Boosting Turnover\nin the Triarylborane-Catalyzed Hydrogenation\nof <i>N</i>‑Substituted Indoles via Olefin-to-Nitrogen\nLewis Base Switching in H<sub>2</sub>‑Cleavage Steps

The shelf-stable heteroleptic borane B(2,6-Cl<sub>2</sub>C<sub>6</sub>H<sub>3</sub>)(3,5-Br<sub>2</sub>-2,6-F<sub>2</sub>C<sub>6</sub>H)<sub>2</sub> (<b>B</b><sup><b>7</b></sup>) efficiently\ncatalyzes the solvent-free hydrogenation of various substituted indoles\nto indolines with an unprecedented turnover number of 8,500, which\nis more than 400-fold higher than that reported for B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> under diluted conditions. Mechanistic\nstudies revealed that this hydrogenation proceeds via an olefin-to-nitrogen\nswitching of Lewis bases involved in the H<sub>2</sub>-cleavage steps:\ninitially, H<sub>2</sub> cleavage is mediated by a frustrated Lewis\npair (FLP) comprising the indole C3-carbon and boron atoms, which\nthen switches to an FLP system comprising the indoline nitrogen and\nboron atoms after formation of the indoline. This study demonstrates\nthe potential of relatively benign main-group elements for the catalytic\nsynthesis of valuable N-containing molecules using H<sub>2</sub>.