Synthesis of Iminoboryl <i>o</i>-Carboranes by Lewis Base Promoted Aminoborirane-to-Iminoborane Isomerization

Abstract

The iminoboryl <i>o</i>-carboranes (Me₃Si)-Cb-B≡N-R (Cb = B₁₀C₂H₁₀, <b>3a</b>, R = SiMe₃; <b>3b</b>, R = ^<i>t</i>Bu) have been successfully synthesized by tetrahydrofuran (THF)-promoted isomerization from the corresponding <i>o</i>-carborane-fused aminoboriranes Cb{BN(SiMe₃)R} (<b>2</b>). The synthetic protocol of the previously reported borirane <b>2a</b> was optimized. The borirane Cb{BN(SiMe₃)^<i>t</i>Bu} (<b>2b</b>) and the iminoboranes <b>3a</b> and <b>3b</b> were fully characterized by NMR, IR, and single-crystal X-ray diffraction analyses. The borirane <b>2a</b> isomerizes more readily than <b>2b</b>. 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 <b>3a</b> dimerized in the presence of a catalytic amount of ^<i>t</i>BuNC to give the cyclodimer <b>4</b>. Quick equilibrium between <b>4</b> and the isonitrile adduct <b>4·</b>^{<b><i>t</i></b>}<b>BuNC</b> was observed in solution.