ELECTROPHILIC HALOGENATION OF nido-5,6-C_2B_8H_(12)

摘要

Room-temperature chlorination of the dicarbaborane nido-5,6-C_2B_8H_(12) (1) with CCl_4 in the presence of anhydrous AlCl_3 yielded a mixture of 7-Cl-5,6-C_2B_8H_(11) (7-Cl-1) (yield 34%),4-Cl-5,6-C_2B_8H_(11) (4-Cl-1) (yield 20%),3-Cl-5,6-C_2B_8H_(11) (3-Cl-1) (yield 21%),and 3,4-Cl_2-5,6-C_2B_8H_(10) (3,4-Cl_2-1) (yield 26%),while a trisubstituted derivative,3,4,7-Cl_3-5,6-C_2B_8H_9 (3,4,7-Cl_3-1) (yield 80%),was isolated as a sole product at reflux.Bromination of compound 1 with elemental Br_2 in the presence of Al powder at ambient temperature in CS_2 gave a mixture of 7-Br-5,6-C_2B_8H_(11) (7-Br-1) (yield 31%) and 4,7-Br_2-5,6-C_2B_8H_(10) (4,7-Br_2-1) (yield 52%).The most selective was AlCl_3-catalyzed iodination of 1 in refluxing benzene,which resulted in the formation of 7-I-5,6-C_2B_8H_(11) (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 (~(11)B,~1H) combined with two-dimensional [~(11)B-~(11)B]-COSY and ~1H-{~(11)B(selec-tive)} NMR techniques.Various NMR effects of halo-substitution are discussed for the series of monosubstituted 7-X-5,6-C_2B_8H_(11) (7-X-1) compounds (X=Cl,Br,and I).