Collect. Czech. Chem. Commun. 2006, 71, 1549-1556

Electrophilic Halogenation of nido-5,6-C2B8H12

Josef Holub, Mario Bakardjiev and Bohumil Štíbr*

Institute of Inorganic Chemistry, Academy of Sciences of the Czech Republic, 250 68 Řež, Czech Republic


Room-temperature chlorination of the dicarbaborane nido-5,6-C2B8H12 (1) with CCl4 in the presence of anhydrous AlCl3 yielded a mixture of 7-Cl-5,6-C2B8H11 (7-Cl-1) (yield 34%), 4-Cl-5,6-C2B8H11 (4-Cl-1) (yield 20%), 3-Cl-5,6-C2B8H11 (3-Cl-1) (yield 21%), and 3,4-Cl2-5,6-C2B8H10 (3,4-Cl2-1) (yield 26%), while a trisubstituted derivative, 3,4,7-Cl3-5,6-C2B8H9 (3,4,7-Cl3-1) (yield 80%), was isolated as a sole product at reflux. Bromination of compound 1 with elemental Br2 in the presence of Al powder at ambient temperature in CS2 gave a mixture of 7-Br-5,6-C2B8H11 (7-Br-1) (yield 31%) and 4,7-Br2-5,6-C2B8H10 (4,7-Br2-1) (yield 52%). The most selective was AlCl3-catalyzed iodination of 1 in refluxing benzene, which resulted in the formation of 7-I-5,6-C2B8H11 (7-I-1) (yield 85%). Although performed under different conditions, the experiments point to the following order of reactivity of individual positions in 1 in electrophilic halogenation: 7 > 4 > 3. Individual compounds were isolated and purified by liquid chromatography and characterized by mass spectrometry and NMR spectroscopy (11B, 1H) combined with two-dimensional [11B-11B]-COSY and 1H-{11B(selective)} NMR techniques. Various NMR effects of halo-substitution are discussed for the series of monosubstituted 7-X-5,6-C2B8H11 (7-X-1) compounds (X = Cl, Br, and I).

Keywords: Boranes; Carboranes; Dicarbaboranes; Electrophilic substitutions; Halogenations; NMR spectroscopy.

References: 23 live references.