Replacing C6F5 groups with Cl and H atoms in frustrated Lewis pairs: H2 additions and catalytic hydrogenations

Abstract

2-(Dialkylamino)phenylboranes containing the BXZ group, where X, Z = C₆F₅, Cl, and H, were prepared in a few synthetic steps and demonstrated the cleavage of H₂ under mild conditions. Depending on the nature of the dialkylamino group, X, and Z, the stability of the produced zwitterionic H₂ adducts varies from isolated solids indefinitely stable in an inert atmosphere to those quickly equilibrating with the initial aminoborane and H₂. Using a combined experimental/computational approach on a series of isostructural aminoboranes (dialkylamino = 2,2,6,6-tetramethylpiperid-1-yl), it was demonstrated that the electronegativity and the steric effect of the substituents generally follow the trend C₆F₅ ∼ Cl ≫ H. This observation is useful for designing new FLPs for practical applications. As an example, we demonstrated the hydrogenation of alkynes to cis-alkenes under mild conditions that was catalyzed by a chloro-analogue of the C₆F₅-substituted aminoborane developed previously. The presence of a BHCl group in the aminochloroboranes or in their H₂ adducts features facile redistribution of the H and Cl atoms and the formation of polychloro and polyhydrido species.