Borylated Tetrazoles from Cycloaddition of Azide Anions to Nitrilium Derivatives of closo-Decaborate Clusters

Interaction of the closo-decaborate clusters [Bun4N][B10H9(NCR)] (R = Me 1a, Et 1b, But 1c, Ph 1d) with the azide [Ph3PNPPh3]N3 proceeds immediately upon mixing the reagents in an MeCN solution at RT, giving the borylated 1,5-disubstituted tetrazoles [B10H9(N4CR)]2– in essentially quantitative yield. On a synthetic scale, sodium azide, NaN3, reacts similarly with the nitrile functionality of 1a–d in an acetonitrile suspension under mild conditions (RT, 15 h) to afford selectively the borylated tetrazoles [Bun4N]2[B10H9(N4CR)] (2a–d; 88–96% isolated yields) and the water-soluble Na2[B10H9(N4CR)] species (3a–d; ca. 95% isolated yield) after the metathetical reaction with NaBPh4 in MeOH/H2O. The reaction with N3– represents the first example of the propargyl-allenyl anion type dipole cycloaddition (CA) to the nitrilium derivatives of any boron clusters. Reactions of 1a–d with alkyl or aryl azides (p-Me-C6H4-COCH2N3, p-NO2-C6H4-CH2N3, PhN3) do not proceed even under harsh conditions (2 d, dry EtCN, 100 °C, un...