Magnesiothermic reduction can directly convert SiO2 into Si nanostructures. Despite intense efforts, efficient fabrication of highly nanoporous silicon by Mg still remains a significant challenge due to the exothermic reaction nature. By employing table salt (NaCl) as a heat scavenger for the magnesiothermic reduction, we demonstrate an effective route to convert diatom (SiO2) and SiO2/GeO2 into nanoporous Si and Si/Ge composite, respectively. Fusion of NaCl during the reaction consumes a large amount of heat that otherwise collapses the nano-porosity of products and agglomerates silicon domains into large crystals. Our methodology is potentially competitive for a practical production of nanoporous Si-based materials.
The reaction conditions for the synthesis of Cu-BTC (BTC = benzene-1,3,5-tricarboxylic acid) were elucidated using a continuous-flow microreactor-assisted solvothermal system to achieve crystal size and phase control. A high-rate synthesis of Cu-BTC metal-organic frameworks with a BET surface area of more than 1600 m(2) g(-1) (Langmuir surface area of more than 2000 m(2) g(-1)) and with a 97% production yield could be achieved with a total reaction time of 5 minutes.
The metal organic framework, MOF-74IJNi), was synthesized in a continuous flow microwave-assisted reactor obtaining a high space-time yield (~90 g h −1 L −1 ) and 96.5% conversion of reagents. Separation of the nucleation and growth steps was performed by using uniform and rapid microwave heating to induce nucleation, which allowed a substantial increase in conversion for shorter reaction times under mild pressure. High yields were achieved in minutes, as opposed to days for typical batch syntheses, with excellent control over the material's properties due to more uniform nucleation, and the separation of the nucleation and growth steps. Optimization of the microwave reactor parameters led to improvements in crystallinity, reagent conversion, and production rates. Differences in MOF-74(Ni) crystallinity were observed as smaller grains were formed when higher microwave zone temperatures were used. Crystallinity differences led to different final adsorption properties and surface areas. Herein we show that a continuous high space-time yield synthesis of MOF-74(Ni) allows control over nucleation using microwave heating.
Enynes are important
motifs in bioactive compounds. They can be
synthesized by alkyne–alkyne couplings for which a number of
mechanisms have been suggested depending on catalyst type and dominant
product isomers. Regarding bimetallic pathways, hydrometalations and
anti-carbopalladations have been discussed to account for the formation
of geminally substituted and (Z)-configured enynes,
respectively. Here we report a bimetallic alkyne–alkyne coupling
that yields (E)-configured enynes. An unusual type
of acetylide Pd bridging was found in putative catalytic intermediates
which is arguably responsible for the regio- and stereochemical reaction
outcome. Mechanistic studies suggest that a double μ–κ:η2 acetylide bridging enables a bimetallic syn-carbometalation.
Interestingly, depending on the reaction conditions, it is also possible
to form the geminal regioisomer as major product with the same catalyst.
This regiodivergent outcome is explained by bi- versus monometallic
reaction pathways.
Enantiopure fluorine
containing β-amino acids are of large
biological and pharmaceutical interest. Strategies to prepare β-amino
acid derivatives possessing a F-containing tetrasubstituted stereocenter
at the α-C atom in a catalytic asymmetric sense are rare, in
particular using an enantioselective electrophilic C–F bond
formation. In the present study, a highly enantioselective palladacycle-catalyzed
fluorination of isoxazolinones was developed. It is demonstrated that
isoxazolinones are useful precursors toward enantiopure β-amino
acid derivatives by diastereo- and chemoselective reduction. The formed
γ-aminoalcohols served as valuable precursors toward β-amino
acids, β-amino acid esters, and β-lactams, all featuring
tetrasubstituted fluorinated stereocenters. In addition, by this work,
enantioenriched fluorinated azetidines were accessible for the first
time.
A diastereospecific enantiodivergent allylation of pyrazolones is reported which is catalyzed by a planar chiral pentaphenylferrocene based palladacycle. With the same catalyst batch both product enantiomers were selectively available. The method is applicable to structurally diverse substrates and gave products with enantiomeric excesses between 85 and 94%. In addition, we could show that pyrazolones are transformable into β-aminoamides.
Chiral acyclic tertiary allylic alcohols are very important synthetic building blocks, but their enantioselective synthesis is often challenging. A major limitation in catalytic asymmetric 1,2‐addition approaches to ketones is the enantioface differentiation by steric distinction of both ketone residues. Herein we report the development of a catalytic asymmetric Meisenheimer rearrangement to overcome this problem, as it proceeds in a stereospecific manner. This allows for high enantioselectivity also for the formation of products in which the residues at the generated tetrasubstituted stereocenter display a similar steric demand. Low catalyst loadings were found to be sufficient and the reaction conditions were mild enough to tolerate even highly reactive functional groups, such as an enolizable aldehyde, a primary tosylate, or an epoxide. Our investigations suggest an intramolecular rearrangement pathway.
Organic compounds featuring a chlorine substituted stereocenter are frequently found in nature and are interesting for pharmaceutical applications and as synthetic building blocks. Catalytic methods to generate such stereocenters by C,H bond functionalization are still relatively rare. Here we report the first catalytic asymmetric chlorination of isoxazolinones, a synthetically and biologically interesting class of heterocycles, which can be considered as precursors for β-aminoacids. The title reaction was catalyzed with high enantioselectivity by a planar chiral ferrocene based palladacycle in high to excellent yields. It is showcased that the products are valuable for post-synthetic transformations. An S N 2 reaction proceeded with smooth inversion of the absolute configuration. The substitution product could then be transformed into an α-azido β-aminoacid derivative via a reductive, diastereoselective ring opening.
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