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

The reaction of 8-(trimethylsiloxy)quinoline (QOTMS) with BCl(3) and (aryl)BCl(2) forms QOBCl(2) and QOBCl(aryl). The subsequent addition of stoichiometric AlCl(3) 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, AlCl(3) preferentially binds with QOBCl(2) at oxygen, forming QOBCl(2)⋅AlCl(3), 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 AlCl(3) back to chloride abstraction, allowing formation of [Image: see text]QOBCl(+). All the prepared borenium cations are highly chlorophilic and exhibit significant interaction with AlCl(4)(−) resulting in an equilibrium concentration of Lewis acidic “AlCl(3)” species. The presence of “AlCl(3)(”) species limits the alkyne substrates compatible with these borenium systems, with reaction of [QOBPh][AlCl(4)] 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.