Stereodynamic Quinone-Hydroquinone Molecules That Enantiomerize at sp3-Carbon via Redox-Interconversion

In situ real-time assessment of wavelength dependent degradation of methyl orange on rGO-TiO2 photocatalyst

Titanium dioxide (TiO2) has been extensively incorporated with reduced graphene oxide (rGO) to synthesize visible light (Vis) active photocatalysts. Here, we synthesized such rGO-TiO2 nanocomposite with 10% rGO by mass. The photocatalytic activity of rGO-TiO2 was quantitatively evaluated using quartz crystal microbalance (QCM). The in situ real-time QCM data resulted in different photocatalytic degradation percentages of methyl orange (MO) at the solid-air interface under ultraviolet (UV) and Vis irradiations. Conventional UV-Vis studies at the solid-liquid interface revealed that a unique hypsochromic effect occurs selectively under Vis irradiation. Radical scavenger studies confirmed that photogenerated holes are the primary active species contributing to this wavelength dependence. The degradation mechanisms under both irradiations are proposed based on the co-catalyzing and photosensitizing dual nature of rGO. The results of this study enhance the empirical knowledge of modified TiO2 photocatalysts while signifying the impact of irradiation wavelength on the extent and mechanism of photocatalytic degradation.

An anthracene-containing crown ether: synthesis, host-guest properties and modulation of solid state luminescence

Organic solid state vapochromic materials are of great significance for the development of supramolecular chemistry and materials science. Herein, we synthesize a crown ether derivative (An34C10) containing two anthracene units and construct new crown ether-based vapochromic host-guest co-crystals. Due to the presence of anthracene, An34C10 not only shows good fluorescence properties but also displays mechanochromism. Single crystal structural analysis, powder X-ray diffraction and differential scanning calorimetry experiments demonstrate that the transformation between different stacking modes of An34C10 is responsible for mechanochromism. In addition, An34C10 can complex with 1,2,4,5-tetracyanobenzene (TCNB) to form host-guest complex (An34C10@TCNB) co-crystals. Because organic solvent fuming alters charge-transfer interactions in An34C10@TCNB, the fluorescence of the co-crystals can be turned on and off by 4-methylpyridine and chloroform vapors, respectively, realizing selective detection with opposite emission outputs. Meanwhile, the stimuli-responsive properties of An34C10 and An34C10@TCNB possess good cycling performance. This work provides a new strategy for the construction of organic solid state luminescent materials.

Using the phospha-Michael reaction for making phosphonium phenolate zwitterions

The reactions of 2,4-di-tert-butyl-6-(diphenylphosphino)phenol and various Michael acceptors (acrylonitrile, acrylamide, methyl vinyl ketone, several acrylates, methyl vinyl sulfone) yield the respective phosphonium phenolate zwitterions at room temperature. Nine different zwitterions were synthesized and fully characterized. Zwitterions with the poor Michael acceptors methyl methacrylate and methyl crotonate formed, but could not be isolated in pure form. The solid-state structures of two phosphonium phenolate molecules were determined by single-crystal X-ray crystallography. The bonding situation in the solid state together with NMR data suggests an important contribution of an ylidic resonance structure in these molecules. The phosphonium phenolates are characterized by UV-vis absorptions peaking around 360 nm and exhibit a negative solvatochromism. An analysis of the kinetics of the zwitterion formation was performed for three Michael acceptors (acrylonitrile, methyl acrylate, and acrylamide) in two different solvents (chloroform and methanol). The results revealed the proton transfer step necessary to stabilize the initially formed carbanion as the rate-determining step. A preorganization of the carbonyl bearing Michael acceptors allowed for reasonable fast direct proton transfer from the phenol in aprotic solvents. In contrast, acrylonitrile, not capable of forming a similar preorganization, is hardly reactive in chloroform solution, while in methanol the corresponding phosphonium phenolate is formed.

Control of dynamic sp3-C stereochemistry

Stereogenic sp³-hybridized carbon centres are fundamental building blocks of chiral molecules. Unlike dynamic stereogenic motifs, such as sp³-nitrogen centres or atropisomeric biaryls, sp³-carbon centres are usually fixed, requiring intermolecular reactions to undergo configurational changes. Here we report the internal enantiomerization of fluxional carbon cages and the consequences of their adaptive configurations for the transmission of stereochemical information. The sp³-carbon stereochemistry of the rigid tricyclic cages is inverted through strain-assisted Cope rearrangements, emulating the low-barrier configurational dynamics typical for sp³-nitrogen inversion or conformational isomerism. This dynamic enantiomerization can be stopped, restarted or slowed by external reagents, while the configuration of the cage is controlled by neighbouring, fixed stereogenic centres. As part of a phosphoramidite-olefin ligand, the fluxional cage acts as a conduit to transmit stereochemical information from the ligand while also transferring its dynamic properties to chiral-at-metal coordination environments, influencing catalysis, ion pairing and ligand exchange energetics.

Pyrogallate-Based Metal-Organic Framework with a Two-Dimensional Secondary Building Unit

We report a metal-organic framework (MOF) with a rare two-dimensional (2D) secondary building unit (SBU). The SBU comprises mixed-valent Fe2+ and Fe3+ metal ions bridged by oxygen atoms pertaining to the polytopic ligand 3,3',4,4',5,5'-hexahydroxybiphenyl, which also define the iron-oxide 2D layers. Overall, the anionic framework exhibits rare topology and evidences strong electronic communication between the mixed-valence iron sites. These results highlight the importance of dimensionality control of MOF SBUs for discovering new topologies in reticular chemistry, and especially for improving electronic communication within the MOF skeleton.

Microbial paracetamol degradation involves a high diversity of novel amidase enzyme candidates

Pharmaceuticals are relatively new to nature and often not completely removed in wastewater treatment plants (WWTPs). Consequently, these micropollutants end up in water bodies all around the world posing a great environmental risk. One exception to this recalcitrant conversion is paracetamol, whose full degradation has been linked to several microorganisms. However, the genes and corresponding proteins involved in microbial paracetamol degradation are still elusive. In order to improve our knowledge of the microbial paracetamol degradation pathway, we inoculated a bioreactor with sludge of a hospital WWTP (Pharmafilter, Delft, NL) and fed it with paracetamol as the sole carbon source. Paracetamol was fully degraded without any lag phase and the enriched microbial community was investigated by metagenomic and metatranscriptomic analyses, which demonstrated that the microbial community was very diverse. Dilution and plating on paracetamol-amended agar plates yielded two Pseudomonas sp. isolates: a fast-growing Pseudomonas sp. that degraded 200 mg/L of paracetamol in approximately 10 h while excreting 4-aminophenol, and a slow-growing Pseudomonas sp. that degraded paracetamol without obvious intermediates in more than 90 days. Each Pseudomonas sp. contained a different highly-expressed amidase (31% identity to each other). These amidase genes were not detected in the bioreactor metagenome suggesting that other as-yet uncharacterized amidases may be responsible for the first biodegradation step of paracetamol. Uncharacterized deaminase genes and genes encoding dioxygenase enzymes involved in the catabolism of aromatic compounds and amino acids were the most likely candidates responsible for the degradation of paracetamol intermediates based on their high expression levels in the bioreactor metagenome and the Pseudomonas spp. genomes. Furthermore, cross-feeding between different community members might have occurred to efficiently degrade paracetamol and its intermediates in the bioreactor. This study increases our knowledge about the ongoing microbial evolution towards biodegradation of pharmaceuticals and points to a large diversity of (amidase) enzymes that are likely involved in paracetamol metabolism in WWTPs.

Diastereomeric dinickel(II) complexes with non-innocent bis(octaazamacrocyclic) ligands: isomerization, spectroelectrochemistry, DFT calculations and use in catalytic oxidation of cyclohexane

Diastereomeric dinickel(II) complexes with bis-octaazamacrocyclic 15-membered ligands [Ni(L1-3-L1-3)Ni] (4-6) have been prepared by oxidative dehydrogenation of nickel(II) complexes NiL1-3 (1-3) derived from 1,2- and 1,3-diketones and S-methylisothiocarbohydrazide. The compounds were characterized by elemental analysis, ESI mass spectrometry, and IR, UV-vis, 1H NMR, and 13C NMR spectroscopy. Single crystal X-ray diffraction (SC-XRD) confirmed the isolation of the anti and syn isomers of bis-octaazamacrocyclic dinickel(II) complexes 4a and 4s, the syn-configuration of 5s and the anti-configuration of the dinickel(II) complex 6a. Dimerization of prochiral nickel(II) complexes 1-3 generates two chiral centers at the bridging carbon atoms. The anti-complexes were isolated as meso-isomers (4a and 6a) and the syn-compounds as racemic mixtures of R,R/S,S-enantiomers (4s and 5s). The syn-anti isomerization (epimerization) of the isolated complexes in chloroform was disclosed. The isomerization kinetics of 5a was monitored at five different temperatures ranging from 20 °C to 50 °C by 1H NMR spectroscopy indicating the clean conversion of 5a into 5s. The activation barrier determined from the temperature dependence of the rate constants via the Eyring equation was found to be ΔH‡ = 114 ± 1 kJ mol-1 with activation entropy ΔS‡ = 13 ± 3 J K-1 mol-1. The complexes contain two low-spin nickel(II) ions in a square-planar coordination environment. The electrochemical behavior of 4a, 4s, 5s and 6a and the electronic structure of the oxidized species were studied by UV-vis-NIR-spectroelectrochemistry (SEC) and DFT calculations indicating the redox non-innocent behavior of the complexes. The dinickel(II) complexes 4a, 4s, 5s and 6a/6s were investigated as catalysts for microwave-assisted solvent-free oxidation of cyclohexane by tert-butyl hydroperoxide to produce a mixture of cyclohexanone and cyclohexanol (KA oil). The best value for KA oil yield (16%) was obtained with a mixture of 6a/6s after 2 h of microwave irradiation at 100 °C.

Selective Extraction, Recovery, and Sensing of Hydroquinone Mediated by a Supramolecular Pillar[5]quinone Quinhydrone Charge-Transfer Complex

Intermolecular interactions between an electron-rich aromatic hydroquinone (HQ) with its electron deficient counterpart, benzoquinone (BQ), result in the formation of a quinhydrone charge-transfer complex. Herein, we report a novel quinhydrone-type complex between pillar[5]quinone (P[5]Q) and HQ. Characterized by a suite of spectroscopic techniques including ¹H NMR, UV-visible, and FTIR together with PXRD, SEM, BET, CV, and DFT modeling studies, the stability of the complex is determined to be due to an electron-proton transfer reaction coupled with a complementary donor-acceptor interaction. The selectivity of P[5]Q toward HQ over other dihydroxybenzene isomers allows for not only the naked-eye detection of HQ but also its selective liquid-liquid extraction and recovery from aqueous media.

Molecular rotor based on an oxidized resorcinarene

Rate of rotation of substituents in a molecular single stator-double rotor based on an oxidized resorcinarene with unsaturated hemiquinonoid groups at its meso positions (i.e., a fuchsonarene) has been controlled according to solvent polarity and acidity.

Modification of N-terminal α-amine of proteins via biomimetic ortho-quinone-mediated oxidation

Naturally abundant quinones are important molecules, which play essential roles in various biological processes due to their reduction potential. In contrast to their universality, the investigation of reactions between quinones and proteins remains sparse. Herein, we report the development of a convenient strategy to protein modification via a biomimetic quinone-mediated oxidation at the N-terminus. By exploiting unique reactivity of an ortho-quinone reagent, the α-amine of protein N-terminus is oxidized to generate aldo or keto handle for orthogonal conjugation. The applications have been demonstrated using a range of proteins, including myoglobin, ubiquitin and small ubiquitin-related modifier 2 (SUMO2). The effect of this method is further highlighted via the preparation of a series of 17 macrophage inflammatory protein 1β (MIP-1β) analogs, followed by preliminary anti-HIV activity and cell viability assays, respectively. This method offers an efficient and complementary approach to existing strategies for N-terminal modification of proteins.

Methods for selective modification of the N-terminus of proteins are of high interest, but mostly require specific amino acid residues. Here, the authors report a selective and fast method for N-terminal modification of proteins based on quinone-mediated oxidation of the alpha-amine to aldehyde or ketone, and apply it to diverse proteins.

Nanomolecular singlet oxygen photosensitizers based on hemiquinonoid-resorcinarenes, the fuchsonarenes

Singlet oxygen sensitization involving a class of hemiquinonoid-substituted resorcinarenes prepared from the corresponding 3,5-di-t-butyl-4-hydroxyphenyl-substituted resorcinarenes is reported. Based on variation in the molecular structures, quantum yields comparable with that of the well-known photosensitizing compound meso-tetraphenylporphyrin were obtained for the octabenzyloxy-substituted double hemiquinonoid resorcinarene reported herein. The following classes of compounds were studied: benzyloxy-substituted resorcinarenes, acetyloxy-substituted resorcinarenes and acetyloxy-substituted pyrogallarenes. Single crystal X-ray crystallographic analyses revealed structural variations in the compounds with conformation (i.e., rctt, rccc, rcct) having some influence on the identity of hemiquinonoid product available. Multiplicity of hemiquinonoid group affects singlet oxygen quantum yield with those doubly substituted being more active than those containing a single hemiquinone. Compounds reported here lacking hemiquinonoid groups are inactive as photosensitizers. The term 'fuchsonarene' (fuchson + arene of resorcinarene) is proposed for use to classify the compounds.

Multimodal switching of a redox-active macrocycle

Molecules that can exist in multiple states with the possibility of toggling between those states based on different stimuli have potential for use in molecular switching or sensing applications. Multimodal chemical or photochemical oxidative switching of an antioxidant-substituted resorcinarene macrocycle is reported. Intramolecular charge-transfer states, involving hemiquinhydrones are probed and these interactions are used to construct an oxidation-state-coupled molecular switching manifold that reports its switch-state conformation via striking variation in its electronic absorption spectra. The coupling of two different oxidation states with two different charge-transfer states within one macrocyclic scaffold delivers up to five different optical outputs. This molecular switching manifold exploits intramolecular coupling of multiple redox active substituents within a single molecule.

A Stereodynamic Redox-Interconversion Network of Vicinal Tertiary and Quaternary Carbon Stereocenters in Hydroquinone-Quinone Hybrid Dihydrobenzofurans

Reversible redox processes involving hydroquinones and quinones are ubiquitous in biological reaction networks, materials science, and catalysis. While extensively studied in intermolecular settings, less is known about intramolecular scenarios. Herein, we report hydroquinone-quinone hybrid molecules that form two-stereocenter dihydrobenzofurans via intramolecular cyclization under thermodynamic control. A π-methylhistidine peptide-catalyzed kinetic resolution allowed us to study the stereodynamic behavior of enantio- and diastereo-enriched dihydrofurans. In the course of this study, it was revealed that a reversible intramolecular redox-interconversion network connects all four possible stereoisomers via inversion of a quaternary carbon stereocenter without achiral intermediates. As a result, these findings on hydroquinone-quinone hybrid molecules provide insights into potential natural origin and synthetic access of the common dihydrobenzofuran motif.

Molecular Dynamics Simulations of a Conformationally Mobile Peptide-Based Catalyst for Atroposelective Bromination

It is widely accepted that structural rigidity is required to achieve high levels of asymmetric induction in catalytic, enantioselective reactions. This fundamental design principle often does not apply to highly selective catalytic peptides that often exhibit conformational heterogeneity. As a result, these complex systems are particularly challenging to study both experimentally and computationally. Herein, we utilize molecular dynamics simulations to investigate the role of conformational mobility on the reactivity and selectivity exhibited by a catalytic, β-turn-biased peptide in an atroposelective bromination reaction. By means of cluster analysis, multiple distinct conformers of the peptide and a catalyst-substrate complex were identified in the simulations, all of which were corroborated by experimental NMR measurements. The simulations also revealed that a shift in the conformational equilibrium of the peptidic catalyst occurs upon addition of substrate, and the degree of change varies among different substrates. On the basis of these data, we propose a correlation between the composition of the peptide conformational ensemble and its catalytic properties. Moreover, these findings highlight the importance of conformational dynamics in catalytic, asymmetric reactions mediated by oligopeptides, unveiled through high-level, state-of-the-art computational modeling.