Electrophilic Halogenation of closo-1,2-C₂B₈H₁₀

Initial studies on electrophilic halogenation of the dicarbaborane closo-1,2-C₂B₈H₁₀ (1) have been carried out to reveal that the substitution takes place at B7 and B10 vertexes, which are the most removed from the CH positions. The course of the halogenation is strongly dependent on the nature of the halogenation agent and reaction conditions. Individual reactions led to the isolation of the monosubstituted compounds 1,2-C₂B₈H₉-10-X (2) (where X = F, I) and 1,2-C₂B₈H₉-7-X (3) (where X = Cl, I). Disubstituted carboranes 1,2-C₂B₈H₈-7,10-X₂ (4) (where X = Cl, Br, I) were obtained under more forcing conditions. Individual halo derivatives were characterized by mass spectrometry and high-field NMR (¹¹B, ¹H,¹³C) spectroscopy combined with two-dimensional [¹¹B-¹¹B]-COSY, ¹H{¹¹B(selective)}, and [¹¹B-¹H]-correlation NMR techniques. All of the derivatives bearing a halogen substituent in the B10 position exhibit a remarkable antipodal ¹³C and ¹H NMR shielding at the CH1 vertex, increasing in the order H < I < Br < Cl < F. The structures of 1,2-C₂B₈H₈-7,10-X₂ derivatives (where X = Cl, I, 4b,d) were established by X-ray diffraction analyses.