Michael Addition-Elimination Ring-Opening Polymerization.

A cyclic thioenone system capable of controlled ring-opening polymerization (ROP) is presented that leverages a reversible Michael addition-elimination (MAE) mechanism. The cyclic thioenone monomers are easy to access and modify and for the first time incorporate the dynamic reversibility of MAE with chain-growth polymerization. This strategy features mild polymerization conditions, tunable functionalities, controlled molecular weights (Mn), and narrow dispersities. The obtained polythioenones exhibit excellent optical transparency and good mechanical properties and can be depolymerized to recover the original monomers. Density functional theory (DFT) calculations of model reactions offer insights into the role of monomer conformation in the polymerization process, as well as explaining divergent reactivity observed in seven-membered thiepane (TP) and eight-membered thiocane (TC) ring systems. Collectively, these findings demonstrate the feasibility of MAE mechanisms in ring-opening polymerization and provide important guidelines toward future monomer designs.

Chemically Recyclable Polymer System Based on Nucleophilic Aromatic Ring-Opening Polymerization.

The development of chemically recyclable polymers with desirable properties is a long-standing but challenging goal in polymer science. Central to this challenge is the need for reversible chemical reactions that can equilibrate at rapid rates and provide efficient polymerization and depolymerization cycles. Based on the dynamic chemistry of nucleophilic aromatic substitution (SNAr), we report a chemically recyclable polythioether system derived from readily accessible benzothiocane (BT) monomers. This system represents the first example of a well-defined monomer platform capable of chain-growth ring-opening polymerization through an SNAr manifold. The polymerizations reach completion in minutes, and the pendant functionalities are easily customized to tune material properties or render the polymers amenable to further functionalization. The resulting polythioether materials exhibit comparable performance to commercial thermoplastics and can be depolymerized to the original monomers in high yields.

Precise Introduction of the -CHnX3-n (X = F, Cl, Br, I) Moiety to Target Molecules by a Radical Strategy: A Theoretical and Experimental Study.

Addition of halomethyl radicals to form bioactive molecules has recently become an efficient strategy. The reaction has a bottleneck, however, which is the effective and selective generation of the proper halomethyl •CHnX3-n radical by combining CHnX4-n with a carbon radical. Understanding the reactivity and selectivity of carbon radicals in the hydrogen atom transfer (HAT) and halogen atom transfer (XAT) reactions of CHnX4-n is key to the development of such an attractive method. With the help of the emerging data-driven strategy, DFT calculations were used to explore various correlations. For selectivity, the relative energy barriers between HAT and XAT reactions (ΔG⧧H - ΔG⧧X) correlate reasonably well with the three parameters ΔGH, ΔGX, and IP, and the correlation studies reveal that the calculated IPinver and the experimental ΔBDE can be used to conveniently predict the selectivity. Predicted selectivities are consistent with experimental determinations. This work not only provides a possibility for selecting carbon radicals with the known or easily obtained physicochemical data but also demonstrates that the informatic workflow such as generating data and identifying correlations has potential applications in mining reaction rules.

Generation of Diazomethyl Radicals by Hydrogen Atom Abstraction and Their Cycloaddition with Alkenes.

A general catalytic methodology for the synthesis of pyrazolines from α-diazo compounds and conjugated alkenes is reported. The direct hydrogen atom transfer (HAT) process of α-diazo compounds promoted by the tert-butylperoxy radical generates electrophilic diazomethyl radicals, thereby reversing the reactivity of the carbon atom attached with the diazo group. The regiocontrolled addition of diazomethyl radicals to carbon-carbon double bonds followed by intramolecular ring closure on the terminal diazo nitrogen and tautomerization affords a diverse set of pyrazolines in good yields with excellent regioselectivity. This strategy overcomes the limitations of electron-deficient alkenes in traditional dipolar [3+2]-cycloaddition of α-diazo compounds with alkenes. Furthermore, the straightforward formation of the diazomethyl radicals provides umpolung reactivity, thus opening new opportunities for the versatile transformations of diazo compounds.

Radical-Mediated Strategies for the Functionalization of Alkenes with Diazo Compounds.

One of the most common reactions of diazo compounds with alkenes is cyclopropanation, which occurs through metal carbene or free carbene intermediates. Alternative functionalization of alkenes with diazo compounds is limited, and a methodology for the addition of the elements of Z-CHR2 (with Z = H or heteroatom, and CHR2 originates from N2═CR2) across a carbon-carbon double bond has not been reported. Here we report a novel reaction of diazo compounds utilizing a radical-mediated addition strategy to achieve difunctionalization of diverse alkenes. Diazo compounds are transformed to carbon radicals with a photocatalyst or an iron catalyst through PCET processes. The carbon radical selectively adds to diverse alkenes, delivering new carbon radical species, and then forms products through hydroalkylation by thiol-assisted hydrogen atom transfer (HAT), or forms azidoalkylation products through an iron catalytic cycle. These two processes are highly complementary, proceed under mild reaction conditions, and show high functional group tolerance. Furthermore, both transformations are successfully performed on a gram-scale, and diverse γ-amino esters, γ-amino alcohols, and complex spirolactams are easily prepared with commercially available reagents. Mechanistic studies reveal the plausible pathways that link the two processes and explain the unique advantages of each.

Catalytic Oxidative Cleavage Reactions of Arylalkenes by tert-Butyl Hydroperoxide - A Mechanistic Assessment.

Oxidative cleavage reactions of arylalkenes by tert-butyl hydroperoxide that occur by free radical processes provide access to carboxylic acid or ketone products. However, the pathway to these cleavage products is complex, initiated by regioselective oxygen radical addition to the carbon-carbon double bond. Subsequent reactions of the initially formed benzyl radical lead eventually to carbon-carbon cleavage. Thorough investigations of these reactions have identified numerous reaction intermediates that are on the pathways to final product formation, and they have identified a new synthetic methodology for the synthesis of peroxy radical addition-induced hydroperoxide formation.

Generation of Halomethyl Radicals by Halogen Atom Abstraction and Their Addition Reactions with Alkenes.

α-Aminoradicals undergo halogen atom abstraction to form halomethyl radicals in reactions initiated by the combination of tert-butyl hydroperoxide, aliphatic trialkylamine, halocarbon, and copper(I) iodide. The formation of the α-aminoradical circumvents preferential hydrogen atom transfer in favor of halogen atom transfer, thereby releasing the halomethyl radical for addition to alkenes. The resulting radical addition products add the tert-butylperoxy group to form α-peroxy-ß,ß-dichloropropylbenzene products that are convertible to their corresponding ß,ß-dichloro-alcohols and to novel pyridine derivatives. Computational analysis clearly explains the deviation from traditional HAT of chloroform and also establishes formal oxidative addition/reductive elimination as the lowest energy pathway.

Assembly of Tetrahydropyran Derivatives from Aldehydes, Allylboronates, and Syngas by Asymmetric Relay Catalytic Cascade Reaction.

An efficient synthesis of highly enantio-enriched tetrahydropyrans from readily available aldehydes, allylboronates, and syngas has been established by multiply relay catalysis of rhodium and chiral phosphoric acid. The cascade reaction integrates the asymmetric allylboration of aldehydes and alkene hydroformylation, providing a structurally diverse range of products with different workup procedures. The concise synthesis of key chiral building blocks to access herboxidiene and leucascandrolide A demonstrates the high synthetic utility of this method. The cascade reaction employing alkenes to replace aldehydes was also successful.

[Comparative research for micro-push-out bond strengths of glass fiber posts treated by poly-dopamine or silane coupling agent].

OBJECTIVE: To evaluate the micro-push-out bond strengths of prefabricated glass fiber posts with poly-dopamine functionalized to root dentin using resin cements, contrasted with silane treatment. METHODS: In the study, 30 glass fiber posts were randomly divided into 3 groups (10 posts in each group) for different surface treatments. Group 1, treated with poly-dopa; Group 2, treated with silane coupling agent for 60s; Group 3, no surface treatment (Control group). The 30 extracted human, single-rooted teeth were endodontically treated and a 9 mm post space was prepared in each tooth with post drills provided by the manufacturer. Following post cementation, the specimens were stored in distilled water at 37 °C for 7 days. The micro-push-out bond strengths were tested using a universal testing machine (0.5 mm/min), and the failure modes were examined with a stereomicroscope. The data of the three groups were statistically analyzed using the one-way ANOVA test(α= 0.05). RESULTS: The bond strengths were (7.909 ± 1.987) MPa for Group 1, (5.906 ± 0.620) MPa for Group 2, and 4.678 ± 0.910 MPa for Group 3. The bond strength of poly-dopamine group was significantly higher than that of the silane group (P<0.05). CONCLUSION: Contrasted with silane treatment, surface poly-dopamine functionalization was confirmed to be a more reliable method for improving the bond strength of resin luting agents to fiber posts.

Chemical Circularity in 3D Printing with Biobased Δ-Valerolactone.

Digital Light Processing (DLP) is a vat photopolymerization-based 3D printing technology that fabricates parts typically made of chemically crosslinked polymers. The rapidly growing DLP market has an increasing demand for polymer raw materials, along with growing environmental concerns. Therefore, circular DLP printing with a closed-loop recyclable ink is of great importance for sustainability. The low-ceiling temperature alkyl-substituted δ-valerolactone (VL) is an industrially accessible biorenewable feedstock for developing recyclable polymers. In this work, acrylate-functionalized poly(δ-valerolactone) (PVLA), synthesized through the ring-opening transesterification polymerization of VL, is used as a platform photoprecursor to improve the chemical circularity in DLP printing. A small portion of photocurable reactive diluent (RD) turns the unprintable PVLA into DLP printable ink. Various photocurable monomers can serve as RDs to modulate the properties of printed structures for applications like sacrificial molds, soft actuators, sensors, etc. The intrinsic depolymerizability of PVLA is well preserved, regardless of whether the printed polymer is a thermoplastic or thermoset. The recovery yield of virgin quality VL monomer is 93% through direct bulk thermolysis of the printed structures. This work proposes the utilization of depolymerizable photoprecursors and highlights the feasibility of biorenewable VL as a versatile material platform toward circular DLP printing.

One-Pot Synthesis of Depolymerizable δ-Lactone Based Vitrimers.

A depolymerizable vitrimer that allows both reprocessability and monomer recovery by a simple and scalable one-pot two-step synthesis of vitrimers from cyclic lactones is reported. Biobased δ-valerolactone with alkyl substituents (δ-lactone) has low ceiling temperature; thus, their ring-opening-polymerized aliphatic polyesters are capable of depolymerizing back to monomers. In this work, the amorphous poly(δ-lactone) is solidified into an elastomer (i.e., δ-lactone vitrimer) by a vinyl ether cross-linker with dynamic acetal linkages, giving the merits of reprocessing and healing. Thermolysis of the bulk δ-lactone vitrimer at 200 °C can recover 85-90 wt% of the material, allowing reuse without losing value and achieving a successful closed-loop life cycle. It further demonstrates that the new vitrimer has excellent properties, with the potential to serve as a biobased and sustainable replacement of conventional soft elastomers for various applications such as lenses, mold materials, soft robots, and microfluidic devices.

Modulating Polymerization Thermodynamics of Thiolactones Through Substituent and Heteroatom Incorporation.

A central challenge in the development of next-generation sustainable materials is to design polymers that can easily revert back to their monomeric starting material through chemical recycling to monomer (CRM). An emerging monomer class that displays efficient CRM are thiolactones, which exhibit rapid rates of polymerization and depolymerization. This report details the polymerization thermodynamics for a series of thiolactone monomers through systematic changes to substitution patterns and sulfur heteroatom incorporation. Additionally, computational studies highlight the importance of conformation in modulating the enthalpy of polymerization, leading to monomers that display high conversions to polymer at near-ambient temperatures, while maintaining low ceiling temperatures (Tc). Specifically, the combination of a highly negative enthalpy (-19.3 kJ/mol) and entropy (-58.4 J/(mol·K)) of polymerization allows for a monomer whose equilibrium polymerization conversion is very sensitive to temperature.

Catalyst-Free Formation of Nitrile Oxides and Their Further Transformations to Diverse Heterocycles.

The formation of nitrile oxides with diazocarbonyl compounds by nitrosyl transfer from tert-butyl nitrite under mild conditions and without the use of a catalyst or an additive is reported. This transformation is broadly applicable to the synthesis of furoxans by dimerization and isoxazoles and isoxazolines by cycloaddition. This methodology is also applied for the millimole-scale synthesis of two biologically active compounds. The formation of the nitrile oxide from a diazoacetamide is stable and confirmed experimentally.

Asymmetric α-Allylation of Aldehydes with Alkynes by Integrating Chiral Hydridopalladium and Enamine Catalysis.

A palladium-catalyzed asymmetric α-allylation of aldehydes with alkynes has been established by integrating the catalysis of enamine and chiral hydridopalladium complex that is reversibly formed from the oxidative addition of Pd(0) to chiral phosphoric acid. The ternary catalyst system, consisting of an achiral palladium complex, a primary amine, and a chiral phosphoric acid allows the reaction to tolerate a wide scope of α,α-disubstituted aldehydes and alkynes, affording the corresponding allylation products in high yields and with excellent levels of enantioselectivity.

Ir/PTC cooperatively catalyzed asymmetric umpolung allylation of α-imino ester enabled synthesis of α-quaternary amino acid derivatives bearing two vicinal stereocenters.

A novel Ir/PTC (phase-transfer catalyst) cooperatively catalyzed asymmetric umpolung allylation of simple α-imino esters is developed and it provides facile access to α-quaternary amino acid derivatives bearing two vicinal stereocenters. Both the metal catalyst and the phase-transfer catalyst are crucial for this methodology. The reaction features good yields, high diastereo- and enantio-selectivities, ease of scale-up to gram scale and further transformation of the products (e.g. to quaternary proline analogues with multi stereocenters).

Inhibition of Enterococcus faecalis by Calcium Peroxide.

OBJECTIVE: To investigate the inhibition of Enterococcus faecalis by calcium peroxide (CaO2). METHODS: The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of Enterococcus faecalis by CaO2 and calcium hydroxide (Ca(OH)2) were determined by direct exposure tests (n = 10). The inhibition zone of E. faecalis mycoderm treated with CaO2 and Ca(OH)2 paste (53% w/w) was observed using agar diffusion tests (n = 20). The inhibition of E. faecalis biofilms by CaO2/phosphate-buffered saline (PBS) and Ca(OH)2/PBS suspensions were observed using confocal laser scanning microscopy and the percentages of live bacteria in the biofilms calculated. RESULTS: The MIC of Ca(OH)2 (4.5 to 5.5 mg/ml) was higher than the MIC of CaO2 (2.0 to 2.5 mg/ml) (P < 0.05), and the MBC of Ca(OH)2 (14.5 to 15.5 mg/ml) was higher than that of CaO2 (3.0 to 3.5 mg/ml) (P < 0.05). No inhibition zone was observed for Ca(OH)2 in agar diffusion tests, while the diameter of the inhibition zone around CaO2 was 8.6 ± 0.4 mm. There were significant differences between groups in the percentages of surviving bacteria in E. faecalis biofilms after treatment (P < 0.05): group CaO2 < group Ca(OH)2 < group PBS < group BHI. CONCLUSION: The inhibition of E. faecalis by CaO2 was greater than that by Ca(OH)2.

[Induction effects of pentachlorophenol on vitellogenin and p53 in Chinese rare minnow (Gobiocypris rarus)].

Taking the Chinese rare minnow (Gobiocypris rarus) as the test animal, the studies were designed to investigate induction effects of pentachlorophenol (PCP) on vitellogenin (VTG) protein, VTG gene and tumor suppressor p53 gene in the liver of Gobiocypris rarus. The endocrine disrupting of PCP was evaluated by detecting VTG, and sensitive biomarkers of PCP were screened at both protein and mRNA levels. Gobiocypris rarus were exposed to PCP at 1.5, 15, 40, 80, 120, 150, 160 microg x L(-1) respectively, while setting blank, solvent control and 17alpha-ethynylestradiol (EE2) as positive control. Using sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), enzyme-linked immunosorbent assay (ELISA), VTG protein expression differences were detected in the liver of Gobiocypris rarus after exposure to PCP. Cloning the VTG and p53 gene new fragments of Gobiocypris rarus based on conserved regions, mRNA expression levels of VTG and p53 gene in the liver of Gobiocypris rarus were determined by quantitative real-time PCR assay after PCP treatment. The results showed that 40, 80, 120, 160 microg x L(-1) PCP induced the liver of male and female Gobiocypris rarus to produce VTG protein, and had a significant concentration effect. VTG and p53 mRNA levels significantly increased in the liver of Gobiocypris rarus after exposure to 1.5, 15, 150 microg x L(-1) PCP, and had remarkable concentration and time effects. The studies suggested that PCP had estrogenic effects, and VTG protein, VTG and p53 gene in the liver of Gobiocypris rarus could be used as candidate sensitive biomarkers for detecting PCP.