Computational Studies of Lewis Acidity and Basicity in Frustrated Lewis Pairs

Computational studies that characterize the effects of Lewis acidity/basicity on FLP formation and reactivity are reviewed. Formation of the FLP encounter complex "cage" depends on Lewis acidities and basicities of substituent "external" atoms, and their abilities to interact intramolecularly. Computations indicate that these interactions are worth 9-18 kcal mol(-1) for partly fluorinated FLPs such as (F5C6)(3)B center dot center dot center dot P(t-Bu)(3), and less for less fluorinated species such as (H5C6)(3)B center dot center dot center dot P (t-Bu)(3). Reactivity within the cage depends on the "classical" Lewis acidities/basicities of the internal atoms. Energetics here fall into the range of 5-50 kcal mol(-1); the larger the value, the greater the ability of the FLP to capture or split a substrate. In several cases the computationally predicted reaction barriers differ little with internal Lewis acidity/basicity, indicating that the rate-determining step involves the substrate entering the cage rather than attack by the Lewis acid/base atoms. In others, barriers vary sizably with Lewis acidity/basicity, indicating the opposite. In one case it was shown that these effects cancel, such that the three component barriers are identical for a range of substituted Lewis acid FLP components.