Synthesis of a Transient Cationic Phosphaborene and its Trapping by Facile Aliphatic C-H Bond Activation

The synthesis of a transient cationic phosphaborene [(Mes*)P=B(CAAC)]⁺ (Mes*=2,4,6,-tri-tert-butylphenyl, CAAC=cyclic alkylamino carbene) by halide abstraction from the B-brominated analogue is reported. This species was found to undergo rapid and selective intramolecular aliphatic C-H bond activation to yield a phosphinoborenium cation, which undergoes facile deprotonation to give a cyclic base-stabilized phosphaborene. Computational investigation of the mechanism of C-H activation indicates a boron-centred activation route with an exceptionally low barrier of 8 kJ mol^-1, followed by a nearly barrierless hydride migration from boron to phosphorus. This underscores the enhanced reactivity of phosphaborenes compared to their lighter iminoborane analogues and indicates that steric stabilization alone is likely insufficient to isolate persistent cationic phosphaborenes, necessitating the use of alternative design strategies.