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

• Published in: Journal of the American Chemical Society Vol. 128; no. 11; pp. 3737 - 3747
• Main Authors: Bass, John D, Solovyov, Andrew, Pascall, Andrew J, Katz, Alexander
• Format: Journal Article
• Language: English
• Published: WASHINGTON American Chemical Society 22.03.2006; Amer Chemical Soc
• Subjects: Acids - chemistry; Amines - chemistry; Carbamates - chemistry; Catalysis; Chemistry; Chemistry, Multidisciplinary; Electrochemistry; Lysine - chemistry; Physical Sciences; Science & Technology; Silicon Dioxide - chemistry; Spectrophotometry, Ultraviolet; Thermodynamics; MICHAEL REACTION; ANTIBODY CATALYSIS; DEHYDRATION; STRUCTURAL-CHARACTERIZATION; CONDENSATION; AROMATIC-ALDEHYDES; SILICA; FORMING REACTIONS; EFFICIENT; SOLID BASE
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja057395c

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.