Unsymmetrical Diaryl Borinium Cation: From Solvent-Dependent Radical Behavior to an Elusive Chloroborinium Intermediate.

The diaryl borinium cation ([Ar2B]+), featuring two vacant p-orbitals at the boron center, represents one of the most reactive species among known boranylium cations. To harness its extreme electron deficiency, we utilized a sterically demanding 2,6-dimesitylphenyl (Dmp) substituent to stabilize an unsymmetrically substituted dicoordinate boron cation, [Dmp-B-Mes]+ ([2]+). We demonstrate that this structural asymmetry unlocks the unusual reactivity of borinium cations, ranging from distinct radical reactions to regioselective metathesis processes. The one-electron reduction of [2]+ generates a partially saturated 9-borafluorene radical 4•, which exhibits solvent-dependent decomposition pathways. In addition to this neutral radical reactivity, the incorporation of the Dmp group enables a highly regioselective B-CMes metathesis reaction with Et3SiH and tBuCl. This strategy provides synthetic access to elusive hydrido- and chloroborinium intermediates. The existence of the transient [Dmp-B-Cl]+ intermediate ([9]+) was verified by fluoride-trapping experiments and supported by computational analysis, which revealed a stabilizing intramolecular borinium-π interaction. Overall, this study highlights the unique reactivity of unsymmetrical diaryl borinium cations and demonstrates the experimental accessibility to highly reactive borinium species bearing H and Cl substituents.