Probing the Impact of Solvent on Lewis Acid Catalysis via Fluorescent Lewis Adducts

Over the years, various multiparameter methods have been developed to measure the strength of a Lewis acid. However, a major challenge for these measurements lies in the complexity that arises from variables such as solvent and other fundamental interactions, as well as perturbations of Lewis acids as their reaction environment changes. Herein, we evaluate the impact of solvent effects on the Fluorescent Lewis Adduct (FLA) method using a series of representative Lewis acids. The solution-state nature of the FLA method in particular offers the ability to correlate Lewis Acid Units (LAUs) obtained from the FLA measurement with reactivity. The binding of a Lewis acid in various solvents reveals a measurable dichotomy between both polarity and donor ability of the solvent. While not strictly separable, as solvent polarity increases observed LAUs increase; however, as solvent donor ability increases the observed LAUs decrease. This dichotomy was confirmed by titration data and catalytic Diels-Alder cycloaddition and hydrosilylation reactions with neutral boranes, illustrating that solvation effects can be appropriately gauged by LAU value determined from the FLA method.