Lactone Research Articles

Full valorisation of waste PET into dimethyl terephthalate and cyclic arylboronic esters

Developing efficient and cost-effective methodologies for high value-added conversion of waste plastic delivers substantial environmental and economic benefits. Herein, we develop a novel approach utilizing boric acid in the methanolysis of waste polyethylene terephthalate (PET) to derive pure dimethyl terephthalate (DMT) and boronic acid esters through in-situ capture of ethylene glycol (EG). It not only upcycles waste PET but also eliminates intricate EG purification processes. Catalyzed by magnalium-aluminum-layered double oxides (Mg4Al1-LDO), this method achieved 100 % conversions of PET with 96 % and 99 % yields of arylboronic esters and DMT, respectively. Kinetic studies and in-situ Fourier-transform infrared spectroscopy (FT-IR) demonstrated the pivotal role of the monodentate methoxy species, generated through the interaction of medium basic Mg–O ion pairs and methanol. This method demonstrates applicability for the upcycling of assorted discarded PET wastes, polyesters, and polycarbonates with EG units, highlighting its potential as a comprehensive solution for waste plastic management.

Molecular dynamics simulation of bonding role of cyclic borate ester in improving mechanical properties of the HTPB propellant

Excellent mechanical properties are the fundamental requirement for the practical application of solid propellants. However, solid oxidant fillers such as hexahydro-1,3,5-trinitros triazine (RDX) seriously weaken the mechanical strength of the propellant due to poor adhesion with the binder matrix. In this work, novel cyclic borate ester (CBE) bonding agents with long-chain alkyl moiety were designed by the molecular dynamics (MD) method to improve the unfavorable interfacial bonding effect. The interface models, which consist of RDX substrate and binder matrix where hydroxyl-terminated polybutadiene (HTPB) crosslinked with curing agents, were constructed to reveal the interface enhancement mechanism by CBE bonding agents. The results indicate that the designed CBE bonding agents with appropriate solubility parameters (SPs) can migrate from the binder matrix to the RDX surface. Besides, CBE bonding agents exhibit strong interaction with RDX compared to the binder matrix due to the formation of hydrogen bonds. Finally, the simulated tensile tests with two tensile modes showed that CBE bonding agents can enhance the interfacial strength and reduce stress softening rates by the calculation of debonding work. Moreover, this improvement can be controlled by adjusting the chain length of CBE bonding agents. The results obtained in this work deepen the understanding of the interfacial strengthening mechanism of bonding agents for solid propellants and provide a framework for designing high-performance bonding agents based on the MD method.

Two Lactones Non-toxic to Humans Are Effective Repellents for the German Cockroach.

Lactones are cyclic esters of hydroxy carboxylic acids, present in several fruits and animals consumed by humans. There is evidence that some lactones modify insect behavior. The aim of the present study was to evaluate the repellent effect of four lactones (γ- and δ-nonalactone, and γ- and δ-dodecalactone) in first instar nymphs of the German cockroach, Blattella germanica (Linnaeus). To assess repellency, a nymph was placed on a circle of filter paper, half of which had been treated with lactone dissolved in acetone and the other half with acetone alone. The behavior of the nymph was recorded and the time the nymph spent in each half of the paper was quantified using Ethovision XT 10.1 software. Values of Distribution Coefficient (DC) were calculated: DC = (Tt - TA) / Tt, where Tt is the experimental time and TA is the time the nymph spent in the area treated with the repellent agent. DC can vary between 0 and 1. Values significantly higher than 0.5 indicate repellency. N,N-diethyl-meta-toluamide (DEET) was used as a positive control. DEET, δ- and γ-nonalactone caused repellency as from 77.9µg/cm2, whereas γ- and δ-dodecalactone had a repellent effect starting at 779.0µg/cm2. The values of DC for these concentrations were 0.89 (DEET), 0.86 (γ-nonalactone), 0.87 (δ-nonalactone), 0.83 (γ-dodecalactone), and 0.72 (δ-dodecalactone). To our knowledge, this is the first report of repellency produced by lactones in the German cockroach. This work allowed to identify two lactones that have a repellent effect similar to DEET.

Synthesis of β-cyclodextrin derivatives substituted at larger or smaller rims via amine-catalyzed ring-opening oligomerization of ε-caprolactone

The involvement of cyclodextrins in transesterification reactions with active esters has been described to mimic enzyme-catalyzed reactions, making cyclodextrin molecules suitable as enzyme models. Cyclodextrin-catalyzed ring-opening of cyclic esters in bulk reaction conditions was considered to proceed similarly. However, the mechanism of activating cyclic esters through inclusion in the cyclodextrin cavity remains incompletely understood to date. The present research is focused on observing the transesterification of ε-caprolactone in the presence of β-cyclodextrin and additional amine organocatalysts within dimethyl sulfoxide solutions. The conducted experiments provide insights into the structural changes caused by various catalytic conditions in terms of the substitution pattern of the cyclodextrins. Our results are supported by a deep structural characterization through NMR and MALDI MS, which revealed the prospect of promoting rim-selective substitution of β-cyclodextrin at either secondary or primary hydroxyl groups using 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) organocatalysts. This offers the possibility to prepare cyclodextrin derivatives with specific substitution patterns. Based on the acquired structural information, the particular pathway in which β-cyclodextrin influences the ring-opening of ε-caprolactone is delineated as follows: monomer complexation, substitution at the larger rim, chain elongation, and intramolecular transfer toward the smaller rim.

Torsional Strain Enabled Ring‐Opening Polymerization towards Axially Chiral Semiaromatic Polyesters with Chemical Recyclability

AbstractThe development of new chemically recyclable polymers via monomer design would provide a transformative strategy to address the energy crisis and plastic pollution problem. Biaryl‐fused cyclic esters were targeted to generate axially chiral polymers, which would impart new material performance. To overcome the non‐polymerizability of the biaryl‐fused monomer DBO, a cyclic ester Me‐DBO installed with dimethyl substitution was prepared to enable its polymerizability via enhancing torsional strain. Impressively, Me‐DBO readily went through well‐controlled ring‐opening polymerization, producing polymer P(Me‐DBO) with high glass transition temperature (Tg >100 °C). Intriguingly, mixing these complementary enantiopure polymers containing axial chirality promoted a transformation from amorphous to crystalline material, affording a semicrystalline stereocomplex with a melting transition temperature more than 300 °C. P(Me‐DBO) were capable of depolymerizing back to Me‐DBO in high efficiency, highlighting an excellent recyclability.

Torsional Strain Enabled Ring-Opening Polymerization towards Axially Chiral Semiaromatic Polyesters with Chemical Recyclability.

The development of new chemically recyclable polymers via monomer design would provide a transformative strategy to address the energy crisis and plastic pollution problem. Biaryl-fused cyclic esters were targeted to generate axially chiral polymers, which would impart new material performance. To overcome the non-polymerizability of the biaryl-fused monomer DBO, a cyclic ester Me-DBO installed with dimethyl substitution was prepared to enable its polymerizability via enhancing torsional strain. Impressively, Me-DBO readily went through well-controlled ring-opening polymerization, producing polymer P(Me-DBO) with high glass transition temperature (Tg >100 °C). Intriguingly, mixing these complementary enantiopure polymers containing axial chirality promoted a transformation from amorphous to crystalline material, affording a semicrystalline stereocomplex with a melting transition temperature more than 300 °C. P(Me-DBO) were capable of depolymerizing back to Me-DBO in high efficiency, highlighting an excellent recyclability.

A perspective into ring-opening polymerization of ε-caprolactone and lactides: effect of, ligand, catalyst structure and system dynamics, on catalytic activity and polymer properties

AbstractPolylactic acid (PLAs) and polycaprolactones (PCLs) are emerging to be polyesters of choice because they are biodegradable and biocompatible. The industrial viable method currently used for their production is catalytic ring-opening polymerization (ROP) of their respective cyclic ester monomers. The catalyst is at the core of ROP chemical transformation and as such research is dedicated towards the innovation of more efficient and selective catalysts. Catalyst structural features greatly influence catalyst performance in ROP, and this review discusses the effects of ligand and catalyst structure and system dynamics, on catalytic activity and polymer properties. The effects of associated catalyst components such as initiating groups, ligand chirality, and stereochemical in relation to catalyst activity and polymer properties were also reviewed. The effects of metal nuclearity and catalyst flexibility on catalytic activity were also reviewed. The effect of solvent and temperature was briefly considered. Graphical abstract

Kinetically Controlled Stereoselective Synthesis of 2-Oxo-2-aryl-1,3,2-dioxaphosphorinane Derivatives via a Palladium-Catalyzed Reaction.

Chiral phosphonate esters have been widely applied in the fields of organic chemistry, medicine, and photoelectric materials. However, it requires the challenging enantioselective synthesis of cyclic phosphonate esters with the desired chiral configuration. The two epimers of 2-oxo-2H-1,3,2-dioxaphosphorinane derivatives should have different reactivities in Pd-catalyzed coupling reactions, which could lead to an effective methodology for the asymmetric synthesis of 2-oxo-2-aryl-1,3,2-dioxaphosphorinane derivatives. A thorough investigation of the coupling reactions both computationally and experimentally led to the stereoselective synthesis of chiral cyclic phosphonate esters. The axial isomer of products can be obtained with both high diastereoselectivity and good chemical yields from the mixture of 2-oxo-2H-1,3,2-dioxaphosphorinane derivatives under kinetically controlled conditions.

Ring-Opening Polymerization of Cyclic Esters and Carbonates with (Thio)urea/Cyclopropenimine Organocatalytic Systems.

Organocatalyzed ring-opening polymerization is a powerful tool for the synthesis of a variety of functional, readily degradable polyesters and polycarbonates. We report the use of (thio)ureas in combination with cyclopropenimine bases as a unique catalyst for the polymerization of cyclic esters and carbonates with a large span of reactivities. Methodologies of exceptionally effective and selective cocatalyst combinations were devised to produce polyesters and polycarbonates with narrow dispersities (Đ = 1.01-1.10). Correlations of the pKa of the various ureas and cyclopropenimine bases revealed the critical importance of matching the pKa of the two cocatalysts to achieve the most efficient polymerization conditions. It was found that promoting strong H-bonding interactions with a noncompetitive organic solvent, such as CH2Cl2, enabled greatly increased polymerization rates. The stereoselective polymerization of rac-lactide afforded stereoblock poly(lactides) that crystallize as stereocomplexes, as confirmed by wide-angle X-ray scattering.

To Be or Not To Be… Cooperative? Trimetallic Catalysts for the Ring‐Opening Polymerisation of Cyclic Esters

AbstractMultimetallic catalysts have emerged as a promising route to enhance catalyst performance in cyclic ester ring‐opening polymerisation (ROP), as these complexes can outperform the monometallic analogues in terms of reactivity and/or polymerisation control. Such enhancements are often attributed to “multimetallic cooperativity”, yet the origins of this cooperativity often remain unclear. Here, we report the synthesis and characterisation of two trimetallic Al‐salen complexes (L1(AlMe)3 and L1(AlEt)3), containing three Al‐salen subunits in close proximity, and their monometallic analogues (L2(AlMe) and L2(AlEt)). These complexes have been applied as catalysts for cyclic ester ROP under a range of conditions, to probe their potential for cooperative behaviour. Trimetallic L1(AlMe)3 and L1(AlEt)3 both display high activities in rac‐lactide ROP, giving kobs values of up to 11×10−3 min−1. Yet careful benchmarking against the monometallic complexes reveals that whether these systems display multimetallic cooperativity or not depends upon the reaction conditions. Detailed kinetic and spectroscopic studies into the origins of cooperativity revealed that the neighbouring metals play a key role in the initiation step, rather than propagation. Overall, these studies highlight the importance of understanding the reaction conditions in order to accurately define whether a catalyst displays multimetallic cooperativity.

Efficient and Selective Adsorption of cis-Diols via the Suzuki-Miyaura Cross-Coupling-Modified Phenylboronic-Acid Functionalized Covalent Organic Framework.

In this work, a functional group (boronic acid) was modified onto a covalent organic framework (COF) using the Suzuki-Miyaura cross-coupling reaction to obtain a phenylboronic acid-functionalized covalent organic framework (BrCOF-PBA). This product was used as a selective adsorbent and largely as an efficient solid-phase extractant of flavonoids containing cis-diol structures like quercetin (QUE). Five or six-membered cyclic esters generated from the COF were characterized, and some physicochemical studies were performed, resulting in excellent chemical stability and crystallinity, high specific surface area, stable pore structure, and regular pore size. Unique selectivity of BrCOF-PBA was observed toward QUE and exhibited a huge adsorption capacity (213.96 mg g-1) in a relatively short time (90 min). In contrast, the adsorption properties of morin (MOR) and kaempferol (KAE) with a certain degree of chemical similarity to QUE were only 27.62 and 21.76 mg g-1, respectively. BrCOF-PBA also demonstrated good reusability and robustness, making it an attractive composite material for further analytical applicability.

Exploiting Multimetallic Cooperativity in the Ring-Opening Polymerization of Cyclic Esters and Ethers.

The use of multimetallic complexes is a rapidly advancing route to enhance catalyst performance in the ring-opening polymerization of cyclic esters and ethers. Multimetallic catalysts often outperform their monometallic analogues in terms of reactivity and/or polymerization control, and these improvements are typically attributed to "multimetallic cooperativity". Yet the origins of multimetallic cooperativity often remain unclear. This review explores the key factors underpinning multimetallic cooperativity, including metal-metal distances, the flexibility, electronics and conformation of the ligand framework, and the coordination environment of the metal centers. Emerging trends are discussed to provide insights into why cooperativity occurs and how to harness cooperativity for the development of highly efficient multimetallic catalysts.

Enzymatic Assembly of Diverse Lactone Structures: An Intramolecular C-H Functionalization Strategy.

Lactones are cyclic esters with extensive applications in materials science, medicinal chemistry, and the food and perfume industries. Nature's strategy for the synthesis of many lactones found in natural products always relies on a single type of retrosynthetic strategy, a C-O bond disconnection. Here, we describe a set of laboratory-engineered enzymes that use a new-to-nature C-C bond-forming strategy to assemble diverse lactone structures. These engineered "carbene transferases" catalyze intramolecular carbene insertions into benzylic or allylic C-H bonds, which allow for the synthesis of lactones with different ring sizes and ring scaffolds from simple starting materials. Starting from a serine-ligated cytochrome P450 variant previously engineered for other carbene-transfer activities, directed evolution generated a variant P411-LAS-5247, which exhibits a high activity for constructing a five-membered ε-lactone, lactam, and cyclic ketone products (up to 5600 total turnovers (TTN) and >99% enantiomeric excess (ee)). Further engineering led to variants P411-LAS-5249 and P411-LAS-5264, which deliver six-membered δ-lactones and seven-membered ε-lactones, respectively, overcoming the thermodynamically unfavorable ring strain associated with these products compared to the γ-lactones. This new carbene-transfer activity was further extended to the synthesis of complex lactone scaffolds based on fused, bridged, and spiro rings. The enzymatic platform developed here complements natural biosynthetic strategies for lactone assembly and expands the structural diversity of lactones accessible through C-H functionalization.

Development of a H-bond donor–Lewis basic anion bifunctional organocatalyst for ring-opening polymerizations

Squaramide–phenolate exemplified one-component H-bond donor–anion bifunctional catalyst for ring-opening polymerizations of various cyclic esters, where the Lewis basic phenolate activated initiator and the H-bond donor squaramide activated monomer.

Visualized sensing of erythritol using a simple enzyme-free catechol-based hydrogel film.

Based on the versatile properties of bio-derived materials, non-enzymatic assays in combination with electronic devices have attracted increasing interest. Here, we report a novel enzyme-free visualization approach for the detection of erythritol, which is a zero-calorie natural sweetener and serves as an ideal sucrose substitute for diabetics or overweight people who need sugar control. The recognition element of the electrochemical biosensor was constructed by catechol modification on a chitosan-based hydrogel film. The signal transduction was achieved by the competitive binding assay of sweeteners. The results show that 2-fluorophenylboronic acid (FPBA) can form a cyclic boronate ester with the ortho-hydroxyls of both reduced catechol and oxidized quinone, impeding the electron transfer and leading to redox signal attenuation. The addition of sweeteners caused a competitive reaction resulting in bonding between the 1,2-diols and FPBA moieties, and in the recovery of the redox signals. Importantly, the pattern of redox signal changes of catechol can be detected optically, as the oxidized quinone state is darker in color than the reduced catechol state. Using a simple cell phone imaging application, we demonstrate that erythritol can be distinguished from other sweeteners in real samples using the oxidized catechol-Chit0/agarose hydrogel film. Thus, we envision that this method could allow diabetics and people who need to control their sugar intake to detect whether the product contains only erythritol in the field or at home. In addition, this work further illustrates the potential of bio-derived materials for performing redox-based functions and enzyme-free visualization assays.

Grignard reagents as deprotonation agents for oxazoline-amido-phenolate ligands: structural and catalytic implications with the role of halogen ions.

In this study, various halogen-substituted Grignard reagents were assessed as deprotonating agents for the oxazoline-amido-phenolate ligand, leading to the formation of magnesium complexes. The newly synthesized complexes with halogen substituents displayed three distinct coordinative modes, all extensively characterized through crystallographic methods. The introduction of halogen substituents induced changes in the Lewis acid properties of the complexes, thereby impacting their structural attributes and catalytic behavior during the initiation and propagation of ring polymerization of cyclic esters.

Evaluation of Chemical Constituents and Antioxidant Activity of Extracts of Nardostachys jatamansi (D. Don) DC.

Nardostachys jatamansi is an endangered medicinal plant of the Caprifoliaceae family. The main aim of this study was to evaluate the metabolites and their biological activities. The shade dried, powdered plant materials were subjected to successive ultrasonic extraction with solvents hexane, chloroform, ethyl acetate, and methanol in the increasing order of polarity. Flavonoids, terpenoids, saponins, phenolic volatile oils, and alkaloids were discovered in the phytochemical screening. Gas chromatography mass spectrometry analysis identified 13, 10, and 6 chemical compounds in hexane, chloroform, and ethyl acetate extracts, respectively. Cyclic saturated hydrocarbons, organic acids, alcohol, and ester were the main constituents of extracts verified by GC-MS. Chloroform extracts had a higher phenolic content (49.53 mg GAE/g) than ethyl acetate extracts (44.41 mg GAE/g).  Chloroform extracts had a higher total flavonoid concentration (18.42 mg QE/g) than ethyl acetate extracts (10.57 mg QE/g).  The half-maximum inhibitory concentration (IC50) of the chloroform and ethyl acetate extracts revealed moderate results to antioxidant activity.

Identification of compounds and functional groups of n-hexane seed extracts of Citrullus lanatus and Elaeis guineensis using GC-MS and FT-IR

Citrullus lanatus and Elaeis guineensis are regarded as two of the most significant edible fruits in terms of nutrition and medicine. Utilizing FTIR and GC-MS spectroscopy, the current work aims at identifying bioactive compounds and functional groups present in n-hexane seed extracts of Citrullus lanatus and Elaeis guineensis. The GC-MS approach is used to screen and identify phytocompounds. Using a Buck Scientific M530 USA spectrophotometer, the FTIR technique was used to identify the functional groups. Twenty-five (25) and twenty-nine (29) bioactive compounds were identified in Citrullus lanatus and Elaeis guineensis respectively, by Gas Chromatographic-Mass Spectrometry (GC-MS). Linoelaidic acid (58.79%) was the most prevalent compound identified in Citrullus lanatus seed extract. Gamma-tocopherol has the most diverse pharmacological role. n-hexadecanoic acid (16.82%), dodecanoic acid (0.12%), 9, 12-octadecadienoic acid methyl ester (3.45%) and 9, 12-octadecadienoic acid (Z, Z) (0.91%) are some of the other bioactive compounds detected. The most prevalent phytocompound identified in Elaeis guineensis seed extract is dodecanoic acid (37.20%), whereas the most diverse pharmacological activity was found in n-hexadecanoic acid. N-decanoic acid (6.32%), 6-octadecenoic acid (11.95%), octanoic acid (5.27%) and tetradecanoic acid (17.59%) are some other bioactive compounds identified. Ether, ethene, primary amine, carboxylic acid, cyclic ester, nitrile, methylene, primary, secondary and tertiary alcohols were detected at various wavelengths in both seed extracts by Fourier transform infrared (FTIR) spectrophotometer. Both extracts contain phytocompounds with antioxidant, anti-inflammatory and antimicrobial properties that are useful in preventing and treating diseases.

Modulation of bacterial infection and inflammation by release of lactide from a silicone elastomer device

The aim of this project is to assess the anti-bacterial and anti-inflammation properties of a matrix-type silicone elastomer device providing sustained release of DL-lactide (a cyclic ester dimer of lactic acid). Lactide-loaded matrices were prepared by injection molding, and the devices were assessed for of in vitro release, anti-bacterial activity, and anti-inflammatory activity. The devices provided sustained release of lactide/lactic acid over five days and were effective against E. coli and S. aureus. Further, the devices reduced inflammation through activation of the GPR81 receptor via the NF-κB and JAK-STAT signalling pathway. These lactide-releasing silicone elastomer devices could improve clinical outcomes with certain short-term medical devices.

Organocatalyzed Synthesis of Bicyclic γ‐Lactam Derivatives via Asymmetric Conjugate Addition of Cyclic β‐Keto Esters to Benzoyl Acrylonitriles

AbstractA diaminomethylenemalononitrile organocatalyst efficiently promoted the asymmetric conjugate addition of cyclic β‐keto esters to benzoyl acrylonitriles to afford the corresponding adducts. This was followed by the transformation of the nitrile group into an amide group, resulting in bicyclic γ‐lactam derivatives in good yields with excellent enantioselectivities (up to 99 % ee). As far as we know, this is the first study to synthesize chiral bicyclic γ‐lactam derivatives from cyclic β‐keto esters and benzoyl acrylonitriles.