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.