A stable but highly reactive phosphine-coordinated borenium: Metal-free dihydrogen activation and alkyne 1,2-carboboration
Résumé
Borenium cations have been found to be valuable analogues of boranes as a result of their cationic character which imparts high electrophilicity. Herein, we report the synthesis, characterization, and reactivity of a new type of borenium cation employing a naphthyl bridge and a strong intramolecular P→B interaction. The cation reacts with H\textlessinf\textgreater2\textless/inf\textgreater in the presence of PtBu\textlessinf\textgreater3\textless/inf\textgreater (frustrated Lewis pair (FLP) approach) but also on its own. The mechanism of the reaction between the borenium cation and H\textlessinf\textgreater2\textless/inf\textgreater in the absence of PtBu\textlessinf\textgreater3\textless/inf\textgreater has been investigated using deuterium-labeling experiments and DFT calculations. Both experiments and calculations imply the side-on coordination of H\textlessinf\textgreater2\textless/inf\textgreater to the B center, followed by heterolytic splitting and B-C bond cleavage. An uncommon syn 1,2-carboboration has also been observed upon reaction of the borenium ion with 3-hexyne. Versatile reactivity: A new type of borenium cation is reported in which a naphthyl bridge supports a strong P→B interaction. Borenium reacts with H\textlessinf\textgreater2\textless/inf\textgreater through side-on coordination of H\textlessinf\textgreater2\textless/inf\textgreater to boron, heterolytic splitting, and concomitant cleavage of the B-Mes bond. The molecule also reacts with 3-hexyne through a syn 1,2-carboboration reaction. NTf\textlessinf\textgreater2\textless/inf\textgreater-=triflimide. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.