Synthesis of Mono- and Acyclic Bis-aminoboranes via Controlled Hydroboration 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[BArCl4]; 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.