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 center dot 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 center dot NMe2H to give the amino-borane H2B = NMe2, dimerization of this to give the final product, formation of the linear diborazane H3B center dot NMe2BH2 center dot 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)(2)H2Cl; the dimerization of H2B = NMe2 can be accelerated by MeCN; and complementary nonclassical BH center dot center dot center dot 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 center dot NMe2H.