Mechanistic Studies of the Dehydrocoupling and Dehydropolymerization of Amine–Boranes Using a [Rh(Xantphos)]+ Catalyst

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Abstract

A detailed catalytic, stoichiometric, and mechanistic study on the dehydrocoupling of H3B·NMe2H and dehydropolymerization of H3B·NMeH2 using the [Rh(Xantphos)](+) fragment is reported. At 0.2 mol % catalyst loadings, dehydrocoupling produces dimeric [H2B-NMe2]2 and poly(methylaminoborane) (M(n) = 22,700 g mol(-1), PDI = 2.1), respectively. The stoichiometric and catalytic kinetic data obtained suggest that similar mechanisms operate for both substrates, in which a key feature is an induction period that generates the active catalyst, proposed to be a Rh-amido-borane, that reversibly binds additional amine-borane so that saturation kinetics (Michaelis-Menten type steady-state approximation) operate during catalysis. B-N bond formation (with H3B·NMeH2) or elimination of amino-borane (with H3B·NMe2H) follows, in which N-H activation is proposed to be turnover limiting (KIE = 2.1 ± 0.2), with suggested mechanisms that only differ in that B-N bond formation (and the resulting propagation of a polymer chain) is favored for H3B·NMeH2 but not H3B·NMe2H. Importantly, for the dehydropolymerization of H3B·NMeH2, polymer formation follows a chain growth process from the metal (relatively high degrees of polymerization at low conversions, increased catalyst loadings lead to lower-molecular-weight polymer), which is not living, and control of polymer molecular weight can be also achieved by using H2 (M(n) = 2,800 g mol(-1), PDI = 1.8) or THF solvent (M(n) = 52,200 g mol(-1), PDI = 1.4). Hydrogen is suggested to act as a chain transfer agent in a similar way to the polymerization of ethene, leading to low-molecular-weight polymer, while THF acts to attenuate chain transfer and accordingly longer polymer chains are formed. In situ studies on the likely active species present data that support a Rh-amido-borane intermediate as the active catalyst. An alternative Rh(III) hydrido-boryl complex, which has been independently synthesized and structurally characterized, is discounted as an intermediate by kinetic studies. A mechanism for dehydropolymerization is suggested in which the putative amido-borane species dehydrogenates an additional H3B·NMeH2 to form the "real monomer" amino-borane H2B═NMeH that undergoes insertion into the Rh-amido bond to propagate the growing polymer chain from the metal. Such a process is directly analogous to the chain growth mechanism for single-site olefin polymerization.

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