Boronation of 1,8-Bis(diphenylphosphino)naphthalene: Formation of Cyclic Boronium Salts

Treatment of 1,8-bis(diphenylphosphino)naphthalene (1) with borane−dimethyl sulfide complex in THF/Et2O solution affords monoborane adduct 3 irrespective of the molar ratio of the reagents. The monoboronation product has been characterized by multinuclear (1H, 31P, 13C, 11B) NMR spectroscopy and MS-FAB measurements. Optimization of its structure with DFT methods at the B3LYP/6-31+G(d) level of theory revealed that steric hindrance around the diphenylphosphino group prevents formation of the bis-borane adduct. In dichloromethane or chloroform solution boronation of bis-phosphine 1 with H3B·Me2S results in the unexpected formation of a cyclic dihydroboronium chloride, 4·Cl. The corresponding cyclic iodide 4·I and triflate 4·TfO are formed by treatment of bis-phosphine 1 with H3B·Me2S followed by methyl iodide and methyl triflate, respectively, in THF/Et2O solution. The hexafluorophosphate 4·PF6 prepared by anion exchange from the chloride 4·Cl has also been characterized by single-crystal X-ray diffraction. It has been demonstrated that the cyclic boronium salts are formed in two steps. The first is the formation of monoborane adduct 3, which in the second step undergoes cyclization with concomitant reduction of the chlorohydrocarbon solvents, methyl iodide or triflate. Some aspects of this unprecedented reductive cyclization have been elucidated by DFT calculations.