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(mu-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(mu-H)GaR3}(-)], 2 proved to be unstable in solution at room temperature, evolving to the abnormal NHC-Ga complex [BPinC{{N(tBu)](2)CHCGa(R)(3)}] (3). Interestingly, the structural isomer of 2, with the borenium cation residing at the C4 position of the carbene, [{aItBu-BPin}(+){GaR3(mu-H)GaR3}(-)] (4), was obtained when the abnormal NHC complex [aItBu center dot GaR3] (1) was heated to 70 degrees 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.

Automated summary

This study explores the B-H bond activation of pinacolborane using the steric incompatibility of tris(alkyl) gallium GaR3 and N-heterocyclic carbene 1,3-bis(tert-butyl)imidazol-2-ylidene. A novel ion pair was isolated and structurally authenticated, and the reaction mechanism was elucidated through stoichiometric and catalytic studies. The results demonstrate the potential for efficient hydroboration of aldehydes and ketones under mild reaction conditions.