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

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).