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Abstract

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.