Enhanced Stibine Oxide Lewis Basicity Overcomes Steric Frustration

The characteristic electronic structure of the phosphoryl group in phosphine oxides confers great stability on the P⁺-O^- bond, in part because of back-bonding from O-based lone pairs into the P-C antibonding orbitals. The partial nature of this donation allows the O atom in the phosphoryl unit to exhibit Lewis basicity. This backbonding weakens as the atomic number of the pnictogen increases, which results in a significant enhancement in basicity for the heavier stiboryl congener. Here, we compare the ability of R₃PnO (Pn = P, As, Sb) species to bind to main-group Lewis acids. As the steric bulk of the R group increases, R₃PO and R₃AsO lose this capacity; Dipp₃PO and Dipp₃AsO (where Dipp = 2,6-diisopropylphenyl) are unable to bind even the very strong Lewis acid B-(C₆F₅)₃. In contrast, the enhanced basicity of the stibine oxides allows them to overcome this steric hindrance and form adducts, even in the case of the very hindered Dipp₃SbO·B-(C₆F₅)₃.