Acid−Base Bifunctional and Dielectric Outer-Sphere Effects in Heterogeneous Catalysis: A Comparative Investigation of Model Primary Amine Catalysts

Dielectric and acid−base bifunctional effects are elucidated in heterogeneous aminocatalysis
using a synthetic strategy based on bulk silica imprinting. Acid−base cooperativity between silanols and
amines yields a bifunctional catalyst for the Henry reaction that forms α,β-unsaturated product via quasi-equilibrated iminium intermediate. Solid-state UV/vis spectroscopy of catalyst materials treated with
salicylaldehyde demonstrates zwitterionic iminium ion to be the thermodynamically preferred product in
the bifunctional catalyst. This product is observed to a much lesser extent relative to its neutral imine tautomer
in primary amine catalysts having outer-sphere silanols partially replaced by aprotic functional groups.
One of these primary amine catalysts, consisting of a polar outer-sphere environment derived from cyano-terminated capping groups, has activity comparable to that of the bifunctional catalyst in the Henry reaction,
but instead forms the β-nitro alcohol product in high selectivity (∼99%). This appears to be the first
observation of selective alcohol formation in primary amine catalysis of the Henry reaction. A primary amine
catalyst with a methyl-terminated outer-sphere also produces alcohol, albeit at a rate that is 50-fold slower
than the cyano-terminated catalyst, demonstrating that outer-sphere dielectric constant affects catalyst
activity. We further investigate the importance of organizational effects in enabling acid−base cooperativity
within the context of bifunctional catalysis, and the unique role of the solid surface as a macroscopic ligand
to impose this cooperativity. Our results unequivocally demonstrate that reaction mechanism and product
selectivity in heterogeneous aminocatalysis are critically dependent on the outer-sphere environment.