Recent Synthetic Advances on the Use of Diazo Compounds Catalyzed by Metalloporphyrins

Diazo compounds are organic substances that are often used as precursors in organic synthesis like cyclization reactions, olefinations, cyclopropanations, cyclopropenations, rearrangements, and carbene or metallocarbene insertions into C-H, N-H, O-H, S-H, and Si-H bonds. Typically, reactions from diazo compounds are catalyzed by transition metals with various ligands that modulate the capacity and selectivity of the catalyst. These ligands can modify and enhance chemoselectivity in the substrate, regioselectivity and enantioselectivity by reflecting these preferences in the products. Porphyrins have been used as catalysts in several important reactions for organic synthesis and also in several medicinal applications. In the chemistry of diazo compounds, porphyrins are very efficient as catalysts when complexed with low-cost metals (e.g., Fe and Co) and, therefore, in recent years, this has been the subject of significant research. This review will summarize the advances in the studies involving the field of diazo compounds catalyzed by metalloporphyrins (M-Porph, M = Fe, Ru, Os, Co, Rh, Ir) in the last five years to provide a clear overview and possible opportunities for future applications. Also, at the end of this review, the properties of artificial metalloenzymes and hemoproteins as biocatalysts for a broad range of applications, namely those concerning carbene-transfer reactions, will be considered.

Long-Term In-Use NOx Emissions from London Buses with Retrofitted NOx Aftertreatment

Buses constitute a significant source of air pollutant emissions in cities. In this study, we present real-world NO_x emissions from 97 diesel-hybrid buses measured using on-board diagnostic systems over 44 months and 6.35 million km in London. Each bus had previously been retrofitted with a selective catalytic reduction (SCR) aftertreatment system to reduce emissions of nitrogen oxides (NO_x). On average, parallel hybrid (PH) and series hybrid (SH) buses emitted 3.80 g of NO_x/km [standard deviation (SD) of 1.02] and 2.37 g of NO_x/km (SD of 0.51), respectively. The SCR systems reduced engine-out emissions by 79.8% (SD of 5.0) and 87.2% (SD of 2.9) for the PHs and SHs, respectively. Lower ambient temperatures (0-10 °C) increased NO_x emissions of the PHs by 24.2% but decreased NO_x emissions of the SHs by 27.9% compared to values found at more moderate temperatures (10-20 °C). To improve emissions inventories, we provide new distance-based NO_x emissions factors for different ranges of ambient temperature. During the COVID-19 pandemic, the emissions benefits of reduced congestion were largely offset by more frequent route layovers leading to lower SCR temperatures and effectiveness. This study shows that continuous in-service measurements enable quantification of real-world vehicle emissions over a wide range of operations that complements conventional testing approaches.

Catalytic Co-Conversion of CH4 and CO2 Mediated by Rhodium-Titanium Oxide Anions RhTiO2. Angew Chem Int Ed Engl 2021;60:13788-13792. [PMID: 33890352 PMCID: PMC8251526 DOI: 10.1002/anie.202103808]

Catalytic co‐conversion of methane with carbon dioxide to produce syngas (2 H₂+2 CO) involves complicated elementary steps and almost all the elementary reactions are performed at the same high temperature conditions in practical thermocatalysis. Here, we demonstrate by mass spectrometric experiments that RhTiO₂⁻ promotes the co‐conversion of CH₄ and CO₂ to free 2 H₂+CO and an adsorbed CO (CO_ads) at room temperature; the only elementary step that requires the input of external energy is desorption of CO_ads from the RhTiO₂CO⁻ to reform RhTiO₂⁻. This study not only identifies a promising active species for dry (CO₂) reforming of methane to syngas, but also emphasizes the importance of temperature control over elementary steps in practical catalysis, which may significantly alleviate the carbon deposition originating from the pyrolysis of methane.

Advanced Understanding of the Electron Transfer Pathway of Cytochrome P450s

Cytochrome P450s are heme-thiolate enzymes that participate in carbon source assimilation, natural compound biosynthesis and xenobiotic metabolism in all kingdoms of life. P450s can catalyze various reactions by using a wide range of organic compounds, thus exhibiting great potential in biotechnological applications. The catalytic reactions of P450s are driven by electron equivalents that are sourced from pyridine nucleotides and delivered by cognate or matching redox partners (RPs). The electron transfer (ET) route from RPs to P450s involves one or more redox center-containing domains. As the rate of ET is one of the main determinants of P450 efficacy, an in-depth understanding of the P450 ET pathway should increase our knowledge of these important enzymes and benefit their further applications. Here, the various P450 RP systems along with current understanding of their ET routes will be reviewed. Notably, state-of-the-art structural studies of the two main types of self-sufficient P450 will also be summarized.

Ultrastable and Highly Catalytically Active N-Heterocyclic-Carbene-Stabilized Gold Nanoparticles in Confined Spaces

Controlling the size and surface functionalization of nanoparticles (NPs) can lead to improved properties and applicability. Herein, we demonstrate the efficiency of the metal-carbene template approach (MCTA) to synthesize highly robust and soluble three-dimensional polyimidazolium cages (PICs) of different sizes, each bearing numerous imidazolium groups, and use these as templates to synthesize and stabilize catalytically active, cavity-hosted, dispersed poly-N-heterocyclic carbene (NHC)-anchored gold NPs. Owing to the stabilization of the NHC ligands and the effective confinement of the cage cavities, the as-prepared poly-NHC-shell-encapsulated AuNPs displayed promising stability towards heat, pH, and chemical regents. Most notably, all the Au@PCCs (PCC=polycarbene cage) exhibited excellent catalytic activities in various chemical reactions, together with high stability and durability.

Flow-Through Catalytic Reactors Based on Metal Nanoparticles Immobilized within Porous Polymeric Gels and Surfaces/Hollows of Polymeric Membranes

State-of-the-art of flow-through catalytic reactors based on metal nanoparticles immobilized within the pores of nano-, micro- and macrosized polymeric gels and in the surface or hollow of polymeric membranes is discussed in this mini-review. The unique advantages of continuous flow-through nanocatalysis over the traditional batch-type analog are high activity, selectivity, productivity, recyclability, continuous operation, and purity of reaction products etc. The methods of fabrication of polymeric carriers and immobilization technique for metal nanoparticles on the surface of porous or hollow structures are considered. Several catalytic model reactions comprising of hydrolysis, decomposition, hydrogenation, oxidation, Suzuki coupling and enzymatic reactions in the flow system are exemplified. Realization of “on-off” switching mechanism for regulation of the rate of catalytic process through controlling the mass transfers of reactants in liquid media with the help of stimuli-responsive polymers is demonstrated. Comparative analysis of the efficiency of different flow-through catalytic reactors for various reactions is also surveyed.

Unexpected quenching effect on new particle formation from the atmospheric reaction of methanol with SO3

Despite the high abundance in the atmosphere, alcohols in general and methanol in particular are believed to play a small role in atmospheric new particle formation (NPF) largely due to the weak binding abilities of alcohols with the major nucleation precursors, e.g., sulfuric acid (SA) and dimethylamine (DMA). Herein, we identify a catalytic reaction that was previously overlooked, namely, the reaction between methanol and SO₃, catalyzed by SA, DMA, or water. We found that alcohols can have unexpected quenching effects on the NPF process, particularly in dry and highly polluted regions with high concentrations of alcohols. Specifically, the catalytic reaction between methanol and SO₃ can convert methanol into a less-volatile species--methyl hydrogen sulfate (MHS). The latter was initially thought to be a good nucleation agent for NPF. However, our simulation results suggest that the formation of MHS consumes an appreciable amount of atmospheric SO₃, disfavoring further reactions of SO₃ with H₂O. Indeed, we found that MHS formation can cause a reduction of SA concentration up to 87%, whereas the nucleation ability of MHS toward new particles is not as good as that of SA. Hence, a high abundance of methanol in the atmosphere can lower the particle nucleation rate by as much as two orders of magnitude. Such a quenching effect suggests that the recently identified catalytic reactions between alcohols and SO₃ need to be considered in atmospheric modeling in order to predict SA concentration from SO₂, while also account for their potentially negative effect on NPF.

Direct Conversion of Methane with Carbon Dioxide Mediated by RhVO3 - Cluster Anions

Direct conversion of methane with carbon dioxide to value-added chemicals is attractive but extremely challenging because of the thermodynamic stability and kinetic inertness of both molecules. Herein, the first dinuclear cluster species, RhVO₃ ^- , has been designed to mediate the co-conversion of CH₄ and CO₂ to oxygenated products, CH₃ OH and CH₂ O, in the temperature range of 393-600 K. The resulting cluster ions RhVO₃ CO^- after CH₃ OH formation can further desorb the [CO] unit to regenerate the RhVO₃ ^- cluster, leading to the successful establishment of a catalytic cycle for methanol production from CH₄ and CO₂ (CH₄ +CO₂ →CH₃ OH+CO). The exceptional activity of Rh-V dinuclear oxide cluster (RhVO₃ ^- ) identified herein provides a new mechanism for co-conversion of two very stable molecules CH₄ and CO₂ .

Rational Engineering of a Dynamic, Entropy-Driven DNA Nanomachine for Intracellular MicroRNA Imaging

We rationally engineered an elegant entropy-driven DNA nanomachine with three-dimensional track and applied it for intracellular miRNAs imaging. The proposed nanomachine is activated by target miRNA binding to drive a walking leg tethered to gold nanoparticle with a high density of DNA substrates. The autonomous and progressive walk on the DNA track via the entropy-driven catalytic reaction of intramolecular toehold-mediated strand migration leads to continuous disassembly of DNA substrates, accompanied by the recovery of fluorescence signal due to the specific release of a dye-labeled substrate from DNA track. Our nanomachine outperforms the conventional intermolecular reaction-based gold nanoparticle design in the context of an improved sensitivity and kinetics, attributed to the enhanced local effective concentrations of working DNA components from the proximity-induced intramolecular reaction. Moreover, the nanomachine was applied for miRNA imaging inside living cells.

Inferring catalysis in biological systems

In systems biology, one is often interested in the communication patterns between several species, such as genes, enzymes or proteins. These patterns become more recognisable when temporal experiments are performed. This temporal communication can be structured by reaction networks such as gene regulatory networks or signalling pathways. Mathematical modelling of data arising from such networks can reveal important details, thus helping to understand the studied system. In many cases, however, corresponding models still deviate from the observed data. This may be due to unknown but present catalytic reactions. From a modelling perspective, the question of whether a certain reaction is catalysed leads to a large increase of model candidates. For large networks the calibration of all possible models becomes computationally infeasible. We propose a method which determines a substantially reduced set of appropriate model candidates and identifies the catalyst of each reaction at the same time. This is incorporated in a multiple-step procedure which first extends the network by additional latent variables and subsequently identifies catalyst candidates using similarity analysis methods. Results from synthetic data examples suggest a good performance even for non-informative data with few observations. Applied on CD95 apoptotic pathway our method provides new insights into apoptosis regulation.

Precisely Controllable Core-Shell Ag@Carbon Dots Nanoparticles: Application to in Situ Super-Sensitive Monitoring of Catalytic Reactions

Carbon dots (CDs) have attracted extensive interest owing to their unparalleled physical and chemical characteristics. CDs based nanocomposites have also drawn increasing attention because the combination of different characteristics could offer additional brilliant properties (such as photocatalysis and Raman scattering). In this work, we developed a fast, facile, and controllable method for fabricating core-shell Ag@CDs nanoparticles (NPs) based on the ability of CDs to directly reduce Ag⁺ to Ag NPs without an external photoirradiation process or additional reductants. The as-prepared Ag@CDs NPs caused efficient CDs fluorescence quenching, and the typical bands of carbon species were obtained in the Raman spectrum of CDs. In addition, we found that the Ag@CDs NPs could be utilized as an efficient surface-enhanced Raman scattering (SERS) substrate, showing a discernible detection concentration as low as 10^-8 M by using p-aminothiophenol (PATP) as the probe molecules. The as-prepared Ag@CDs NPs used as the SERS substrate also exhibited excellent peroxidase-like catalytic activity for in situ super-sensitive monitoring of the oxidation of 3,3',5,5'-tetramethylbenzidine by H₂O₂, a plasmon-enhanced driven photocatalytic reaction of p-nitrothiophenol (PNTP) dimerizing into 4,4'-dimercaptoazobenzene, and catalytic driven reduction of PNTP to PATP in the presence of NaBH₄ in real time. Moreover, the determination of H₂O₂ with a significantly lower discernible detection concentration was obtained. This work demonstrated that the hybrid nanostructures not only exhibited unique SERS properties but also showed excellent catalytic activities, especially as an ultrasensitive SERS substrate for monitoring heterogeneous catalytic reactions in real time. This would make it possible to not only obtain the information about catalytic molecular changes but also conduct quantitative and qualitative analysis, and widen the application of CDs in SERS and catalytic reactions.

The Search for Candidate Relevant Subsets of Variables in Complex Systems

We describe a method to identify relevant subsets of variables, useful to understand the organization of a dynamical system. The variables belonging to a relevant subset should have a strong integration with the other variables of the same relevant subset, and a much weaker interaction with the other system variables. On this basis, extending previous work on neural networks, an information-theoretic measure, the dynamical cluster index, is introduced in order to identify good candidate relevant subsets. The method does not require any previous knowledge of the relationships among the system variables, but relies on observations of their values over time. We show its usefulness in several application domains, including: (i) random Boolean networks, where the whole network is made of different subnetworks with different topological relationships (independent or interacting subnetworks); (ii) leader-follower dynamics, subject to noise and fluctuations; (iii) catalytic reaction networks in a flow reactor; (iv) the MAPK signaling pathway in eukaryotes. The validity of the method has been tested in cases where the data are generated by a known dynamical model and the dynamical cluster index is applied in order to uncover significant aspects of its organization; however, it is important that it can also be applied to time series coming from field data without any reference to a model. Given that it is based on relative frequencies of sets of values, the method could be applied also to cases where the data are not ordered in time. Several indications to improve the scope and effectiveness of the dynamical cluster index to analyze the organization of complex systems are finally given.

Preparation of catalytic materials using ionic liquids as the media and functional components

Ionic liquids (ILs) have attracted much attention due to their unique properties and wide application potential in a variety of fields. The unusual properties of ILs provide numerous opportunities to design and prepare arious advanced materials, including highly efficient catalysts. In recent years, synthesis of different kinds of catalytic materials and their applications in chemical reactions have been studied extensively and have become a very interesting area. Herein, we present a review on the synthesis of catalytic materials using ILs as the media and/or functional components; the important and widely investigated topics are discussed, including mainly metal nanocatalysts/IL, functional IL/support, metals or metal oxides/IL/support, polymeric ILs (PILs) catalysts, and the performances of catalytic systems are highlighted. An outlook for this interesting area is also given at the end of the article.