Abstract
Acetic anhydride (AA) is usually considered a stable molecule but is shown here to be able to polymerize in closed reactors to a cross-linked polyketone condensate. By using this chemistry, it was possible to copolymerize AA with l-histidine, which gives a nitrogen-doped functional nanoporous polymer that can act as an acid–base heterogeneous catalyst. The polymer acidic and basic sites were screened by running an acetal hydrolysis Knoevenagel condensation reaction cascade to optimize catalyst synthesis. Furthermore, it was possible to catalyze CO2 cycloaddition to epoxides to the corresponding cyclic carbonate with complete conversion without cocatalysts.
Highlights
Despite the tremendous progress in the recycling of homogeneous organocatalysts, heterogeneous systems still provide the advantage of a convenient recovery and easier implementation in flow systems
A reference polymer was prepared by heating neat Acetic anhydride (AA) in a sealed reactor at temperatures between 200 and 250 °C with variable reaction times
With initial conditions of 220 °C for 72 h (AA-220_72 h), a heterogenous dispersion is obtained composed of a black precipitate with an elemental composition C, 70.7% and H, 4.5%
Summary
Despite the tremendous progress in the recycling of homogeneous organocatalysts, heterogeneous systems still provide the advantage of a convenient recovery and easier implementation in flow systems. Nanostructured materials with a high specific surface area have proven to be strong candidates to perform such heterogeneous catalysis for several decades.[1] On top, there is a need for metal-free systems, drawing attention to heterogeneous organocatalysis. The first generation of strategies consisted of immobilizing catalytic active moieties on inorganic surfaces[2] or organic polymers.[3,4] On the other hand, efforts were described for directly synthetizing organic polymers and copolymers, combining porosity with catalytic functionality, for applications in catalysis and gas adsorption.[5,6] Two main categories of porous organic materials have since been described: crystalline covalent organic frameworks[7,8] and their amorphous counterparts, porous organic polymers[9−12] englobing multiple subclasses.[13−17] only scarce examples have been able to convey into heterogeneous catalysis without postsynthetic metalation. Synthetic procedures generally use already known catalytic-active building blocks such as Tröger’s base,[18] Jørgensen−Hayashi,[19] Proline[20] DABCO,[21] BINOL,[22] or postfunctionalization like sulfonation,[23] exceptions being covalent triazine frameworks[14,24] or polyheptazine frameworks showing catalytic activity as such.[25,26]
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