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

Ian A. Cade, Michael J. Ingleson*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

Abstract

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 QOBCl2AlCl3, 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 Me2QOBCl+. 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.

Original languageEnglish
Pages (from-to)12874-12880
Number of pages7
JournalChemistry - A European Journal
Volume20
Issue number40
DOIs
Publication statusPublished - 26 Sep 2014

Keywords

  • 1 2-carboboration
  • borenium
  • borylation
  • migratory aptitude
  • vinyl boronates

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