syn‐1,2‐Carboboration of Alkynes with Borenium Cations

The reaction of 8‐(trimethylsiloxy)quinoline ( QOTMS ) with BCl₃ and (aryl)BCl₂ forms QOBCl₂ and QOBCl(aryl). The subsequent addition of stoichiometric AlCl₃ follows one of two paths, dependent on the steric demands of the QO ligand and the electrophilicity of the resulting borenium cation. The phenyl‐ and 5‐hexylthienylborenium cations, QOBPh⁺ and QOBTh⁺ , are formed, whereas QOBCl⁺ is not. Instead, AlCl₃ preferentially binds with QOBCl₂ at oxygen, forming QOBCl₂?AlCl₃ , rather than abstracting chloride. A modest increase in the steric demands around oxygen, by installing a methyl group at the 7‐position of the quinolato ligand, switches the reactivity with AlCl₃ back to chloride abstraction, allowing formation of QOBCl⁺ . All the prepared borenium cations are highly chlorophilic and exhibit significant interaction with AlCl₄^? resulting in an equilibrium concentration of Lewis acidic “AlCl₃” species. The presence of “AlCl₃^” species limits the alkyne substrates compatible with these borenium systems, with reaction of [ QOBPh][AlCl₄] with 1‐pentyne exclusively yielding the cyclotrimerised product, 1,3,5‐tripropylbenzene. In contrast, QOBPh⁺ and QOBTh⁺ systems effect the syn‐1,2‐carboboration of 3‐hexyne. DFT calculations at the M06‐2X/6‐311G(d,p)/PCM(DCM) level confirm that the higher migratory aptitude of Ph versus Me leads to a lower barrier to 1,2‐carboboration relative to 1,1‐carboboration.