Unsymmetrical 9-borafluorenes via low-temperature C-H activation of m-terphenylboranes

The reaction of 2,6-(4-t-BuC6H4)(2)C6H3Li and with H2ClB.SMe2 or HCl2B.SMe2 in hexane solution afforded the m-terphenyl-substituted unsymmetrical 9-borafluorene 1-(4-tertbutylphenyl)-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)(2)C6H3Li 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)(2)C6H3Li or 2,6-Mes(2)C(6)H(3)Li (which possess either two or no o- and o"-hydrogens) with H2ClB.SMe2 gave the primary boranes [2,6-(2-MeC6H4)(2)C6H3BH2](2), 4, and [2,6-Mes(2)C(6)H(3)BH(2)](2), 5, respectively. Quenching of the reaction of 2,6-(4-tBuC(6)H(4))(2)C6H3Li with H2ClB.SMe2 after 1.5 h with pyridine resulted in the isolation of the primary borane [2,6-(4-t-BuC6H4)(2)C6H3BH2](2), 2, as the pyridine adduct 2.py, which after thermolysis at 190 degreesC gave 1-(4-tert-butylphenyl)-7-tert-butyl-9-borafluorene-pyridine, 7.py. Heating a C6D6 solution of 4 to 60-70 degreesC 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)(2)C6H3Li with H2ClB.SMe2 in hexane solution followed by addition of THF gave the very crowded diterphenyl borate [2,6-(4-t-BuC6H4)(2)C6H3](2)B(mu-H)(2)Li(THF)(2)](2), 11, which can be converted to I by simple addition of water. Prolonged exposure of I to concentrated aqueous HCl led to B-C bond cleavage and formation of the sterically very crowded diterphenylborinic acid [2,6-(4-t-BuC6H4)(2)C6H3](2)BOH, 15. All compounds have been characterized by H-1, C-13, and B-11 NMR spectroscopy and mass spectrometry, and compounds 1, 4, and 11 have also been characterized by single-crystal X-ray crystallography.