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 H(3)B·NMe(2)H, to give the cyclic amino-borane [H(2)BNMe(2)](2), has been explored using catalysts based upon cationic [Rh(PCy(3))(2)](+) (Cy = cyclo-C(6)H(11)). 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 H(3)B·NMe(2)H to give the amino-borane H(2)B═NMe(2), dimerization of this to give the final product, formation of the linear diborazane H(3)B·NMe(2)BH(2)·NMe(2)H as an intermediate, and its consumption by both B-N bond cleavage and dehydrocyclization. Subtleties of the system include the following: the product [H(2)BNMe(2)](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(PCy(3))(2)H(2)Cl; the dimerization of H(2)B═NMe(2) 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 H(3)B·NMe(2)H.