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

The synthesis, characterization and thermolytic behavior of a new class of phosphinite–borane adducts containing the bulky 2,4,6-t-Bu3C6H2O (Mes*O) group is reported, including the description of the first example of a primary phosphinite–borane adduct isolable at room temperature (Mes*O)PH2·BH31. Adduct 1 and the phenylated derivative (Mes*O)PhPH·BH33 formed highly cross-linked solids, 7 and 8, when heated to 100 and 140 °C, respectively. Characterization of these solids by solid state and solution NMR spectroscopy suggest that both dehydrocoupling (H2 elimination) and Mes*OH elimination occur simultaneously to give the complex products [(Mes*O)xRPHy–BH(2x + 2y + z)]n (R = H or Ph, x + y + z = 1). The tertiary adduct (Mes*O)Ph2P·BH35 and the phosphite–borane (PhO)3P·BH36 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 Mes*OH from 1 and 3 occurs via α-elimination from a phosphorus center. Adducts 1, 3, 5 and 6 were also structurally characterized by single-crystal X-ray diffraction.