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

Abstract

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 ΔG 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.

Synthesis of New Bioinorganic Systems Based on Nitrilium Derivatives of closo-Decaborate Anion and meso-Arylporphyrins with Pendant Amino Groups

“…[13][14][15][16][17][18] Previously we discovered convenient method of synthesis of disubstituted oxonium derivatives using CF 3 SO 3 H as electrophilic inductor. [19] This method is quite universal and allows to produce disubstituted derivatives containing various exo-polyhedral bonds.…”

Azide Chemistry – An Inorganic Perspective, Part II[‡][3+2]‐Cycloaddition Reactions of Metal Azides and Related Systems

“…[358] Just as for the reactions of [Co III (N 3 )(chelate)L] with RCN a three-centred transition state can be ruled out on grounds of coordinative saturation. So it would appear that in both cases the initial products form directly, viz.…”

New Method for Synthesis of Substituted N-Borylated Dipeptides Based on Acetonitrile Derivative of the closo-Dodecaborate Anion

“…It was previously shown that the addition of nucleophilic reagents of various natures to nitrile functional groups in anionic boron clusters can be considered as a convenient way of directed synthesis of their substituted derivatives [12][13][14][15][16][17][18][19][20][21][22][23][24], including those based on biologically active substances [6,25] and biomimetic systems [26,27]. It was found that the addition of tertbutyl esters of proteinogenic amino acids to nitrile derivatives of the closo-decaborate anion and their subsequent selective hydrolysis is a convenient method for the preparation of N-borylated peptides [28].…”