Acid-Base Bifunctional and Dielectric Outer-Sphere Effects in Heterogeneous Catalysis: A Comparative Investigation of Model Primary Amine Catalysts
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- 作者：John D. Bass ; Andrew Solovyov ; Andrew J. Pascall ; and Alexander Katz
- 刊名：Journal of the American Chemical Society
- 出版年：2006
- 出版时间：March 22, 2006
- 年：2006
- 卷：128
- 期：11
- 页码：3737 - 3747
- 全文大小：446K
- 年卷期：v.128,no.11(March 22, 2006)
- ISSN：1520-5126
文摘
Dielectric and acid-base bifunctional effects are elucidated in heterogeneous aminocatalysisusing a synthetic strategy based on bulk silica imprinting. Acid-base cooperativity between silanols andamines yields a bifunctional catalyst for the Henry reaction that forms 
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-unsaturated product via quasi-equilibrated iminium intermediate. Solid-state UV/vis spectroscopy of catalyst materials treated withsalicylaldehyde demonstrates zwitterionic iminium ion to be the thermodynamically preferred product inthe bifunctional catalyst. This product is observed to a much lesser extent relative to its neutral imine tautomerin 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 -nitro alcohol product in high selectivity (~99%). This appears to be the firstobservation of selective alcohol formation in primary amine catalysis of the Henry reaction. A primary aminecatalyst with a methyl-terminated outer-sphere also produces alcohol, albeit at a rate that is 50-fold slowerthan the cyano-terminated catalyst, demonstrating that outer-sphere dielectric constant affects catalystactivity. We further investigate the importance of organizational effects in enabling acid-base cooperativitywithin the context of bifunctional catalysis, and the unique role of the solid surface as a macroscopic ligandto impose this cooperativity. Our results unequivocally demonstrate that reaction mechanism and productselectivity in heterogeneous aminocatalysis are critically dependent on the outer-sphere environment.
,-unsaturated product via quasi-equilibrated iminium intermediate. Solid-state UV/vis spectroscopy of catalyst materials treated withsalicylaldehyde demonstrates zwitterionic iminium ion to be the thermodynamically preferred product inthe bifunctional catalyst. This product is observed to a much lesser extent relative to its neutral imine tautomerin 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 -nitro alcohol product in high selectivity (~99%). This appears to be the firstobservation of selective alcohol formation in primary amine catalysis of the Henry reaction. A primary aminecatalyst with a methyl-terminated outer-sphere also produces alcohol, albeit at a rate that is 50-fold slowerthan the cyano-terminated catalyst, demonstrating that outer-sphere dielectric constant affects catalystactivity. We further investigate the importance of organizational effects in enabling acid-base cooperativitywithin the context of bifunctional catalysis, and the unique role of the solid surface as a macroscopic ligandto impose this cooperativity. Our results unequivocally demonstrate that reaction mechanism and productselectivity in heterogeneous aminocatalysis are critically dependent on the outer-sphere environment.