Although there exists a variety of different catalysts for hydroboration of organic substrates such as aldehydes, ketones, imines, nitriles etc., recent evidence suggests that tetra-coordinate borohydride species, formed by activation, redistribution, or decomposition of boron reagents, are the true hydride donors. We then proposed that Me2S-BH3 could also act as a hydride donor for the reduction of various imines, as similar compounds have been observed to reduce carbonyl substrates. This boron reagent was shown to be an effective and chemoselective hydroboration reagent for a wide variety of imines.
Controlled hydroboration of imines using Me2S-BH3 has been developed as an alternative approach toward the synthesis of both mono- and acyclic bis-aminoboranes. One of the key synthetic steps involved isolation of the imine-BH3 adducts followed by their conversion to the corresponding mono-aminoboranes (R2N-BH2; R = alkyl, aryl). This particular transformation could be achieved thermally, but the best results in terms of product purities were obtained in the presence of catalytic amounts of a β-diketiminate stabilized aluminum complex (i.e., LAl(OTf)2/Na[BArCl 4]; Tf = O2SCF3, L = CH(CCH3)2(N(2,6-iPr2-C6H3))2, ArCl = 3,5-Cl2-C6H3). Symmetrical bis-aminoboranes ((R2N)2BH) could be prepared by mixing either imine-BH3 adducts or mono-aminoboranes with the corresponding free imines under the catalytic reaction conditions. Nevertheless, the most appropriate approach for the synthesis of unsymmetrical bis-aminoboranes was realized when imines were subjected to controlled hydroboration using mono-aminoboranes in the presence of the aluminum catalyst. Even though a majority of unsymmetrical bis-aminoboranes underwent the expected redistribution to their corresponding mono-aminoboranes, this process was averted in a few examples, which was attributed to the enhanced steric properties of the amino substituents.
A crucial step in the formation of mono-aminoboranes (R2N-BH2) from the corresponding imine-BH3 adducts, under mild reaction conditions, is the 1,3-hydride shift. We offer experimental and theoretical insights into this...
Reduction of various unsaturated organic substrates via hydroboration represents one of the most commonly used procedures. Nevertheless, controversial information about the mechanistic insights regarding this particular transformation still exists in the literature. In this work, we show that three-coordinate mono-aminoboranes do not act as effective hydroboration reagents as recently reported unless these boron-containing species are able to form four-coordinate analogues either through dimerization or addition of a base. Furthermore, we also demonstrate that imine-BH3 adduct(s) and mono-amino boranes do not represent key intermediates during Me2S-BH3 catalyzed hydroboration of imines using HBpin as recently reported. In fact, we believe that in both of these reported studies, the Clark mechanism is mainly, if not solely, responsible for the observed/examined hydroboration reactions.
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