Synthesis of a family of peracid-silica materials and their use as alkene epoxidation reagents

• A library of silica-carboxylic acids are synthesized from co-condensation methods.
• Supported carboxylates are converted to supported peracids at up to 70% yield.
• Epoxidation depends strongly on peracid length and surface density.
• Epoxidation selectivity is tunable by adding additional organosilanes.
• Select materials are unusually selective for bis-epoxidation of divinylbenzene.

Alkene epoxidation is an important process for the production of resin monomers and other chemical intermediates. The challenges of and high costs associated with purifying these reactive intermediates motivates the search for new reagents that can afford epoxides in high yields and selectivity with no soluble byproducts. Here, we synthesize a family of silica-supported peracids with variations in the peracid surface density and surface chemistry and describe their use as epoxidation reagents. Materials in this study were synthesized from either sol–gel co-condensation with cyanoalkylsilanes or by grafting of silyl esters to pre-formed silica, giving titrated peracid loadings from 0.3 to 1.5 mmol/g. At constant peracid:alkene ratios, epoxide yields increased monotonically with increasing surface density, up to 1.0 mmol/g for the highest loading materials. Mixed silica surfaces possessing both alkyl and peracid moieties gave, at similar peracid surface densities, 2-3-fold greater olefin conversion, and doubled epoxide selectivity from 45 to >95%, as compared to materials with peracids alone. Hydrophobic, mixed silane-supported peracids give unusual, high selectivity to dioxide syntheses from dienes even at low conversion, suggesting strong adsorption of reactive intermediates, which is not possible for soluble peracids. These experiments suggest tunable parameters that lead to improvements in selectivity and yields in epoxidation with these easily-handled, immobilized peracids.

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