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Reference type: Journal
Authors: Bass JD, Solovyov A, Pascall AJ, Katz A
Article Title: Acid-base bifunctional and dielectric outer-sphere effects in heterogeneous catalysis: A comparative investigation of model primary amine catalysts.
Publication date: 2006
Journal: Journal of the American Chemical Society
Volume: 128
Issue: (11)
Page numbers: 3737-3747.
DOI: 10.1021/ja057395c

Abstract: 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 (similar to 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

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