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, under Ar). However, corresponding 1,4,5-trisubstituted tetrazoles 4a–d, 5, and 6 and 1,3,5-isomers 4′a–d, 5′, and 6′ were obtained by alkylation of 2a–d. The isomers were separated by column chromatography and identified by 2D NOESY NMR and X-ray crystallography (for 5 and 6′). Compounds 2a–d, 3a–d, 4a,b, 4′a–d, 5, 6, 5′, and 6′ were characterized by ICP-MS-based B analysis, high-resolution ESI-MS, molar conductivity, IR, and 1H, 13C­{1H}, and 11B­{1H} NMR spectroscopies. The structures of 2c, 5, and 6′ were elucidated by single-crystal X-ray diffraction. Theoretical calculations at the DFT level (B3LYP and M06-2X functionals) allowed the establishment of the reaction mechanism, which is stepwise in the case of the azide-ion CA and concerted asynchronous (by 30–43%) for the hypothetical CA of RN3. The higher reactivity of N3– toward the borylated nitriles in comparison with organic azides RN3 (by 10.8–18.7 kcal/mol in terms of ΔGs⧧ values calculated at M06-2X) is mostly accounted for by the solvent effects, and these reactions are controlled by kinetic rather than thermodynamic factors.