Borenium Cations as Catalysts for the Reduction of Organic Molecules and Mechanistic Investigations into their Mode of Operation
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The generation and isolation of two novel borenium cations has been described. The observation that the reaction of the Lewis acid B(C6F5)3 and the Lewis base diazabicyclo[2.2.2]octane (DABCO) with pinacol borane (HBpin) resulted in the activation of the B–H bond of HBpin and formation of a stable borenium cation/borohydride salt. This stable salt was used as a catalyst in the hydroboration reaction. It was shown to catalytically reduce a wide array of substrates including imines, N-heterocycles, nitriles, and ketones using pinacol borane as the source of hydride. Another borenium ion, synthesized from trityl tetrakis-pentafluorophenyl borate, DABCO, and HBpin did not contain a nucleophilic borohydride counterion and it was isolated in the solid state. This salt was also found to reduce the same substrates with similar yields and reaction times. The mechanisms of both of these catalysts were investigated and were found to be proceeding by a similar borenium catalyzed process. Quantitative analysis of the initial rates of each catalyst under identical conditions yielded rate constants on the same order of magnitude which strongly suggested that both catalysts operated via similar mechanisms. Stoichiometric experiments and isotope labelling using deuterated pinacol borane demonstrated that the nucleophilic counterion was not a kinetically relevant reducing agent under the reaction conditions. Furthermore, these reactions and the use of an isolable iminium ion as a hydride acceptor indicated that the hydride delivery agent was a DABCO•HBpin adduct. The DABCO•HBpin adduct was observed spectroscopically at ambient and subzero temperatures. Lastly, the rate of reduction using pinacol borane and [d1]-pinacol borane were significantly different and produced a high kinetic isotope effect (KIE = kH/kD = 6.6 ± 0.2). This high KIE strongly indicates that hydride delivery is the rate limiting step in the catalytic cycle. With this knowledge an asymmetric model is discussed and the beginnings of the development of an asymmetric borenium cation catalyzed process are described.