Bifurcated Hydrogen-Bond-Stabilized Boron Analogues of Carboxylic Acids.

The reactivity of a bulky -terphenylboronic acid, DmpB(OH) [; Dmp = 2,6-bis(2,4,6-trimethylphenyl)phenyl], toward three different N-heterocyclic carbenes has been examined. The reaction of with 1 equiv of bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) leads to the formation of a hydrogen-bonded carbene boronic acid adduct, , featuring strong O-H···C contacts. In contrast, more basic 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (IPrMe) and 1,3-di--butylimidazol-2-ylidene (IBu) deprotonate smoothly to afford the rare anionic boranuidacarboxylic acids and , respectively. Structural determination reveals that and bear unprecedented bifurcated hydrogen bonds with a BO unit as a double hydrogen-bond acceptor, which contribute significantly to stabilization of the highly reactive B═O double bond. Quantum-mechanical calculations were conducted to disclose the unique electronic properties of the multiple bonds, as well as the important hydrogen bonds in these compounds.