Synthesis and characterization of bifunctional surfaces with tunable functional group pairs

Abstract

Grafting of pairs of functional groups onto a silica surface was demonstrated by tethering both terminals of an organochlorosilane precursor molecule, Cl 2 (CH 3)Si(CH 2) 4 (CO)(OSi(i-Pr) 2)(CH 2) 2 Si(CH 3)Cl 2 , that possess a cleavable silyl ester bond, onto a silica surface. Hydrolytic cleavage of the silyl ester bond of the grafted molecule resulted in the generation of organized pairs of carboxylic acid and organosilanol groups. This organosilanol moiety was easily transformed into other functional groups through condensation reactions to form, together with the neighboring acid group, pairs such as carboxylic acid/secondary amine, carboxylic acid/pyridine, and carboxylic acid/phosphine. In the case of carboxylic acid/amine pairing, there was evidence of the formation of amide. A sample grafted with amine-carboxylic acid pairs was three times more active (per free amine) than a sample without such pairs for the nitroaldol condensation of 4-nitrobenzaldehyde and nitromethane.

Designing Sites in Heterogeneous Catalysis: Are We Reaching Selectivities Competitive With Those of Homogeneous Catalysts?

“…More commonly, two organo­functionalities may be added to the solid support in a concerted fashion. Several strategies have been tested to control the distribution of the different tethered functionalities within the solid at a molecular level in those cases. , One is by using an imprinting agent during the synthesis of the solid, as, for instance, in the case of the sol-gel synthesis of porous silica where a carbamate-protected APTES with two or three of such units bonded to a central aromatic ring was added to the regular tetraethoxysilane (TEOS) sol-gel reagent; once the solid was formed, methanol/aqueous sodium bicarbonate washing was used to break the carbamate bonds and release the aromatic ring scaffold, leaving behind two or three amine groups tethered to the silica surface in predetermined relative positions (Figure ). A second approach here is site pairing.…”

New materials for catalysis and energy storage devices

“…After attachment to the surface and subsequent hydrolytic cleavage of the silyl ester bond, a carboxylic acid‐silanol pair was formed whose functional ends were separated by the length of the molecule—an adjustable parameter. The silanol group could be further modified, and the examples in Figure demonstrated successful modification to form carboxylic acid/silanol, carboxylic acid/amine, carboxylic acid/pyridine, and carboxylic acid/phosphine pairs . Undoubtedly, other modifications are possible.…”

“…The catalytic activity of the sample with the carboxylic acid/amine pairing was evaluated for the Henry reaction of 4‐nitobenzaldehyde with nitromethane at 50°C to form primarily β‐nitroalcohol with small amounts of β‐nitroalkene. On the per amine basis, this sample was slightly more than twice as active as a sample where the carboxylic acid and amine groups were randomly placed, and thrice as active as a sample with only amine groups …”

Synthesis of Solid Catalysts with Spatially Resolved Acidic and Basic Molecular Functionalities

“…There are many recent citations of heterogeneous tandem and cascade catalysis in the literature that follow this third approach, many involving acid–base combined chemistry. ,− These report different and interesting synthetic methodologies for the addition of the several desired functionalities to a porous solid, but they do not yet offer a universal protocol for doing this. In addition, only a handful of them address the issue of the spatial distribution of the different added groups, − an important consideration when dealing with cascade processes involving unstable intermediates. It is desirable to expand the library of synthetic routes by which such multifunctional catalysts can be made.…”