Boosting Turnover in the Triarylborane-Catalyzed Hydrogenation of N-Substituted Indoles via Olefin-to-Nitrogen Lewis Base Switching in H2-Cleavage Steps

The shelf-stable heteroleptic borane B(2,6-Cl₂C₆H₃)(3,5-Br₂-2,6-F₂C₆H)₂ (B⁷) efficiently catalyzes the solvent-free hydrogenation of various substituted indoles to indolines with an unprecedented turnover number of 8,500, which is more than 400-fold higher than that reported for B(C₆F₅)₃ under diluted conditions. Mechanistic studies revealed that this hydrogenation proceeds via an olefin-to-nitrogen switching of Lewis bases involved in the H₂-cleavage steps: initially, H₂ cleavage is mediated by a frustrated Lewis pair (FLP) comprising the indole C3-carbon and boron atoms, which then switches to an FLP system comprising the indoline nitrogen and boron atoms after formation of the indoline. This study demonstrates the potential of relatively benign main-group elements for the catalytic synthesis of valuable N-containing molecules using H₂.