Borinine-FLP Ring Expansion: Isolation of Eight-Membered B–P Rings Bridged by µ2 Chalcogenide and Chloronium Ions

Boron-phosphorus (B-P) frustrated Lewis pairs (FLPs) are an important class of compounds for activating various small molecules. Utilizing the ring expansion reactivity of 9-chloro-9-borafluorene, a borinine-based FLP was synthesized. Various five-membered main-group element heterocycles were obtained via the reaction of the FLP with Me3NO, S8, and Se. Subsequent reduction of these species yielded the ring-expanded compounds, each featuring bridging B-E-B (E = O, S, Se) bonds. Similarly, halide abstraction from the FLP with AgNTf2 led to the formation of a cationic ring-expanded compound with a bridging B-Cl-B motif. This motif constitutes one of the first examples of a boron-stabilized chloronium ion, as verified using in-depth bonding analysis methods. Mechanistic pathways for the reduction- and halide abstraction-mediated ring expansion reactions are proposed with the aid of density functional theory. Electronic structure computations were performed to determine the best representation of bonding interactions in each compound, suggesting phosophorus(V)-chalcogen double bonding and chalcogen-boron(iii) dative interactions within the heterocycles.