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

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 alpha,beta-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 beta-nitro alcohol product in high selectivity (approximately 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.

Cooperative catalysis by silica-supported organic functional groups

“…22 For instance, in the report by Bass et al, 22 amine-functionalized silica catalysts were prepared by bulk imprinting with protected amines. Before amine deprotection, the silanols were either capped with propyl groups (leading to a non-polar aprotic surface), capped with propylnitrile groups (leading to a polar aprotic surface), or left unmodified (with a surface both polar and protic).…”

Designing bifunctional acid–base mesoporous hybrid catalysts for cascade reactions

“…In fact, the 29 Si MAS NMR spectra of the bifunctional hybrids (Fig.5 curves b and c) exhibit bands from -50 ppm to -80 ppm assigned to T-type silicon (T 3 , T 2 and T 1 ) 65 species having a Si-C covalent bond, these signals being not present in the spectrum of siliceous material (curve a) synthesized for comparison, and then confirming that the hydrolysis and polycondensation of silylated-DMAN and MPTES occurs through reactive alkoxy terminal groups of the silylated moieties. In the 70 spectrum of DMAN-SO 3 H/SiO 2 -5-5 (curve b), it is possible to identify the T-bands corresponding to the T 1 (SiC(OH) 2 (OSi)), T 2 (SiC(OH)(OSi) 2 ) and T 3 (SiC(OSi) 3 ), whilst in the DMAN-SO 3 H/OMS-5-5 spectrum (curve c), only the band due to T 3 is visible, showing that more complete hydrolysis and poly-75 condensation of alkoxide terminal groups, present in the silane precursors, occur during the one-pot micellar route. In addition, three peaks at -92, -100 and -110 ppm are also present, due to Q 2 (Si(OH) 2 (OSi) 2 ), Q 3 (Si(OH)(OSi) 3 ) and Q 4 (Si(OSi) 4 ) silicon units respectively, 48 which are typical of highly siliceous 80 conventional mesoporous materials, associated to tetrahedrally coordinated silicon atoms conforming the solid network.…”

Substrate inhibition in the heterogeneous catalyzed aldol condensation: A mechanistic study of supported organocatalysts

“…Indeed, the weakly acidic silanol groups on the surface of silica have been previously recognized as capable of assisting various reactions. [23,[45][46][47][48][49] To test the participation of silanol groups in the catalytic process, we treated MAP-MSN-3.5 with hexamethyldisilazane (HMDS). 29 Si NMR showed that this reduced the number of silanols by 39% (from 3.8 to 2.3 mmol/g, Figure 4).…”