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Hierarchically porous monolithic silica shows promise as a carrier material for immobilized organocatalysts. Conventional analysis usually includes physisorption, infrared spectroscopy and elemental analysis, among others, to elucidate the pore space and degree of functionalization of the material. However, these methods do not yield information about the spatial distribution of the organic species inside the monolithic reactor. In this work, time‐of‐flight secondary ion mass spectrometry has been applied to characterize the surface of organically functionalized silica monoliths. Cross sections of a silica monolith functionalized with 4‐dimethylaminopyridine were analyzed and the results were compared with physisorption and elemental analysis experiments of the same material. This way, insight into the radial distribution of the catalyst could be achieved, which might assist in interpreting the performance of such reactors in heterogeneous flow catalysis. [ABSTRACT FROM AUTHOR]

Organocatalysis via the enamine mechanism developed to one of the most relevant tools in carbonyl chemistry and is widely used in asymmetric organic synthesis. In this work, a strategy is presented to conveniently immobilize a peptide‐based catalyst on silica supports for use in continuous flow catalysis reactions. A set of different porous silica supports is investigated spanning from mesoporous silica particles with defined pore sizes suitable for packed bed column reactors to silica monoliths with hierarchical meso‐macropore spaces. While the silica supports are functionalized with norbornene entities, the peptide‐based organocatalyst is modified with a tetrazine moiety, enabling the immobilization via inverse electron‐demand Diels‐Alder (IEDDA) reaction. The ligation results in catalyst loadings up to 0.2 mmol g‐1, without compromising the mesopore network. The catalytic activity of the materials is proven by the asymmetric C−C coupling reaction of n‐butanal to ß‐nitrostyrene proceeding in high yield and enantioselectivity in both batch and continuous flow setups. [ABSTRACT FROM AUTHOR]

We report the immobilization of 4‐dimethylaminopyridine (DMAP), a versatile organocatalyst for sterically demanding esterifications, on mesoporous silica particles and macro‐mesoporous silica monoliths, both possessing optimized properties for continuous flow synthesis. An alkyne‐functionalized DMAP derivative was immobilized via click chemistry; the materials were characterized by physisorption analysis, diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) and elemental analysis. While silica particles were functionalized in batch and packed into a packed‐bed reactor, monoliths were cladded with a polyether ether ketone (PEEK) tube after sol‐gel synthesis and functionalized in a circulating flow process. Samples with three different catalyst loadings were prepared, in order to study the impact of the catalyst amount on the mesopore space as well as the catalytic performance. In continuous flow experiments, complete conversion of 1‐phenylethanol to phenylethylacetate was achieved with both materials and short contact times. Monoliths exhibited far lower pressures than packed bed reactors (7 bar at a flow rate of 1 mL min−1) and reached turnover rates up to 9.3x10−2 s−1, which is almost twice as high as a comparable batch experiment. The absence of diffusion limitations in monoliths made investigations on reaction kinetics with microkinetics‐dominated experiments possible. This study demonstrates that all properties needed for a successful transfer of immobilized organocatalysts to sophisticated flow syntheses with complex organocatalysts can be met with functionalized meso‐macroporous monoliths. [ABSTRACT FROM AUTHOR]