Heterogeneous or homogeneous catalysis? Mechanistic studies of the rhodium-catalyzed dehydrocoupling of amine-borane and phosphine-borane adducts

In depth, comparative studies on the catalytic dehydrocoupling of the amine-borane adduct (Me2NHBH3)-B-. (to form [Me2N-BH2](2)) and the phosphine-borane adduct (Ph2PHBH3)-B-. (to form Ph2PH-BH2-PPh2-BH3) with a variety of Rh (pre)catalysts such as [{Rh(1,5-cod)(mu-Cl)}(2)], Rh/Al2O3, Rh-colloid/[Oct(4)N]Cl, and [Rh(1,5-cod)(2)]OTf have been performed in order to determine whether the dehydrocoupling proceeds by a homogeneous or heterogeneous mechanism. The results obtained suggest that the catalytic dehydrocoupling of (Me2NHBH3)-B-. is heterogeneous in nature involving Rh(0) colloids, while that of (Ph2PHBH3)-B-. proceeds by a homogeneous mechanism even when starting with Rh(0) precursors such as Rh/Al2O3. The catalytic dehydrocoupling reactions are thought to proceed by different mechanisms due to a combination of factors such as (i) the greater reducing strength of amine-borane adducts, (ii) the increased ease of dissociation of phosphine-borane adducts, and (iii) phosphine ligation and/or poisoning of active catalytic sites on metal colloids.