An integrated lab‐on‐a‐chip enables the rapid analysis of heterogenized enantioselective organocatalysis at the microscale. A packed‐bed microreactor was seamlessly integrated with a downstream chiral high pressure liquid chromatography (HPLC) functionality to study enantioselective transformations on a single microfluidic glass chip. Hyphenation to mass spectrometry allows for the rapid investigation of the selectivity and the substrate scope of microgram amounts of catalyst beads. Optimization of reaction conditions is possible with minimal reagent consumption and instant analytical feedback.
Gold nanoparticles (AuNPs) were anchored on alkynyl carbamate-functionalized support materials having the suitable features for application as catalysts in continuous-flow packed bed reactors. The functionalization step was carried out by grafting with the di-functional organosilane [3-(2-propynylcarbamate)propyl]triethoxysilane (PPTEOS) three commercial micrometer-sized oxide supports, i.e. silica, alumina, and titania. The alkynyl-carbamate moieties were capable to straightforwardly reduce the gold precursor HAuCl4 yielding the supported AuNPs systems Au/SiO2@Yne, Au/Al2O3@Yne, and Au/TiO2@Yne. A comparison among the three materials revealed that silica allowed the highest organic functionalization (12 wt%) as well as the highest gold loading (3.7 wt%). Moreover, TEM investigation showed only for Au/SiO2@Yne the presence of homogeneously distributed, spherically shaped AuNPs (av. diameter 15 nm). Au/SiO2@Yne is an efficient catalyst, both in batch and flow conditions, in the oxidation of a large variety of alcohols, using H2O2 as oxidizing agent, at a temperature of 90 °C. Furthermore, under flow conditions, the catalyst worked for over 50 h without any significant decrease in the catalytic activity. The catalytic activity of the three catalysts was evaluated and compared in the oxidation of 1-phenylethanol as a model substrate. We found that the flow approach plays a strategic role in preserving the physical and chemical integrity of the solid catalysts during its use, with remarkable consequences for the reaction conversion (from 2% in batch to 80 % in flow) in the case of Au/TiO2@Yne
The asymmetric synthesis of functionalized nitrocyclopropanes has been achieved by a one-pot, four-step method catalyzed by (S)-diphenylprolinol TMS ether, which joins two sequential domino reactions, namely a domino sulfa-Michael/aldol condensation of α,β-unsaturated aldehydes with 1,4-dithiane-2,5-diol, and a domino Michael/α-alkylation reaction of the derived chiral dihydrothiophenes with bromonitromethane. The title compounds were obtained in 27-45% yields, with high levels of diastereoselectivity (93:7 to 100:0 dr) and generally good enantioselectivities (up to 95:5 er).
The condensation of aromatic α-diketones (benzils) with aromatic aldehydes (benzoin-type reaction) and chalcones (Stetter-type reaction) in DMF in the presence of catalytic (25 mol%) KOtBu is reported. Both types of umpolung processes proceed with good efficiency and complete chemoselectivity. On the basis of spectroscopic evidence (MS analysis) of plausible intermediates and literature reports, the occurrence of different ionic pathways have been evaluated to elucidate the mechanism of a model cross-benzoin-like reaction along with a radical route initiated by an electron-transfer process to benzil from the carbamoyl anion derived from DMF. This mechanistic investigation has culminated in a different proposal, supported by calculations and a trapping experiment, based on double electron-transfer to benzil with formation of the corresponding enediolate anion as the key reactive intermediate. A mechanistic comparison between the activation modes of benzils in KOtBu-DMF and KOtBu-DMSO systems is also described.
An (S)-5-(pyrrolidin-2-yl)-1H-tetrazole organocatalyst has been prepared in the form of a monolithic column through the radical copolymerization of a styryl-functionalized pyrrolidinyl-tetrazole derivative, sty- rene and divinylbenzene in the presence of porogens (dodecanol and toluene). The activity of the mono- lithic pyrrolidinyl-tetrazole organocatalyst (triturated polymer) has been initially tested under batch condi- tions using the asymmetric aldol reaction of cyclohexanone and p-nitrobenzaldehyde as the benchmark. A prerequisite of the study has been the utilization of the eco-friendly water–ethanol mixture as the solvent. After having established the high efficiency and recyclability of the catalyst under these conditions, the effect of the flow regime has been evaluated by fabricating the corresponding monolithic microreactor (pressure-resistant stainless steel column). It has been demonstrated by a brief substrate scope study that the flow regime contributes to preserve the activity of the pyrrolidinyl-tetrazole catalyst over time (5 days on stream) with an almost twofold increase in productivity moving from batch to flow conditions. An added value of the flow procedure has been the optimization of a suitable 2D instrumental setup for simultaneous flow reaction and online flow-injection analysis
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