection is also demonstrated by the subsequent site‐selective iodination of the trimer product within the cage.