The iminoboryl o-carboranes (Me3Si)–Cb–BN–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.
Among the parent borirane, benzoborirene and ortho‐dicarbadodecaborane‐fused borirane, the latter possesses the highest ring strain and the highest Lewis acidity according to our density functional theory (DFT) studies. The synthesis of this class of compounds is thus considerably challenging. The existing examples require either a strong π‐donating group or an extra ligand for B‐coordination, which nevertheless suppresses or completely turns off the Lewis acidity. The title compound, which possesses both features, not only allows the 1,2‐insertion of P=O, C=O or C≡N to proceed under milder conditions, but also enables the heretofore unknown dearomative 1,4‐insertion of Ar−(C=O)− into a B−C bond. The fusion of strained molecular systems to an o‐carborane cage shows great promise for boosting both the ring strain and acidity.
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