Unsymmetrical 9-Borafluorenes via Low-Temperature C−H Activation of m-Terphenylboranes
datasetposted on 27.11.2002, 00:00 by Rudolf J. Wehmschulte, Armando A. Diaz, Masood A. Khan
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The reaction of 2,6-(4-t-BuC6H4)2C6H3Li and with H2ClB·SMe2 or HCl2B·SMe2 in hexane solution afforded the m-terphenyl-substituted unsymmetrical 9-borafluorene 1-(4-tert-butylphenyl)-7-tert-butyl-9-(bis-2,6-(4-tert-butylphenyl)phenyl)-9-borafluorene, 1, in good to moderate yields. The related reaction of 2,6-(3,5-Me2C6H3)2C6H3Li with BH2Cl·SMe2 or BHCl2·SMe2 in toluene solution gave 1-(3,5-dimethylphenyl)-6,8-dimethyl-9-(bis-2,6-(3,5-dimethylphenyl)phenyl)-9-borafluorene, 3. Compounds 1 and 3 are air-stable fluorescent solids. The reactions of 2,6-(2-MeC6H4)2C6H3Li or 2,6-Mes2C6H3Li (which possess either two or no o- and o‘ ‘-hydrogens) with H2ClB·SMe2 gave the primary boranes [2,6-(2-MeC6H4)2C6H3BH2]2, 4, and [2,6-Mes2C6H3BH2]2, 5, respectively. Quenching of the reaction of 2,6-(4-t-BuC6H4)2C6H3Li with H2ClB·SMe2 after 1.5 h with pyridine resulted in the isolation of the primary borane [2,6-(4-t-BuC6H4)2C6H3BH2]2, 2, as the pyridine adduct 2·py, which after thermolysis at 190 °C gave 1-(4-tert-butylphenyl)-7-tert-butyl-9-borafluorene·pyridine, 7·py. Heating a C6D6 solution of 4 to 60−70 °C led to C−H activation and formation of a 1:1 adduct of monomeric 4 and 1-(2-methylphenyl)-5-methyl-9-borafluorene, 12. Reaction of 2 equiv of 2,6-(4-t-BuC6H4)2C6H3Li with H2ClB·SMe2 in hexane solution followed by addition of THF gave the very crowded diterphenyl borate [2,6-(4-t-BuC6H4)2C6H3]2B(μ-H)2Li(THF)2]2, 11, which can be converted to 1 by simple addition of water. Prolonged exposure of 1 to concentrated aqueous HCl led to B−C bond cleavage and formation of the sterically very crowded diterphenylborinic acid [2,6-(4-t-BuC6H4)2C6H3]2BOH, 15. All compounds have been characterized by 1H, 13C, and 11B NMR spectroscopy and mass spectrometry, and compounds 1, 4, and 11 have also been characterized by single-crystal X-ray crystallography.