Frustrated Lewis Acid-Base-Pair-Catalyzed Amine-Borane Dehydrogenation.

Abstract

Metal-free catalysis by sterically encumbered Lewis Acid-Base pairs, popularly known as frustrated Lewis pairs (FLPs), is gaining importance by the day due to its promise of providing a greener alternative to transition-metal-based catalysis. One of the stumbling blocks in achieving catalytic dehydrogenation of amine-boranes is catalyst deactivation by the reaction product. Herein, we have theoretically investigated the routes of a dimethylxanthene-derived B,P-FLP-catalyzed dehydrogenation of dimethylamine-borane (DMAB), a rare instance which avoids catalyst inhibition by the reaction product. Our computational findings reveal that the dehydrogenation proceeds via formation of the ion pair [FLP-H]- and [HMe2N-BH2-H-BH2-NMe2H]+. This step is followed by indirect B-H activation assisted by a second DMAB molecule and further H2 release via deprotonation by the PPh2 center. It is revealed that the binding of NMe2BH2 to the FLP is unfavorable which ensures smooth propagation of the catalytic cycle. Catalytic dehydrogenation by the same mechanistic pathway is somewhat inhibited in the case of ammonia-borane by the same FLP due to the latent stabilization provided by strong hydrogen bonding interaction to FLP-NH2BH2 adduct which renders partial deactivation of the catalyst.