Synthesis, characterization and thermolysis of phosphinite-borane adducts: investigation of an unusual thermally-induced phenol elimination reaction

摘要

The synthesis, characterization and thermolytic behavior of a new class of phosphinite-borane adducts containing the bulky 2,4,6-t-Bu_3C_6H_2O(MesO) group is reported, including the description of the first example of a primary phosphinite-borane adduct isolable at room temperature (MesO)PH_2·BH_3 1. Adduct 1 and the phenylated derivative (MesO)PhPH·BH_3 3 formed highly cross-linked solids, 7 and 8, when heated to 100 and 140 deg C, respectively. Characterization of these solids by solidstate and solution NMR spectroscopy suggest that both dehydrocoupling (H_2 elimination) and MesOH elimination occur simultaneously to give the complex products [(MesO)_xRPH_y-BH_(2x+2y+z)]_n (R = H or Ph, x + y + z = 1). The tertiary adduct (MesO)Ph_2P·BH_3 5 and the phosphite-borane (PhO)_3P·PH_3 6 were found to decompose at high temperatures via P-B bond cleavage without formation of the corresponding phenol. These results suggest that the thermally-induced formation of MesOH from 1 and 3 occurs via alpha-elimination from a phosphorus center. Adducts 1,3,5 and 6 were also structurally characterized by single-crystal X-ray diffraction.