Synthesis of Iminoboryl o‑Carboranes by Lewis Base Promoted Aminoborirane-to-Iminoborane Isomerization

The iminoboryl o-carboranes (Me3Si)–Cb–BN–R (Cb = B10C2H10, 3a, R = SiMe3; 3b, R = t Bu) have been successfully synthesized by tetrahydrofuran (THF)-promoted isomerization from the corresponding o-carborane-fused aminoboriranes Cb­{BN­(SiMe3)­R} (2). The synthetic protocol of the previously reported borirane 2a was optimized. The borirane Cb­{BN­(SiMe3) t Bu} (2b) and the iminoboranes 3a and 3b were fully characterized by NMR, IR, and single-crystal X-ray diffraction analyses. The borirane 2a isomerizes more readily than 2b. The kinetics study revealed a bimolecular mechanism between borirane and THF, which is in good agreement with the computationally proposed reaction pathway. The title compounds are thermally robust, but compound 3a dimerized in the presence of a catalytic amount of t BuNC to give the cyclodimer 4. Quick equilibrium between 4 and the isonitrile adduct 4· t BuNC was observed in solution.