Synthesis of [amine(tert-butylisonitrile) dihydroboron(III)] cations via the antimony pentachloride complexes of amine-dihydrocyanoboranes. A new route to amine-carboxyboranes

The reaction of the cyanoborane complexes L·BH₂CN [where L = R₃N (R = Me, Et, Pr, Bu); quinuclidine (Q) and Ph₃P] with SbCl₅ in CCl₄ afforded 1:1 complexes. Coordination of antimony to the nitrogen atom of the cyano groupp was clearly implied by the 35-42 cm^-1 shift of the ν_CN cyano resonances towards the higher wave-numbers. The L·BH₂CN·SbCl₅ complexes, carrying a bridge-head CN function, readily transformed with Bu^tCl into the [BH₂(L)CNBu^t]SbCl₆ tert-butylnitrilium salts with high crystallization ability. In the case of L = R₃N and Q substituents partial chlorination of the BH bond accompanied the formation of the nitrilium salt. The relatively weak coordination of the Bu^tNC ligand in the above antimony complexes was indicated by the 2282-2292 cm-1 values observed for ν_CN, unexpectedly high for isonitrile complexes, as well as by the exchange reactions with strong donors (i.e. Me₂SO, Q, DMAP), giving rise to the liberation of the Bu^tNC. The synthetic utility of related complexes was shown by the highly efficient transformation of [BH₂(L)CNBu^t]SbCl₆ into Me·BH₂C(O)NHBu^t in an alkaline medium, whose acid hydrolysis furnishes Me₃N·BH₂COOH, a well-known and important compound in boron chemistry with high yield. Preparation of this substance with the presented new process, via the tert-butylnitrilium salt, is much more efficient than by means of the previously reported procedures employing the ethylnitrilium salt. The proposed structures of the new compounds L·BH₂CN·SbCl₅, [BH₂(L)CNBu^t]SbCl₆ and Me₃·BH₂C(O)NHBu^t were derived from IR, ¹H and ¹¹B NMR spectral data.