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

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• 作者：Heather C. Johnson ; Erin M. Leitao ; George R. Whittell ; Ian Manners ; Guy C. Lloyd-Jones ; Andrew S. Weller
• 刊名：Journal of the American Chemical Society
• 出版年：2014
• 出版时间：June 25, 2014
• 年：2014
• 卷：136
• 期：25
• 页码：9078-9093
• 全文大小：835K
• ISSN：1520-5126

文摘

A detailed catalytic, stoichiometric, and mechanistic study on the dehydrocoupling of H₃B路NMe₂H and dehydropolymerization of H₃B路NMeH₂ using the [Rh(Xantphos)]⁺ fragment is reported. At 0.2 mol % catalyst loadings, dehydrocoupling produces dimeric [H₂B鈭扤Me₂]₂ and poly(methylaminoborane) (M_n = 22鈥?00 g mol^鈥?, 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鈥揳mido鈥揵orane, that reversibly binds additional amine鈥揵orane so that saturation kinetics (Michaelis鈥揗enten type steady-state approximation) operate during catalysis. B鈥揘 bond formation (with H₃B路NMeH₂) or elimination of amino鈥揵orane (with H₃B路NMe₂H) follows, in which N鈥揌 activation is proposed to be turnover limiting (KIE = 2.1 卤 0.2), with suggested mechanisms that only differ in that B鈥揘 bond formation (and the resulting propagation of a polymer chain) is favored for H₃B路NMeH₂ but not H₃B路NMe₂H. Importantly, for the dehydropolymerization of H₃B路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^鈥?, PDI = 1.8) or THF solvent (M_n = 52鈥?00 g mol^鈥?, 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鈥揳mido鈥揵orane intermediate as the active catalyst. An alternative Rh(III) hydrido鈥揵oryl 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鈥揵orane species dehydrogenates an additional H₃B路NMeH₂ to form the 鈥渞eal monomer鈥?amino鈥揵orane H₂B鈺怤MeH that undergoes insertion into the Rh鈥攁mido 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.