Probing Bowl-to-Bowl Inversion in Enantiopure Subphthalocyanines: Cofacial S N 2 vs S N 1 Pathways at a Tetracoordinate Boron Center

Understanding the bowl-to-bowl (BtB) inversion of bowl-shaped π-systems is essential to fully harnessing their topology in chiral molecular materials. In boron subphthalocyanines (SubPcs)─contracted porphyrinoids known for their high functional versatility─this conformational flexibility is locked by the tetrahedral boron atom hosted in the macrocycle. However, BtB inversion in SubPcs may become accessible via ligand exchange, offering a unique opportunity to explore not only their configurational stability but also the long-standing question of how substitution at tetracoordinate boron proceeds stereochemically. Herein, using enantiopure SubPcs as stereochemical probes, we demonstrate that a model axial ligand exchange reaction with BF₃ proceeds through a dual mechanistic manifold, in which the primary pathway involves a stereoretentive bimolecular mechanism resembling σ-bond metathesis (cSN2), accompanied by a minor SN1-like component involving macrocycle planarization and inversion. Supported by analysis of different enantiopure derivatives, comparative studies with achiral analogues, kinetic measurements, and DFT calculations, we reveal that the balance between retention and inversion─and the overall reaction kinetics─is governed by the electronic and steric environment of the macrocycle, the nature of the leaving group, and the strength of the Lewis acid. Altogether, these findings deepen our understanding of boron-centered substitution mechanisms, while shedding new light on the still underexplored configurational stability of curved π-systems.