Rational Design of Cationic Silyl Lewis Acids.

The synthesis of a series of selanyl-stabilized cationic silyl Lewis acids with naphthalene or acenaphthalene scaffolds is described. The influence of scaffold modifications, substitution variations at the selanyl donor and at the silicon atom on the strengths of the silyl Lewis acid is evaluated using the p-fluorobenzonitrile (FBN) method. These simple variations of the principal structure allow to adjust the Lewis acidity of the cationic silyl Lewis acids from weak ones to examples that are significantly stronger than tris(pentafluorophenyl)borane (BCF). The solid-state structure of the cationic FBN complex [11(FBN)]⁺ with an acenaphthene backbone and a pentafluorophenylselanyl donor provides evidence for the penta-coordination of the silicon atom in this complex, in agreement with the results of nuclear magnetic resonance (NMR) studies in solution. Finally, this study suggests that the FBN method is well suited to assess even very subtle differences in the strength of Lewis acids.