Progressing the Frustrated Lewis Pair Abilities of N-Heterocyclic Carbene/GaR3 Combinations for Catalytic Hydroboration of Aldehydes and Ketones

Exploiting the steric incompatibility of the tris(alkyl)gallium GaR₃ (R = CH₂SiMe₃) and the bulky N-heterocyclic carbene (NHC) 1,3-bis(tert-butyl)imidazol-2-ylidene (ItBu), here we report the B-H bond activation of pinacolborane (HBPin), which has led to the isolation and structural authentication of a novel ion pair, [{ItBu-BPin}⁺{GaR₃(μ-H)GaR₃}^-] (2). Contrastingly, neither ItBu or GaR₃ was able to react with HBPin under the conditions of this study. Combining an NHC-stabilized borenium cation, [{ItBu-BPin}⁺], with an anionic dinuclear gallate, [{GaR₃(μ-H)GaR₃}^-], 2 proved to be unstable in solution at room temperature, evolving to the abnormal NHC-Ga complex [BPinC{{N(tBu)]₂CHCGa(R)₃}] (3). Interestingly, the structural isomer of 2, with the borenium cation residing at the C4 position of the carbene, [{aItBu-BPin}⁺{GaR₃(μ-H)GaR₃}^-] (4), was obtained when the abnormal NHC complex [aItBu·GaR₃] (1) was heated to 70 °C with HBPin, demonstrating that, under these forced conditions, it is possible to induce thermal frustration of the Lewis base/Lewis acid components of 1, enabling the activation of HBPin. Building on these stoichiometric studies, the frustrated Lewis pair (FLP) reactivity observed for the GaR₃/ItBu combination with HBPin could then be upgraded to catalytic regimes, allowing the efficient hydroboration of a range of aldehydes and ketones under mild reaction conditions. Mechanistic insights into the possible reaction pathway involved in this process have been gained by combining kinetic investigations with a comparative study of the catalytic capabilities of several gallium and borenium species related to 2. Disclosing a new cooperative partnership, reactions are proposed to occur via the formation of a highly reactive monomeric hydride gallate, [{ItBu-BPin}⁺{GaR₃(H)}^-] (I). Each anionic and cationic component of I plays a key role for success of the hydroboration, with the nucleophilic monomeric gallate anion favoring the transfer of its hydride to the C═O bond of the organic substate, which in turn is activated by coordination to the borenium cation.