4-membered Ring Research Articles

Reactivity of NHI-Stabilized Heavier Tetrylenes towards CO2 and N2 O.

A heteroleptic amino(imino)stannylene (TMS2 N)(It BuN)Sn: (TMS=trimethylsilyl, It Bu=C[(N-t Bu)CH]2 ) as well as two homoleptic NHI-stabilized tetrylenes, (It BuN)2 E: (NHI=N-heterocyclic imine, E=Ge, Sn) are presented. VT-NMR investigations of (It BuN)2 Sn: (2) reveal an equilibrium between the monomeric stannylene at room temperature and the dimeric form at -80 °C as well as in the solid state. Upon reaction of the homoleptic tetrylenes with CO2 , both compounds insert two equivalents of CO2 , however differing bonding modes can be observed. (It BuN)2 Sn: (2) inserts one equivalent of CO2 into each Sn-N bond, giving carbamato groups coordinated κ2 O,O' to the metal center. With (It BuN)2 Ge: (3), the Ge-N bonds stay intact upon activation, being bridged by one molecule of CO2 respectively, forming 4-membered rings. Furthermore, the reactivity of 2 towards N2 O was investigated, resulting in partial oxidation to form stannylene dimer [((It BuN)3 SnO)(It BuN)Sn:]2 (6).

The Multivariable Zhang–Zhang Polynomial of Phenylenes

The Zhang–Zhang polynomial of a benzenoid system is a well-known counting polynomial that was introduced in 1996. It was designed to enumerate Clar covers, which are spanning subgraphs with only hexagons and edges as connected components. In 2018, the generalized Zhang–Zhang polynomial of two variables was defined such that it also takes into account 10-cycles of a benzenoid system. The aim of this paper is to introduce and study a new variation of the Zhang–Zhang polynomial for phenylenes, which are important molecular graphs composed of 6-membered and 4-membered rings. In our case, Clar covers can contain 4-cycles, 6-cycles, 8-cycles, and edges. Since this new polynomial has three variables, we call it the multivariable Zhang–Zhang (MZZ) polynomial. In the main part of the paper, some recursive formulas for calculating the MZZ polynomial from subgraphs of a given phenylene are developed and an algorithm for phenylene chains is deduced. Interestingly, computing the MZZ polynomial of a phenylene chain requires some techniques that are different to those used to calculate the (generalized) Zhang–Zhang polynomial of benzenoid chains. Finally, we prove a result that enables us to find the MZZ polynomial of a phenylene with branched hexagons.

Connecting 16- with 4-Membered Rings: Ring-Opening Polymerization of Bio-Based ω-Pentadecalactone with Amino-Alkoxy-Bis(phenolate) Yttrium Initiators and Copolymerization with β-Butyrolactone

Ring-opening polymerization (ROP) of bio-based lactones is an effective way to overcome limited fossil resources. The 16-membered, musk-derived ω-pentadecalactone (PDL) occurs naturally as an ω-hydroxy fatty acid. As PDL is considered a macrolactone, controlled ROP of this transoid lactone is mostly obtained via enzymatic routes. Herein, amino-alkoxy-bis(phenolate) yttrium amido and alkoxide catalysts are reported for controlled metal-catalyzed ROP of PDL. PDL polymerizations are conducted at elevated temperatures from 60 to 100 °C to produce poly(ω-pentadecalactone) (PPDL) with number-average molecular weights of up to 134.5 kg mol−1 and moderate polydispersities (1.5 to 2.1). Utilizing isopropanol as a chain transfer agent (CTA) lead to higher initiator efficiencies and low polydispersities while still showing full monomer conversions with molar masses tunable by the amount of CTA. Block copolymerizations of PDL are performed via sequential addition. Four-membered, cisoid, and rather reluctant, racemic β-butyrolactone (BBL), which can be synthesized from carbon dioxide, is chosen as the second block. After determining that PDL has a lower coordination strength to the yttrium center and has to be added as the first monomer, successful connection between PPDL and poly(3-hydroxybutyrate) (PHB) blocks is shown via SEC and DOSY NMR spectroscopy. Different ratios of PDL and BBL are chosen to obtain PPDL-b-PHB polymers with different block length and molar masses. Powder X-ray diffraction measurements and DSC measurements prove the preservation of crystallinity in the block copolymers.

Uniform Li-ion diffusion and robust solid electrolyte interface construction for kilogram-scale Si@ZIF powder as the anode in Li-ion batteries

The zeolitic imidazolate framework (ZIF) is characterized by a highly ordered structure, large cavities, and thermal/chemical stabilities and has been widely researched for energy storage. In this study, using commercial Si powder with a diameter of 50–200 nm at a kilogram scale, Si@ZIF core-shell particles with dispersed ZIF-8 rhombic dodecahedrons were fabricated by a one-pot method and achieved excellent electrochemical performance. With the specific pore diameter and ordered network formed by membered rings, experimental results demonstrated that the ZIF-8 shell realized a uniform and accelerated Li+ diffusion route, alleviated volumetric expansion of the Si core, and constructed a LiF-concentrated robust solid electrolyte interface film with less unnecessary consumption of electrolyte and Li+. Density functional theory calculations verified the higher adsorption energy for Li-solvated clusters and the de-solvation effect on the surface of ZIF-8, which can increase Li+ concentration along the one-dimensional channels of 4-membered ring with a diameter similar to that of Li+ for high-speed and uniform Li+ intercalation. Furthermore, the large and elastic rhombic dodecahedrons formed by pure ZIF-8 buffered the volume change and maintained the integrity of the Si electrode during cycling. As a result, the Si@8Z anode achieved a reversible capacity of 818.5 mAh g−1 after 650 cycles at 1 A g−1 with a high initial coulombic efficiency of 88.2 % and outstanding rater performance even at 8 A g−1. Nevertheless, the capacity of the Si anode decreased to 0 mAh g−1 after 200 cycles. The present work clarifies the effect of ZIF on the performance of Si anodes and provides a simple method to modify commercial Si powder.

Conditions for higher methanol selectivity for partial CH4 oxidation over Fe-MOR using N2O as the oxidant and comparison to Fe-SSZ-13, Fe-SSZ-39, Fe-FER, and Fe-ZSM-5

Continuous catalytic conversion of methane to methanol was investigated on Fe-MOR at atmospheric pressure and moderate temperatures (270–300 °C) with an emphasis on improving the methanol selectivity. Results showed a relation between the methanol selectivity, iron content and mesopore addition on Fe-MOR as well as H2O concentration in the feed. Decreasing the iron content of Fe-MOR, adding mesopores to MOR or increasing water vapor pressure from 3 to 21 kPa gave higher methanol selectivity. Addition of mesopores suppressed the over-oxidation products (CO and CO2) and coke formation significantly, resulting in an increased catalyst lifetime as well as higher methanol turnover frequency (2.0 vs. 3.9 h−1 at 320 °C). Using DFT methods several active site candidates were analyzed regarding their formation energies, UV–Vis spectra and methanol formation pathways. DFT studies suggest that [Fe2O2]2+ or [HOFe(μ-O)FeOH]2+ sites on the 8 member rings are active for direct methane to methanol conversion.

Toward the complete set of fulleroids with four-membered rings: Predicted topologies and symmetry statistics for Cn (n = 20–60)

The fullerene-like structures can be characterized not only by the classical topology (12 pentagonal and rest hexagonal rings), but can also contain 4-membered rings (typically forbidden due to strong steric stresses). Here we report the numbers and the symmetry point group statistics for all C 20–C 60 fulleroids with 4-membered rings (141325 in total). The most symmetrical structures with 6 to 120 automorphism group orders (251 in total) are drawn in Schlegel diagrams and characterized by facet sets and symmetry point groups in the list. Among them, 19 series of tubulen-like structures constructed from pairs of the same caps separated by belts of hexagons have been found. The obtained results are of practical interest and can be used for the description of the topologies of polyoxometallates with the fullerene-related structures.

Constructing the Mechanism of Dinoflagellate Luciferin Bioluminescence Using Computation.

Dinoflagellate luciferin bioluminescence is unique since it does not rely on decarboxylation but is poorly understood compared to that of firefly, bacteria, and coelenterata luciferins. Here we computationally investigate possible protonation states, stereoisomers, a chemical mechanism, and the dynamics of the bioluminescence intermediate that is responsible for chemiexcitation. Using semiempirical dynamics, time-dependent density functional theory static calculations, and a correlation diagram, we find that the intermediate's functional group that is likely responsible for chemiexcitation is a 4-member ring, a dioxetanol, that undergoes [2π + 2π] cycloreversion and the biolumiphore is the cleaved structure. The simulated emission spectra and luciferase-dependent absorbance spectra agree with the experimental data, giving support to our proposed mechanism and biolumiphore. We also compute circular dichroism spectra of the intermediate's four stereoisomers to guide future experiments in differentiating them.

Ethene-1,1-disulfonyl Difluoride (EDSF) for SuFEx Click Chemistry: Synthesis of SuFExable 1,1-Bissulfonylfluoride Substituted Cyclobutene Hubs.

We present the synthesis of 1,1-bis(fluorosulfonyl)-2-(pyridin-1-ium-1-yl)ethan-1-ide, a bench-stable precursor to ethene-1,1-disulfonyl difluoride (EDSF). The novel SuFEx reagent, EDSF, is demonstrated in the preparation of 26 unique 1,1-bissulfonylfluoride substituted cyclobutenes via a cycloaddition reaction. The regioselective click cycloaddition reaction is rapid, straightforward, and highly efficient, enabling the generation of highly functionalized 4-membered ring (4MR) carbocycles. These carbocycles are valuable structural motifs found in numerous bioactive natural products and pharmaceutically relevant small molecules. Additionally, we showcase diversification of the novel cyclobutene cores through selective Cs2 CO3 -activated SuFEx click chemistry between a single S-F group and an aryl alcohol, yielding the corresponding sulfonate ester products with high efficiency. Finally, density functional theory calculations offer mechanistic insights about the reaction pathway.

Persistent topological Laplacian analysis of SARS-CoV-2 variants.

Topological data analysis (TDA) is an emerging field in mathematics and data science. Its central technique, persistent homology, has had tremendous success in many science and engineering disciplines. However, persistent homology has limitations, including its inability to handle heterogeneous information, such as multiple types of geometric objects; being qualitative rather than quantitative, e.g., counting a 5-member ring the same as a 6-member ring, and a failure to describe non-topological changes, such as homotopic changes in protein-protein binding. Persistent topological Laplacians (PTLs), such as persistent Laplacian and persistent sheaf Laplacian, were proposed to overcome the limitations of persistent homology. In this work, we examine the modeling and analysis power of PTLs in the study of the protein structures of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike receptor binding domain (RBD). First, we employ PTLs to study how the RBD mutation-induced structural changes of RBD-angiotensin-converting enzyme 2 (ACE2) binding complexes are captured in the changes of spectra of the PTLs among SARS-CoV-2 variants. Additionally, we use PTLs to analyze the binding of RBD and ACE2-induced structural changes of various SARS-CoV-2 variants. Finally, we explore the impacts of computationally generated RBD structures on a topological deep learning paradigm and predictions of deep mutational scanning datasets for the SARS-CoV-2 Omicron BA.2 variant. Our results indicate that PTLs have advantages over persistent homology in analyzing protein structural changes and provide a powerful new TDA tool for data science.

Three-dimensional printing of silica glass with sub-micrometer resolution

Silica glass is a high-performance material used in many applications such as lenses, glassware, and fibers. However, modern additive manufacturing of micro-scale silica glass structures requires sintering of 3D-printed silica-nanoparticle-loaded composites at ~1200 °C, which causes substantial structural shrinkage and limits the choice of substrate materials. Here, 3D printing of solid silica glass with sub-micrometer resolution is demonstrated without the need of a sintering step. This is achieved by locally crosslinking hydrogen silsesquioxane to silica glass using nonlinear absorption of sub-picosecond laser pulses. The as-printed glass is optically transparent but shows a high ratio of 4-membered silicon-oxygen rings and photoluminescence. Optional annealing at 900 °C makes the glass indistinguishable from fused silica. The utility of the approach is demonstrated by 3D printing an optical microtoroid resonator, a luminescence source, and a suspended plate on an optical-fiber tip. This approach enables promising applications in fields such as photonics, medicine, and quantum-optics.

Unveiling the low-temperature oxidation mechanism of a carbon-neutral fuel – Monoglyme via theoretical study

Monoglyme is a promising carbon-neutral for engines due to its renewability, compatibility with existing infrastructure, high energy content and cetane number, etc. However, its low-temperature oxidation mechanism is not well understood, which hinders its application in advantage combustion technologies. Therefore, this study carried out an in-depth theoretical analysis to bridge this gap in knowledge. All potential reaction pathways related to oxygen addition, hydrogen transfer, concerted elimination, and QOOH decomposition were identified using the density functional theory method. The corresponding rate constants were calculated by solving the time-dependent master equation based on the Rice-Ramsperger-Kassel-Marcus theory in conjunction with the transition state theory, and were subsequently fitted to Arrhenius expressions. It is discovered that the oxygen addition to the monoglyme radical formed by dehydrogention from the intermediate site is more competitive than that from the first site. The QOOH formed by the initial adduct RO2 via the 6-member ring transition state exerts a stronger impact on the second oxygen addition of monoglyme than that via the 5, 7, and 9-member ring transition states. After comparing the simulated and measured concentration distribution of the key substances during the oxidation of monoglyme in a jet-stirred reactor, the reliability and accuracy of the established kinetic model was confirmed.

Metabolite of chiral cycloxaprid in solvent and in the raw of Puer tea.

Cycloxaprid (CYC) with a chiral oxabridged cis- structure contains a pair of enantiomers. Enantioselective degradation, transformation and metabolite of CYC was performed in different solvents under light and raw Puer tea processing. The results showed that cycloxaprid enantiomers in acetonitrile and acetone was stable over 17day, however the transformation of 1S, 2R-(-)-cycloxaprid or 1R, 2S-(-)-cycloxaprid was founded in methanol. The fastest degradation of cycloxaprid occurred in acetone under light, the metabolites were founded with retention times (TR) at 34.83, 15.78min, which mainly was via the reduce reaction of NO2 to NO, and rearrange reaction to tetrahydropyran. Degradation pathways were via the cleavage of the oxabridge seven member ring and the whole C ring. However, the degradation pathway under raw Puer tea processing was via the cleavage of whole C ring and the cleavage of oxabridge seven member ring and reducing NO2, then it underwent an elimination of nitromethylene and rearrange reaction. This pathway of Puer tea processing was firstly founded.

From Sweet Molecular Giants to Square Sugars and Vice Versa

AbstractThis account describes our recent studies in the field of glycomimetics. Our efforts in understanding the structural basis of multivalent effects in glycosidase inhibition have led to decisive mechanistic insights supported by X-ray diffraction analyses and to the discovery of multimeric iminosugars displaying one of the largest binding enhancements reported so far for a non-polymeric enzyme inhibitor. Pushing the limits of the inhibitory multivalent effect has also driven progress in synthetic methodology. The unexpected observation of side products en route to the synthesis of our targets has been the starting point of several new synthetic methodologies, including metal-free deoxygenation of alcohols and one-pot double thioglycosylation. In parallel to our work on ‘giant’ neoglycoclusters, we have developed access to original constrained glycomimetics based on a 4-membered ring (‘square sugars’). Carbohydrates with a quaternary (pseudo)anomeric position were also synthesized from exo-glycals through catalytic hydrogen atom transfer and a novel oxidative radical-polar crossover process.1 Introduction2 Sweet Giants3 Multivalency Spin-Offs4 Sweet Curiosities4.1 Square Sugars4.2 From C,C-Glycosides to Formal Glycosylation of Quinones5 Conclusion

The Mechanism of Frabot Test for Uric Acid

Uric acid is an Analyte. Excess uric acid ─ hyperuricemia ─ leads to gout and renal stones. Frabot noted that uric acid added to an alkaline tungstate solution developed an intense blue colour. A colorimetric method widely utilized to quantify uric acid depends on the reduction of the chromogen sodium tungstate by uric acid to produce a measurable colour change. The reaction route from the initial products to the coloured compounds and the oxido-degradation of the organic substance has not been advanced. In this communication the electron flow is given in each step of the series of reactions that take place. These are isomerization to imidol, Michael addition to enone, oxirane formation in a redox step, hydrolysis of imido group, oxirane ring opening, ring contraction to five- member ring, ring opening via alkoxide, and finally assisted decarboxylation to the end product, 5-ureido hydantoin (allantoin).

4-Membered Ring Carbocations: A Positive Development in the Synthesis of 3,3-Disubstituted Oxetanes and Azetidines.

4-Membered heterocycles are low molecular weight polar scaffolds with intriguing potential for drug discovery. Despite their unquestionable value, methods to access such heterocycles remain scant. Here, we describe the generation of oxetane- and azetidine- benzylic carbocations as a general strategy to access valuable 3,3-disubstituted derivatives.

Total Synthesis of Taxol Enabled by Inter- and Intramolecular Radical Coupling Reactions.

Taxol is a clinically used drug for the treatment of various types of cancers. Its 6/8/6/4-membered ring (ABCD-ring) system is substituted by eight oxygen functional groups and flanked by four acyl groups, including a β-amino acid side chain. Here we report a 34-step total synthesis of this unusually oxygenated and intricately fused structure. Inter- and intramolecular radical coupling reactions connected the A- and C-ring fragments and cyclized the B-ring, respectively. Functional groups of the A- and C-rings were then efficiently decorated by employing newly developed chemo-, regio-, and stereoselective reactions. Finally, construction of the D-ring and conjugation with the β-amino acid delivered taxol. The powerful coupling reactions and functional group manipulations implemented in the present synthesis provide new valuable information for designing multistep target-oriented syntheses of diverse bioactive natural products.

Active precursor promoting nucleation/growth of MWW zeolite and controlling its morphology

MWW zeolites is an important catalyst in petrochemical industry. However, the efficient preparation of MWW zeolites still faces challenges, and the origin of influential factors for regulating its structure properties also remains obscure. Herein, we designed a nanoscale amorphous silica-alumina species denoted as active precursor (APS), and adopt the APS in the HMI mixture to synthesize MCM-22 zeolite (APS-MWW) successfully. To reveal the distinctive role of APS in promoting the crystallization of MWW zeolites, two crystal materials (ITQ-1 and MCM-22) and one mother liquor (ML) as seeds to synthesize three types of MWW zeolites. Typically, when adding APS in the synthetic mixture, the HMI amount was reduced to less than a quarter and crystallization time was reduced to 36 h. APS-MWW sample provides a smaller particle size (2–4 μm) and thinner stacked layer thickness (5–20 nm). Synchrotron radiation Small Angle X-ray Scattering (SAXS) shows each seed has a different impact on the species’ fractal structure and size distribution in the mixture, which is highly related to the nucleation and growth of MWW zeolites. APS shows a large number of 6 membered ring (MR) structure units which play a significant role in boosting the rapid nucleation and growth of APS-MWW zeolite. Among the synthesized MWW zeolites, the APS-MWW performs the highest ethylbenzene yield in the alkylation reaction of benzene-ethylene, which is attributed to its moderate flake thickness, appropriate texture properties and more external surface acidity. The results will provide a new perspective for producing MWW-types zeolites by using the available and effective active precursor.

Insight into the role of excess hydroxide ions in silicate condensation reactions.

The formation of silicate oligomers in the early stages is key to zeolite synthesis. The pH and the presence of hydroxide ions are important in regulating the reaction rate and the dominant species in solutions. This paper describes the formation of silicate species, from dimers to 4-membered rings, using ab initio molecular dynamics simulations in explicit water molecules with an excess hydroxide ion. The thermodynamic integration method was used to calculate the free energy profile of the condensation reactions. The hydroxide group's role is not only to control the pH of the environment, but also to actively participate in the condensation reaction. The results show that the most favorable reactions are linear-tetramer and 4-membered-ring formation, with overall barriers of 71 kJ mol-1 and 73 kJ mol-1, respectively. The formation of trimeric silicate, with the largest free-energy barrier of 102 kJ mol-1, is the rate-limiting step under these conditions. The excess hydroxide ion aids in the stabilization of the 4-membered-ring structure over the 3-membered-ring structure. Due to a relatively high free-energy barrier, the 4-membered ring is the most difficult of the small silicate structures to dissolve in the backward reaction. This study is consistent with the experimental observation that silicate growth in zeolite synthesis is slower in a very-high-pH environment.

Evaluating the efficacy of zeolites synthesized from natural clay for the methanol-to-hydrocarbon process.

Introducing sustainability into advanced catalytic material design is essential to address growing environmental concerns. Among them, synthesizing inorganic zeolite materials from non-traditional sources (like natural clay) offers several advantages, contributing to sustainability and environmental stewardship. With this objective, we used kaolin to synthesize zeolites with different topologies: SSZ-13 (8-MR with CHA topology), ZSM-5 (10-MR with MFI topology), and Beta (12-MR with BEA topology) (MR: member ring), where a simple and flexible synthetic protocol was adopted without any significant changes. All these zeolites were subjected to catalytic performance evaluation concerning the industrially relevant methanol-to-hydrocarbon (MTH) process. Herein, the kaolin-derived zeolites, especially ZSM-5, led to superior performance and demonstrated enhanced catalyst deactivation-resistant behavior compared to their zeolite counterparts prepared from traditional synthetic routes. Various characterization tools (including under operando conditions) were employed to understand their reactions and deactivation mechanisms. Overall, making zeolites from non-traditional sources presents a pathway for sustainable and environmentally friendly material production, offering benefits such as reduced resource dependence, lower energy consumption, and tailored physicochemical properties beneficial to catalysis. In a broader context, such a research approach contributes to the transition toward a more sustainable and circular economy.

Synthesis of oxetane and azetidine ethers as ester isosteres by Brønsted acid catalysed alkylation of alcohols with 3-aryl-oxetanols and 3-aryl-azetidinols.

Oxetanes and azetidines continue to draw significant interest in medicinal chemistry, as small, polar and non-planar motifs. Oxetanes also represent interesting surrogates for carbonyl-containing functional groups. Here we report a synthesis of 3,3-disubstituted oxetane- and azetidine-ethers, with comparisons made to the ester functional group. The tertiary benzylic alcohols of the 4-membered rings are selectively activated using Brønsted acid catalysis and reacted with simple alcohols to form the ethers and maintain the oxetane ring intact. This approach avoids the use of strong bases and halide alkylating agents and allows alcohol libraries to be leveraged. Oxetane ethers demonstrate excellent chemical stability across a range of conditions and an improved stability vis-à-vis analogous esters under basic and reducing conditions.