| Abstract |
Interaction of the c/oso-decaborate clusters [(Bu4N)-N-n][B10H9(NCR)] (R = Me la, Et lb, Bu-t 1c, Ph 1d) with the azide [Ph3PNPPh3]N-3 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, IS h) to afford selectively the borylated tetrazoles [(Bu4N)-N-n](2)[B10H9(N4CR)] (2a-d; 88-96\% isolated yields) and the water-soluble Na-2[B10H9(N4CR)] species (3a-d; ca. 95\% isolated yield) after the metathetical reaction with NaBPh4 in MeOH/H2O. The reaction with N-3(-) represents the first example of the propargyl-allenyl anion type dipole cycloadclition (CA) to the nitrilium derivatives of any boron clusters. Reactions of la-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 degrees 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 H-1, C-13\H-1\, and B-11\H-1\ 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 N-3- toward the borylated nitriles in comparison with organic azides RN3 (by 10.8-18.7 kcal/mol in terms of Delta G(double dagger) values calculated at M06-2X) is mostly accounted for by the solvent effects, and these reactions are controlled by kinetic rather than thermodynamic factors. |
