DMAP Immobilized on Porous Silica Particles and Monoliths for the Esterification of Phenylethanol in Continuous Flow

Abstract

We report the immobilization of 4-dimethylaminopyridine (DMAP), a versatile organocatalyst for sterically demanding esterifications, on mesoporous silica particles and macromesoporous 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 À 1 ) and reached turnover rates up to 9.3x10 À 2 s À 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.

Peptide‐based Organocatalyst on Stage: Functionalizing Mesoporous Silica by Tetrazine‐Norbornene Ligation

“…Apart from these, also monolithic silica possessing mesopores and macropores with average pore dimensions of 3–10 nm and 4 μm, respectively, were used (Sil‐M, Figure 1). Both the particles and the monolithic materials meet typical porosity parameters, being suitable for the application in HPLC and performed well in previous studies on organocatalysis, using classical, non‐peptidic catalysts such as propylamine or 4‐(dimethylamino) pyridine (DMAP) [21,22] …”

“…Both the particles and the monolithic materials meet typical porosity parameters, being suitable for the application in HPLC and performed well in previous studies on organocatalysis, using classical, nonpeptidic catalysts such as propylamine or 4-(dimethylamino) pyridine (DMAP). [21,22]…”

“…With Sil-P30-Cat, TOF values up to 17 h À 1 were reached at room temperature, which is comparable to results of other functionalized silica materials used in heterogeneous organocatalysis. [21,22] Therefore, the material does show a significant catalytical activity and the relatively low reaction yields rather seem to be caused by the small amount of material that is used in a reactor of that dimension. Indeed, using Sil-P10-Cat, conversions up to 68 % were possible (entries 4 and 5), with slightly higher TOF values of up to 24 h À 1 .…”

Recyclable Polymer Supported DMAP Catalyzed Cascade Synthesis of α-Pyrones

“…We envisioned that base promoted C-5 addition of oxazolone in a conjugate fashion to activated α,β-unsaturated ketone followed by ring-opening/cyclization might lead to highly functionalized 2-pyrone (Scheme ). Heterogeneous organic bases, mostly amines, have been exploited as catalysts/mediators as well as ligands for metal-catalyzed reactions. − The utilization of supported catalysts/reagents is one of the successful ways to make the methods more sustainable and greener. Consequently, we herein disclose first reusable polystyrene-supported DMAP catalyzed cascade synthesis of highly substituted 2-pyrones via 1,4-addition of 5 H -oxazol-4-one to α,β-unsaturated-β-keto esters followed by lactonization–elimination.…”