Reactivity-Tuning in Frustrated Lewis Pairs: Nucleophilicity and Lewis Basicity of Sterically Hindered Phosphines

The nucleophilicity and Lewis basicity of sterically hindered phosphines, widely used in catalysis and in frustrated Lewis pair (FLP) chemistry, have been quantified by determining the rates and equilibrium constants of their associations with reference systems (benzhydrylium and tritylium ions) of calibrated electrophilicities and Lewis acidities. These structure–reactivity investigations allow a rationalization of the Lewis acid–base interactions all along the way from covalent Lewis adducts to FLPs. Comparisons of the association of phosphines of increasing sizes (Ph₃P, (o-tolyl)₃P, and tBu₃P) with the triarylborane B(C₆F₅)₃ and with the isoelectronic tritylium ions Ar₃C⁺ provide detailed insights for the future fine-tuning of the reactivities of FLPs. As a proof of concept, tritylium-ion-derived FLPs were shown to react with alkynes, as reported for the FLPs derived from the benchmark triarylborane B(C₆F₅)₃.