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 journalArticle

10 Citations (Scopus)

Abstract

This study reports the use of the trisalkylgallium GaR3 (R=CH2 SiMe3 ), 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 [ItBuCH2 OGaR3 ] (1) or [ItBuCH(p-Br-C6 H4 )OGaR3 ] (2), or alternatively, at its abnormal (C4) site yielding [aItBuCH(p-Br-C6 H4 )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-C6 H4 )C(CH2 )OGaR3 }(-) ] (8) or [{ItBuH}(+) {Ph2 C=C=NGaR3 }(-) ] (12).

LanguageEnglish
Pages15826-15833
Number of pages8
JournalChemistry - A European Journal
Volume22
Issue number44
Early online date26 Sep 2016
DOIs
Publication statusPublished - 24 Oct 2016

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Carbonyl compounds
Gallium
Ketones
Bearings (structural)
Chemical activation
Lewis Acids
Hydrogen
Nitriles
Aldehydes
Discrete Fourier transforms
Salts
Nuclear magnetic resonance
Thermodynamics
Atoms
Kinetics
Acids
Substrates
carbene

Keywords

  • trisalkylgallium
  • Lewis-acid
  • carbonyl compounds
  • frustrated Lewis pairs
  • boron
  • aluminium
  • cabene ligands
  • cooperative chemistry
  • gallium

Cite this

@article{48479d2f95074532b754bfb322f891d8,
title = "Understanding the subtleties of frustrated Lewis pair activation of carbonyl compounds by N-Heterocyclic carbene/alkyl gallium pairings",
abstract = "This study reports the use of the trisalkylgallium GaR3 (R=CH2 SiMe3 ), 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 [ItBuCH2 OGaR3 ] (1) or [ItBuCH(p-Br-C6 H4 )OGaR3 ] (2), or alternatively, at its abnormal (C4) site yielding [aItBuCH(p-Br-C6 H4 )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-C6 H4 )C(CH2 )OGaR3 }(-) ] (8) or [{ItBuH}(+) {Ph2 C=C=NGaR3 }(-) ] (12).",
keywords = "trisalkylgallium, Lewis-acid, carbonyl compounds, frustrated Lewis pairs, boron , aluminium, cabene ligands, cooperative chemistry, gallium",
author = "Marina Uzelac and Armstrong, {David R.} and Kennedy, {Alan R.} and Eva Hevia",
note = "This is the peer reviewed version of the following article: Uzelac, M., Armstrong, D. R., Kennedy, A. R., & Hevia, E. (2016). Understanding the subtleties of frustrated Lewis pair activation of carbonyl compounds by N-Heterocyclic carbene/alkyl gallium pairings. Chemistry - A European Journal, 22(44), 15826-15833, which has been published in final form at https://dx.doi.org/ 10.1002/chem.201603597. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.",
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Understanding the subtleties of frustrated Lewis pair activation of carbonyl compounds by N-Heterocyclic carbene/alkyl gallium pairings. / Uzelac, Marina; Armstrong, David R.; Kennedy, Alan R.; Hevia, Eva.

In: Chemistry - A European Journal, Vol. 22, No. 44, 24.10.2016, p. 15826-15833.

Research output: Contribution to journalArticle

TY - JOUR

T1 - Understanding the subtleties of frustrated Lewis pair activation of carbonyl compounds by N-Heterocyclic carbene/alkyl gallium pairings

AU - Uzelac, Marina

AU - Armstrong, David R.

AU - Kennedy, Alan R.

AU - Hevia, Eva

N1 - This is the peer reviewed version of the following article: Uzelac, M., Armstrong, D. R., Kennedy, A. R., & Hevia, E. (2016). Understanding the subtleties of frustrated Lewis pair activation of carbonyl compounds by N-Heterocyclic carbene/alkyl gallium pairings. Chemistry - A European Journal, 22(44), 15826-15833, which has been published in final form at https://dx.doi.org/ 10.1002/chem.201603597. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.

PY - 2016/10/24

Y1 - 2016/10/24

N2 - This study reports the use of the trisalkylgallium GaR3 (R=CH2 SiMe3 ), 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 [ItBuCH2 OGaR3 ] (1) or [ItBuCH(p-Br-C6 H4 )OGaR3 ] (2), or alternatively, at its abnormal (C4) site yielding [aItBuCH(p-Br-C6 H4 )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-C6 H4 )C(CH2 )OGaR3 }(-) ] (8) or [{ItBuH}(+) {Ph2 C=C=NGaR3 }(-) ] (12).

AB - This study reports the use of the trisalkylgallium GaR3 (R=CH2 SiMe3 ), 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 [ItBuCH2 OGaR3 ] (1) or [ItBuCH(p-Br-C6 H4 )OGaR3 ] (2), or alternatively, at its abnormal (C4) site yielding [aItBuCH(p-Br-C6 H4 )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-C6 H4 )C(CH2 )OGaR3 }(-) ] (8) or [{ItBuH}(+) {Ph2 C=C=NGaR3 }(-) ] (12).

KW - trisalkylgallium

KW - Lewis-acid

KW - carbonyl compounds

KW - frustrated Lewis pairs

KW - boron

KW - aluminium

KW - cabene ligands

KW - cooperative chemistry

KW - gallium

U2 - 10.1002/chem.201603597

DO - 10.1002/chem.201603597

M3 - Article

VL - 22

SP - 15826

EP - 15833

JO - Chemistry - A European Journal

T2 - Chemistry - A European Journal

JF - Chemistry - A European Journal

SN - 0947-6539

IS - 44

ER -