Ferrocene-Stabilized Silicon Cations as Catalysts for Diels–Alder Reactions: Attempted Experimental Quantification of Lewis Acidity and ReactIR Kinetic Analysis

The 29Si NMR chemical shifts of ferrocene-stabilized silicon cations span a wide range depending on the substituents at the silicon atom. These pronounced differences in deshielding of the silicon atom do not translate into significant differences in their catalytic activity in Diels–Alder reactions. It was shown by Lewis pair formation with Lewis base probes (Et3PO and pyridine-d5) that there is hardly any difference between these silicon cations after coordination to a Lewis base. This finding not only thwarts experimental quantification of the Lewis acidity of the free Lewis acids but also demonstrates that the reactivity differences are largely due to steric effects for a given counteranion. These observations are further verified by a ReactIR kinetic analysis. The Lewis acidity of silicon cations and their performance as catalysts cannot be correlated with 29Si NMR chemical shifts as well as resonances of adducts with Lewis base probes, not even for a subset of silicon Lewis acids.