Synthesis of Electrophiles Derived from Dimeric Aminoboranes and Assessing Their Utility in the Borylation of πnucleophiles

Clément R.P. Millet, Jürgen Pahl, Emily Noone, Kang Yuan, Gary S. Nichol, Marina Uzelac, Michael J. Ingleson

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Abstract

Dimeric aminoboranes, [H2BNR2]2 (R = Me or CH2CH2) containing B2N2 cores, can be activated by I2, HNTf2 (NTf2 = [N(SO2CF3)2]), or [Ph3C][B(C6F5)4] to form isolable H2B(μ-NR2)2BHX (for X = I or NTf2). For X = [B(C6F5)4]- further reactivity, presumably between [H2B(μ-NMe2)2BH][B(C6F5)4] and aminoborane, forms a B3N3-based monocation containing a three-center two electron B-(μ-H)-B moiety. The structures of H2B(μ-NMe2)2BH(I) and [(μ-NMe2)BH(NTf2)]2 indicated a sterically crowded environment around boron, and this leads to the less common O-bound mode of NTf2 binding. While the iodide congener reacted very slowly with alkynes, the NTf2 analogues were more reactive, with hydroboration of internal alkynes forming (vinyl)2BNR2 species and R2NBH(NTf2) as the major products. Further studies indicated that the B2N2 core is maintained during the first hydroboration, and that it is during subsequent steps that B2N2 dissociation occurs. In the mono-boron systems, for example, iPr2NBH(NTf2), NTf2 is N-bound; thus, they have less steric crowding around boron relative to the B2N2 systems. Notably, the monoboron systems are much less reactive in alkyne hydroboration than the B2N2-based bis-boranes, despite the former being three coordinate at boron while the latter are four coordinate at boron. Finally, these B2N2 electrophiles are much more prone to dissociate into mono-borane species than pyrazabole [H2B(μ-N2C3H3)]2 analogues, making them less useful for the directed diborylation of a single substrate.

Original languageEnglish
Pages (from-to)2638-2647
Number of pages10
JournalOrganometallics
Volume41
Issue number18
Early online date14 Sept 2022
DOIs
Publication statusPublished - 26 Sept 2022

Bibliographical note

Funding Information:
This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement no. 769599). We thank the mass spectrometry facility (SIRCAMS) at the University of Edinburgh for carrying out MS analysis.

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Organic Chemistry
  • Inorganic Chemistry

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