Understanding the Subtleties of Frustrated Lewis Pair Activation of Carbonyl Compounds by N-Heterocyclic Carbene/Alkyl Gallium Pairings

Marina Uzelac, David R. Armstrong, Alan R. Kennedy, Eva Hevia

Research output: Contribution to journalArticlepeer-review

23 Citations (SciVal)

Abstract

This study reports the use of the trisalkylgallium GaR3(R=CH2SiMe3), containing sterically demanding monosilyl groups, as an effective Lewis-acid component for frustrated Lewis pair activation of carbonyl compounds, when combined with the bulky N-heterocyclic carbene 1,3-bis(tert-butyl)imidazol-2-ylidene (ItBu) or 1,3-bis(tert-butyl)imidazolin-2-ylidene (SItBu). The reduction of aldehydes can be achieved by insertion into the C=O functionality at the C2 (so-called normal) position of the carbene affording zwitterionic products [ItBuCH2OGaR3] (1) or [ItBuCH(p-Br-C6H4)OGaR3] (2), or alternatively, at its abnormal (C4) site yielding [aItBuCH(p-Br-C6H4)OGaR3] (3). As evidence of the cooperative behaviour of both components, ItBu and GaR3, neither of them alone are able to activate any of the carbonyl-containing substrates included in this study NMR spectroscopic studies of the new compounds point to complex equilibria involving the formation of kinetic and thermodynamic species as implicated through DFT calculations. Extension to ketones proved successful for electrophilic α,α,α-trifluoroacetophenone, yielding [aItBuC(Ph)(CF3)OGaR3] (7). However, in the case of ketones and nitriles bearing acidic hydrogen atoms, C−H bond activation takes place preferentially, affording novel imidazolium gallate salts such as [{ItBuH}+{(p-I-C6H4)C(CH2)OGaR3}] (8) or [{ItBuH}+{Ph2C=C=NGaR3}] (12).

Original languageEnglish
Pages (from-to)15826-15833
Number of pages8
JournalChemistry - A European Journal
Volume22
Issue number44
DOIs
Publication statusPublished - 24 Oct 2016

Bibliographical note

Publisher Copyright:
© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Keywords

  • carbene ligands
  • carbonyl compounds
  • cooperative chemistry
  • frustrated Lewis pairs
  • gallium

ASJC Scopus subject areas

  • Catalysis
  • Organic Chemistry

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