Development of a Generic Mechanism for the Dehydrocoupling of Amine-Boranes: A Stoichiometric, Catalytic, and Kinetic Study of H₃B·NMe₂H Using the [Rh(PCy₃)₂]⁺ Fragment

The multistage Rh-catalyzed dehydrocoupling of the secondary amine-borane H₃B·NMe₂H, to give the cyclic amino-borane [H₂BNMe₂]₂, has been explored using catalysts based upon cationic [Rh­(PCy₃)₂]⁺ (Cy = cyclo-C₆H₁₁). These were systematically investigated (NMR/MS), under both stoichiometric and catalytic regimes, with the resulting mechanistic proposals for parallel catalysis and autocatalysis evaluated by kinetic simulation. These studies demonstrate a rich and complex mechanistic landscape that involves dehydrogenation of H₃B·NMe₂H to give the amino-borane H₂BNMe₂, dimerization of this to give the final product, formation of the linear diborazane H₃B·NMe₂BH₂·NMe₂H as an intermediate, and its consumption by both B–N bond cleavage and dehydrocyclization. Subtleties of the system include the following: the product [H₂BNMe₂]₂ is a modifier in catalysis and acts in an autocatalytic role; there is a parallel, neutral catalyst present in low but constant concentration, suggested to be Rh­(PCy₃)₂H₂Cl; the dimerization of H₂BNMe₂ can be accelerated by MeCN; and complementary nonclassical BH···HN interactions are likely to play a role in lowering barriers to many of the processes occurring at the metal center. These observations lead to a generic mechanistic scheme that can be readily tailored for application to many of the transition-metal and main-group systems that catalyze the dehydrocoupling of H₃B·NMe₂H