Phenonium-Ion-Mediated Skeletal Editing of Paracyclophanes

Phenonium ions have played a central role in physical-organic chemistry, yet their use as strategic intermediates in synthesis remains underexplored. Here, we report a phenonium-ion-mediated strategy for the skeletal editing of paracyclophanes. Installation of a one-carbon handle at the aliphatic bridge, followed by direct alcohol activation with BF₃·OEt₂ under solvolysis-avoiding conditions, generates a paracyclophane-derived phenonium ion that undergoes regioselective ring opening by external nucleophiles. This strategy provides modular access to functionalized higher paracyclophane homologues and tolerates S-, N-, C-, selenium-, and halide-based nucleophiles under operationally simple, gram-scalable conditions. The sequence can be applied iteratively and extended to double bridge expansion, while reductive desulfurization furnishes the corresponding parent hydrocarbons. Mechanistic studies combining nuclear magnetic resonance spectroscopy, kinetic analysis, stereochemical probes, and density functional theory calculations support a self-accelerating mechanism in which product water shifts BF₃ speciation toward BF₃·H₂O, which assists leaving-group departure through hydrogen-bond donation and thereby facilitates phenonium-ion formation. Together, these results establish paracyclophane-derived phenonium ions as controllable intermediates for skeletal editing and provide practical access to higher paracyclophane topologies with application potential in functional materials, as exemplified by a bridge-expanded thermally activated delayed fluorescence (TADF) emitter.