Addition of small amounts of water to the organic solvent may result in different promoting or inhibiting effects in aldol condensation reactions, depending on the distribution of active sites on the surface. We combine liquid-phase catalytic activity measurements of cyclopentanone (CPO) condensation with spectroscopies and theoretical calculations of the structure of water clusters and the CPO reactant around acid active sites. The study confirms the water-assisted water remote bond polarization, explaining the promoting effect of water on silica catalysts functionalized with varying densities of sulfonic acid groups.  Solid-state NMR and computational calculations demonstrate that the water molecules participating in the reaction remain in close proximity of the active site, and their protons are in fast  exchange; this process also involves surface silanols. Accumulation of water around the sulfonic acid sites causes the proton of SO<sub>3</sub>H to delocalize and transfer to nearby water molecules, forming a hydronium ion separated from the active site. Consequently, water clusters around the acid site can form a bridge with a CPO molecule located close to the activated CPO, on another site. The CPO molecule polarized by the hydronium ion acts as an efficient electrophile in the C–C coupling step, which explains the observed water promotion. When the distance between two acid sites is greater than a distance corresponding to about 2 water molecules, the efficiency of the remote bond polarization of the CPO decreases significantly, and the presence of water no longer promotes the reaction. In situ DRIFT measurements indicate that the coverage and structure of water vary with the density of silanols and hydrophobic groups on the surface. Although the silanol groups are not crucial in polarizing the electrophile, a high density of silanol groups enhances the distribution of water on the surface and moderate inhibition at high water concentrations. By contrast, when the silanol groups are substituted by the hydrophobic groups with a higher density and longer alkyl chain, the disrupted water structure on hydrophilic sites leads to a diminished water promotion. This work provides a quantitative assessment of how solvent molecules can bridge isolated active sites, enabling cooperative catalysis.

Acid site proximity and remote polarization by water in carbonyl condensation reactions

ACS Spring 2022

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