Photo-responsive hydrogels based on a ruthenium complex: synthesis and degradation.

We report the synthesis of a photo responsive metallo-hydrogel based on a ruthenium(II) complex as a functional cross-linker. This metal complex contains reactive 4AAMP (= 4-(acrylamidomethyl)pyridine) ligands, which can be cleaved by light-induced ligand substitution. Ru[(bpy)2(4AAMP)2] cross-links 4-arm-PEG-SH macromonomers by thia-Michael-addition to the photocleavable 4AAMP ligand for the preparation of the hydrogel. Irradiation with green light at 529 nm leads to photodegradation of the metallo-hydrogel due to the ligand dissociation, which can be adjusted by adjusting the Ru[(bpy)2(4AAMP)2] concentration. The ligand substitution forming [Ru(bpy)2(L)2]2+ (L = H2O and CH3CN) can be monitored by 1H NMR spectroscopy and UV-visible absorption. The control of degradation by light irradiation plays a significant role in modulating the elasticity and stiffness of the light sensitive metallo-hydrogel network. The photo-responsive hydrogel is a viable substrate for cell cultures.

Lignin Upconversion by Functionalization and Network Formation.

Lignin, a complex and abundant biopolymer derived from plant cell walls, has emerged as a promising feedstock for sustainable material development. Due to the high abundance of phenylpropanoid units, aromatic rings, and hydroxyl groups, lignin is an ideal candidate for being explored in various material applications. Therefore, the demand on lignin valorization for development of value-added products is significantly increasing. This mini-review provides an overview of lignin upconversion, focusing on its functionalization through chemical and enzymatic routes, and its application in lignin-based polymer resins, hydrogels, and nanomaterials. The functionalization of lignin molecules with various chemical groups offers tailored properties and increased compatibility with other materials, expanding its potential applications. Additionally, the formation of lignin-based networks, either through cross-linking or blending with polymers, generates novel materials with improved mechanical, thermal, and barrier properties. However, challenges remain in optimizing functionalization techniques, preserving the innate complexity of lignin, and achieving scalability for industrial implementation. As lignin's potential continues to be unlocked, it is poised to contribute significantly to the shift towards more eco-friendly and resource-efficient industries.

Palladium-Catalyzed Directed Halogenation of Bipyridine N-Oxides.

The palladium-catalyzed directed C-H halogenation of bipyridine N-oxides was investigated. Using NCS or NBS (N-chloro- or N-bromosuccinimide) and 5 mol % Pd(OAc)2 in chlorobenzene (0.10 molar) at 110 °C, pyridine-directed functionalization took place and 3-chloro- or 3-bromobipyridine N-oxides were obtained in high yields. The reaction is sensitive to steric hindrance by 4- and 6'-substituents. Only in the latter case, where coordination of palladium by the pyridine is hindered, 3'-halogenation directed by the N-oxide function was observed. The halogenated products were deoxygenated by PCl3 or PBr3.

Monitoring the Transmembrane Proton Gradient Generated by Cytochrome bo3 in Tethered Bilayer Lipid Membranes Using SEIRA Spectroscopy.

Membrane proteins act as biocatalysts or ion/proton pumps to convert and store energy from ubiquitous environmental sources. Interfacing these proteins to electrodes allows utilizing the energy for enzymatic biofuel cells or other auspicious biotechnological applications. To optimize the efficiency of these devices, appropriate membrane models are required that ensure structural and functional integrity of the embedded enzymes and provide structural insight. We present a spectroelectrochemical surface-enhanced infrared absorption (SEIRA) and electrical impedance spectroscopy (EIS) study of the bacterial respiratory ubiquinol/cytochrome bo3 (cyt bo3) couple incorporated into a tethered bilayer lipid membrane (tBLM). Here, we employed a new lipid tether (WK3SH, dihydrocholesteryl (2-(2-(2-ethoxy)ethoxy)ethanethiol), which was synthesized using a three-step procedure with very good yield and allowed measuring IR spectra without significant spectral interference of the tBLM. The functional integrity of the incorporated cyt bo3 was demonstrated by monitoring the enzymatic O2 reduction current and the formation of the transmembrane proton gradient. Based on a SEIRA-spectroscopic redox titration, a shift of the pH-dependent redox potential of the ubiquinones under turnover conditions was correlated with an alkalinization of the submembrane reservoir by +0.8 pH units. This study demonstrates the high potential of tBLMs and the SEIRA spectroscopic approach to study bioenergetic processes.

Electrocatalytic carbon dioxide reduction by using cationic pentamethylcyclopentadienyl-iridium complexes with unsymmetrically substituted bipyridine ligands.

Eight [Ir(bpy)Cp*Cl](+) -type complexes (bpy= bipyridine, Cp*=1,2,3,4,5-pentamethylcyclopentadienyl) containing differently substituted bipyridine ligands were synthesized and characterized. Cyclic voltammetry (CV) of the complexes in Ar-saturated acetonitrile solutions showed that the redox behavior of the complexes could be fine tuned by the electronic properties of the substituted bipyridine ligands. Further CV in CO2 -saturated MeCN/H2 O (9:1, v/v) solutions showed catalytic currents for CO2 reduction. In controlled potential electrolysis experiments (MeCN/MeOH (1:1, v/v), Eapp =-1.80 V vs Ag/AgCl), all of the complexes showed moderate activity in the electrocatalytic reduction of CO2 with good stability over at least 15 hours. This electrocatalytic process was selective toward formic acid, with only traces of dihydrogen or carbon monoxide and occasionally formaldehyde as byproducts. However, the turnover frequencies and current efficiencies were quite low. No direct correlation between the redox potentials of the complexes and their catalytic activity was observed.

Oxidation of allylic and benzylic alcohols to aldehydes and carboxylic acids.

An oxidation of allylic and benzylic alcohols to the corresponding carboxylic acids is effected by merging a Cu-catalyzed oxidation using O2 as a terminal oxidant with a subsequent chlorite oxidation (Lindgren oxidation). The protocol was optimized to obtain pure products without chromatography or crystallization. Interception at the aldehyde stage allowed for Z/E-isomerization, thus rendering the oxidation stereoconvergent with respect to the configuration of the starting material.

Conversion of pyridine N-oxides to tetrazolopyridines.

An efficient and convenient procedure for the conversion of pyridine N-oxides to tetrazolopyridines by treatment with 4-toluene sulfonyl chloride and sodium azide in toluene at elevated temperature is described.

Synthesis of asymmetrically substituted terpyridines by palladium-catalyzed direct C-H arylation of pyridine N-oxides.

The synthesis of asymmetrically substituted 2,2':6',2''-terpyridines is reported. First, palladium-catalyzed C-H arylation of pyridine N-oxides with substituted bromopyridines gave 2,2'-bipyridine N-oxides, which were further arylated in a second step to form 2,2':6',2''-terpyridine N-oxides. Yields of up to 77% were obtained with N-oxides bearing an electron-withdrawing ethoxycarbonyl substituent in the 4-position. Pd(OAc)2 with either P(tBu)3 or P(o-tolyl)3 was used as the catalyst. Cyclometalated complexes derived from Pd(OAc)2 and these phosphines were also effective. K3PO4 as the base gave better results than K2CO3. Subsequent deoxygenation with H2 and Pd/C as the catalyst gave the asymmetrically substituted 2,2':6',2''-terpyridines in near quantitative yield. This reaction sequence significantly reduces the number of steps required in comparison with known cross-coupling methods and therefore allows convenient and scalable access to substituted terpyridines.

Supramolecular hydrogel capsules based on PEG: a step toward degradable biomaterials with rational design.

Supramolecular microgel capsules based on polyethylene glycol (PEG) are a promising class of soft particulate scaffolds with tailored properties. An approach to fabricate such particles with exquisite control by droplet-based microfluidics is presented. Linear PEG precursor polymers that carry bipyridine moieties on both chain termini are gelled by complexation to iron(II) ions. To investigate the biocompatibility of the microgels, living mammalian cells are encapsulated within them. The microgel elasticity is controlled by using PEG precursors of different molecular weights at different concentrations and the influence of these parameters on the cell viabilities, which can be optimized to exceed 90% is studied. Reversion of the supramolecular polymer cross-linking allows the microcapsules to be degraded at mild conditions with no effect on the viability of the encapsulated and released cells.

Synthesis of unsymmetrically substituted bipyridines by palladium-catalyzed direct C-H arylation of pyridine N-oxides.

Substituted bipyridines were efficiently prepared by direct coupling between pyridine N-oxides and halopyridines using a palladium catalyst. Pyridine N-oxides with electron-withdrawing substituents gave the best yields. This method allows the convenient preparation of 2,2'-, 2,3'-, and 2,4'-bipyridines which are useful as functionalized ligands for metal complexes or as building blocks for supramolecular architectures.

Enantiopure 2-substituted glyceraldehyde derivatives by aza-Claisen rearrangement or C-alkylation of enamines.

2-Alkyl derivatives of butane-2,3-diacetal (BDA) protected glyceraldehyde were stereoselectively prepared by aza-Claisen rearrangement of N-allyl-enammonium ions or C-alkylation of enamines. This allows rapid and convenient access to densely functionalized chiral building blocks.

Arenes to anilines and aryl ethers by sequential iridium-catalyzed borylation and copper-catalyzed coupling.

[reaction: see text] N-Alkyl- and N-arylanilines were synthesized from arenes by a two-step sequence of iridium-catalyzed borylation and copper-catalyzed coupling with amines. Diaryl ethers were obtained by a related sequence of arene borylation, followed by coupling with phenols. In particular, 3,5-disubstituted arylamines and aryl ethers were prepared by initiating this sequence with meta-substituted arenes.

One-pot synthesis of arylboronic acids and aryl trifluoroborates by Ir-catalyzed borylation of arenes.

[reaction: see text] The synthesis of arylboronic acids and aryl trifluoroborates in a one-pot sequence by Ir-catalyzed borylation of arenes is reported. To prepare the arylboronic acids, the Ir-catalyzed borylation is followed by oxidative cleavage of the boronic ester with NaIO4. To prepare the aryltrifluoroborate, the Ir-catalyzed borylation is followed by displacement of pinacol by KHF2. These two-step sequences give products that are more reactive toward subsequent chemistry than the initially formed pinacol boronates.

Amine- and ether-chelated aryllithium reagents-structure and dynamics.

Chelation and aggregation in phenyllithium reagents with potential 6- and 7-ring chelating amine (2, 3) and 5-, 6-, and 7-ring chelating ether (4, 5, 6) ortho substituents have been examined utilizing variable temperature (6)Li and (13)C NMR spectroscopy, (6)Li and (15)N isotope labeling, and the effects of solvent additives. The 5- and 6-ring ether chelates (4, 5) compete well with THF, but the 6-ring amine chelate (2) barely does, and 7-ring amine chelate (3) does not. Compared to model compounds (e.g., 2-ethylphenyllithium 7), which are largely monomeric in THF, the chelated compounds all show enhanced dimerization (as measured by K = [D]/[M](2)) by factors ranging from 40 (for 6) to more than 200 000 (for 4 and 5). Chelation isomers are seen for the dimers of 5 and 6, but a chelate structure could be assigned only for 2-(2-dimethylaminoethyl)phenyllithium (2), which has an A-type structure (both amino groups chelated to the same lithium in the dimer) based on NMR coupling in the (15)N, (6)Li labeled compound. Unlike the dimer, the monomer of 2 is not detectably chelated. With the exception of 2-(methoxymethyl)phenyllithium (4), which forms an open dimer (12) and a pentacoordinate monomer (13), the lithium reagents all form monomeric nonchelated adducts with PMDTA.