Photoredox-Catalyzed Reduction of Halogenated Arenes in Water by Amphiphilic Polymeric Nanoparticles

The use of organic photoredox catalysts provides new ways to perform metal-free reactions controlled by light. While these reactions are usually performed in organic media, the application of these catalysts at ambient temperatures in aqueous media is of considerable interest. We here compare the activity of two established organic photoredox catalysts, one based on 10-phenylphenothiazine (PTH) and one based on an acridinium dye (ACR), in the light-activated dehalogenation of aromatic halides in pure water. Both PTH and ACR were covalently attached to amphiphilic polymers that are designed to form polymeric nanoparticles with hydrodynamic diameter D_H ranging between 5 and 11 nm in aqueous solution. Due to the hydrophobic side groups that furnish the interior of these nanoparticles after hydrophobic collapse, water-insoluble reagents can gather within the nanoparticles at high local catalyst and substrate concentrations. We evaluated six different amphiphilic polymeric nanoparticles to assess the effect of polymer length, catalyst loading and nature of the catalyst (PTH or ACR) in the dechlorination of a range of aromatic chlorides. In addition, we investigate the selectivity of both catalysts for reducing different types of aryl-halogen bonds present in one molecule, as well as the activity of the catalysts for C-C cross-coupling reactions. We find that all polymer-based catalysts show high activity for the reduction of electron-poor aromatic compounds. For electron-rich compounds, the ACR-based catalyst is more effective than PTH. In the selective dehalogenation reactions, the order of bond stability is C-Cl > C-Br > C-I irrespective of the catalyst applied. All in all, both water-compatible systems show good activity in water, with ACR-based catalysts being slightly more efficient for more resilient substrates.     

Radical Anions of Sterically Protected Polyenes: An ESR and ENDOR Study

Owing to the steric protection by four bulky substituents in the terminal positions 1 and n, several conjugated polyenes could be reduced with K or Cs metal in 1,2-dimethoxyethane (DME) or tetrahydrofuran (THF) to fairly persistent radical anions. These compounds, denoted here as 2 , 3 ,…︁ 7 (which corresponds to the number, \2 n=2, 3, …︁7, of their formal double bonds) are 1,1,n,n-tetra(tert-butyl) derivatives of buta-1,3-diene, hexa-1,3,5-triene, octa-1,3,5,7-tetraene, deca-1,3,5,7,9-pentaene, dodeca-1,3,5,7,9,11-hexaene, and tetradeca-1,3,5,7,9,11,13-heptaene. In addition to the six polyenes 2 – 7 with all-trans-configuration, the studies comprised an isomer of 3 , the trans,cis,trans-triene, c -3 . The radical anions 2 ^.− – 7 ^.− and c -3 ^.− were characterized by their hyperfine data acquired with ESR, ENDOR, and TRIPLE-resonance spectroscopy. The ¹H-coupling constants comply with the spin distribution predicted for the radical anions of such `linear' π-systems by simple MO models. Ion pairs formed with K⁺ in DME were loose but became tighter with Cs⁺ in THF. Propensity to ion pairing decreased with the lengthening of the π-system on going from 2 ^.− to 3 ^.− – 7 ^.−. Hyperfine data are likewise reported for the radical anions of all-trans-polyenes 8 and 9 , in which two tert-butyl substituents in one terminal position of 2 and 3 , respectively, were replaced by CN groups.     

Phosphate and Vanadate in Biological Systems: Chemical Relatives or More?

Structural and functional analogies between acid phosphatases and vanadium haloperoxidases are reflected in the conservation of the amino acid residues contributing to the active sites of these enzymes. This has interesting consequences for the research on both enzyme systems. A first example is the newly proposed structure for the active site of human glucose-6-phosphatase: The picture shows parts of six of the nine transmembrane helices as well as the amino acids (black ovals) that presumably participate in the formation of the active site.     

Genome-Guided Discovery of the First Myxobacterial Biarylitide Myxarylin Reveals Distinct C–N Biaryl Crosslinking in RiPP Biosynthesis

Ribosomally synthesized and post-translationally modified peptides (RiPPs) are a structurally diverse group of natural products. They feature a wide range of intriguing post-translational modifications, as exemplified by the biarylitides. These are a family of cyclic tripeptides found in Planomonospora, carrying a biaryl linkage between two aromatic amino acids. Recent genomic analyses revealed that the minimal biosynthetic prerequisite of biarylitide biosynthesis consists of only one ribosomally synthesized pentapeptide precursor as the substrate and a modifying cytochrome-P450-dependent enzyme. In silico analyses revealed that minimal biarylitide RiPP clusters are widespread among natural product producers across phylogenetic borders, including myxobacteria. We report here the genome-guided discovery of the first myxobacterial biarylitide MeYLH, termed Myxarylin, from Pyxidicoccus fallax An d48. Myxarylin was found to be an N-methylated tripeptide that surprisingly exhibits a C–N biaryl crosslink. In contrast to Myxarylin, previously isolated biarylitides are N-acetylated tripeptides that feature a C–C biaryl crosslink. Furthermore, the formation of Myxarylin was confirmed by the heterologous expression of the identified biosynthetic genes in Myxococcus xanthus DK1622. These findings expand the structural and biosynthetic scope of biarylitide-type RiPPs and emphasize the distinct biochemistry found in the myxobacterial realm.     

Umlagerung von 5-Amino-5-X-pentadienalen zu 2-Aminopyrylium-Salzen

Rearrangement of 5-Amino-5-X-pentadienals to 2-Aminopyrylium Salts Various 2-aminopyrylium salts 7 (X=Cl, Br, I) are available in a simple one-pot procedure by reacting `push-pull' enynes 5 with equivalent amounts of HCl, HBr, or HI. On the other hand, reaction of HF or AcOH with `push-pull' enynes 5 is considerably slower so that an excess of HF or AcOH is needed for the reaction to 7 (X=F, AcO). The 2-aminopyrylium salts 7 are the key intermediates in the postulated rearrangement of 5-amino-5-halogeno-pentadienals 6 to 5-halogenopenta-2,4-dienamides 8 (Scheme 1, bottom), which is vinylogous to the well-known rearrangement of 3-amino-3-X-propenals 2 to 3-X-propenamides 3 (Scheme 1, top).     

Diethoxymethane as tailor-made fuel for gasoline controlled autoignition

Within the cluster of excellence “Tailor-Made Fuels from Biomass”  diethoxymethane (DEM) was identified as a promising fuel candidate from a production perspective. Synthesized by combining a bio-based feedstock and $C{O}_2$ as carbon source together with “green hydrogen” from water electrolysis DEM is defined as “bio-hybrid fuel” . To determine the molecules general applicability to a combustion system and to develop up combustion models a rapid screening of the ignition characteristics is performed in a rapid compression machine and a shock tube. Those suggest DEM being a potential fuel for gasoline controlled autoignition (GCAI) because of a relatively wide range of temperature independent ignition delay, a good autoignition behavior compared to conventional gasoline fuel and a multi-stage ignition behavior. To test the suitability of those molecules as a fuel and determine possible improvements to the production side, DEM was used in a single cylinder research engine operated in GCAI combustion mode. Compared to GCAI combustion with conventional RON95 E10 fuel, DME shows a significantly decreased ignition delay. As a consequence, the internal residual gas fraction, whose enthalpy is used to initiate autoignition, can be reduced and combustion stability is increased. Starting from similar combustion phasing using external exhaust gas recirculation to align the ignition behavior of DEM and RON95 E10, a variation of the intake temperature reveals that DEM has the potential to reduce the sensitivity of the combustion system.     

Interstellar Enolization-Acetaldehyde (CH3CHO) and Vinyl Alcohol (H2CCH(OH)) as a Case Study

Owing to the unique conditions in cold molecular clouds, enols—the thermodynamically less stable tautomers of aldehydes and ketones—do not undergo tautomerization to their more stable tautomers in the gas phase because they cannot overcome tautomerization barriers at the low temperatures. Laboratory studies of interstellar analog ices have demonstrated the formation of several keto–enol tautomer pairs in astrochemically relevant ice mixtures over the last years. However, so far only one of them, acetaldehyde−vinyl alcohol, has been detected in deep space. Due to their reactivity with electrophiles, enols can play a crucial role in our understanding of the molecular complexity in the interstellar medium and in comets and meteorites. To study the enolization of aldehydes in interstellar ices by interaction with galactic cosmic rays (GCRs), we irradiated acetaldehyde ices with energetic electrons as proxies of secondary electrons generated in the track of GCRs while penetrating interstellar ices. The results indicate that GCRs can induce enolization of acetaldehyde and that intra- as well as intermolecular processes are relevant. Therefore, enols should be ubiquitous in the interstellar medium and could be searched for using radio telescopes such as ALMA. Once enols are detected and abundances are established, they can serve as tracers for the non-equilibrium chemistry in interstellar ices thus eventually constraining fundamental reaction mechanisms deep inside interstellar ices.     

Phosphetes via Transition Metal Free Ring Closure – Taking the Proper Turn at a Thermodynamic Crossing

A transition metal free route to phosphetes featuring an exocyclic alkene unit is presented. In this approach phosphanides are added to a variety of diynes generating phosphaallylic intermediates which depending on the reaction conditions transform either to phosphetes or the corresponding phospholes. Investigation of the reaction mechanism by combined quantum chemical and experimental means identifies phosphole formation as thermodynamically controlled reaction path, whereas kinetic control furnishes the corresponding phosphetes. Structural and luminescence properties of the rare class of phosphetes are explored, as well as for selected key intermediates.     

Roll-to-Roll Production of Spider Silk Nanofiber Nonwoven Meshes Using Centrifugal Electrospinning for Filtration Applications

Filtration systems used in technical and medical applications require components for fine particle deep filtration to be highly efficient and at the same time air permeable. In high efficiency filters, nonwoven meshes, which show increased performance based on small fiber diameters (e.g., using nanofibers), can be used as fine particle filter layers. Nanofiber nonwoven meshes made by electrospinning of spider silk proteins have been recently shown to exhibit required filter properties. Needle-based electrospinning, however, is limited regarding its productivity and scalability. Centrifugal electrospinning, in contrast, has been shown to allow manufacturing of ultrathin polymer nonwoven meshes in an efficient and scalable manner. Here, continuous roll-to-roll production of nonwoven meshes made of recombinant spider silk proteins is established using centrifugal electrospinning. The produced spider silk nanofiber meshes show high filter efficiency in the case of fine particulate matter below 2.5 µm (PM2.5) and a low pressure drop, resulting in excellent filter quality.