Synthesis and Selective Carboboration Reactivity of a Flexible P-Stabilized Bromoborenium

The P-stabilized bromoborenium i-Pr₂P(o,o'-biphenylene)BBr⁺ was generated by bromide abstraction. Reaction with internal alkynes afforded ring-expanded products arising from 1,2-carboboration of the B-Cspacer bond. The reaction is both chemo- and regioselective. Following the addition of XylNC, all compounds were fully characterized, including by single-crystal X-ray diffraction. DFT calculations provide mechanistic insight and reveal the key factors governing this transformation.