(73d) Controlling Immobilized Amine Reactivity Via Tailored Surface Environments

We have elucidated how silanols can cooperate with grafted primary amine base catalyst sites on silica, within the context of promoting versatile acid-base bifunctional heterogeneous catalysis. By capping silanols with cyano (polar/nonacidic) and methyl-terminated (nonpolar/nonacidic) functional groups as shown below, it was possible to separate outer-sphere dielectric effects from acid-base cooperativity for aldol-related catalysis rate and reaction mechanism. Silanol acidity shuttled heterogeneous aminocatalysis via a covalent iminium ion intermediate, which could be observed upon binding salicylaldehyde as a probe molecule. Primary amines located within an aprotic but polar cyano environment had comparable catalytic activity, but instead bound salicylaldehyde as a neutral imine tautomer and followed a previously postulated but experimentally unverified ion-pair catalysis mechanism. We discuss these differences in reactivity based on environment in comparison with molecular models for amines on silica.