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

Interaction of the closo -decaborate clusters [Bu n 4 N][B 10 H 9 (NCR)] (R = Me 1a, Et 1b, Bu t 1c, Ph 1d ) with the azide [Ph 3 PNPPh 3 ]N 3 proceeds immediately upon mixing the reagents in an MeCN solution at RT, giving the borylated 1,5-disubstituted tetrazoles [B 10 H 9 (N 4 CR)] 2– in essentially quantitative yield. On a synthetic scale, sodium azide, NaN 3, 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 [Bu n 4 N] 2 [B 10 H 9 (N 4 CR)] ( 2a – d; 88–96% isolated yields) and the water-soluble Na 2 [B 10 H 9 (N 4 CR)] species ( 3a – d; ca. 95% isolated yield) after the metathetical reaction with NaBPh 4 in MeOH/H 2 O. The reaction with N 3 – 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-C 6 H 4 -COCH 2 N 3, p -NO 2 -C 6 H 4 -CH 2 N 3, PhN 3 ) 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 1 H, 13 C{ 1 H}, and 11 B{ 1 H} 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 RN 3 . The higher reactivity of N 3 – toward the borylated nitriles in comparison with organic azides RN 3 (by 10.8–18.7 kcal/mol in terms of Δ G s ⧧ values calculated at M06-2X) is mostly accounted for by the solvent effects, and these reactions are controlled by kinetic rather than thermodynamic factors.