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.

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