Stepwise B[triple bond, length as m-dash]N bond cleavage by isocyanides: access to the 3-azaborole structural motif

The reaction of ^TMS oCb-B[triple bond, length as m-dash]N-SiMe₃ (^TMS oCb = 1-trimethylsilyl-ortho-carboranyl) with the sterically demanding (2,4,6-tri-tert-butylphenyl)isocyanide enables the isolation of the corresponding 1 : 1 iminoborane-isocyanide adduct, a species considered to be the key intermediate in isocyanide-catalyzed dimerization. In sharp contrast, the less hindered isocyanides, namely (2,4,6-trimethylphenyl)isocyanide and (2,6-di-iso-propylphenyl)isocyanide, react with the o-carboranyl iminoborane in a 3 : 1 fashion. Two equivalents of isocyanide undergo a [2 + 1 + 1] cycloaddition with the B[triple bond, length as m-dash]N triple bond to furnish a 1,2-azaborete-3,4-diimine four-membered ring, while the third equivalent coordinates to the boron center of the ring. Subsequent reduction with Mg or Ca induces selective insertion of this third isocyanide into the BN bond, generating an unprecedented 3-azaborolediide species. Single crystal structure characterization and quantum chemical calculations reveal that the negative charge is extensively delocalized onto the o-carborane cage. Theoretical studies further establish a clear mechanistic pathway for the entire transformation.