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Green Photocatalysis: A Comprehensive Review of Plant-Based Materials for Sustainable Water Purification
An Iron-Dependent Alcohol Dehydrogenase Is Involved in Ethanol Metabolism of Aromatoleum aromaticum
Transesterification/Esterification Reaction Catalysed by Functional Hybrid MOFs for Efficient Biodiesel Production

Journal Description

Reactions

Reactions is an international, peer-reviewed, open access journal on reaction chemistry and engineering published quarterly online by MDPI.

Open Access— free for readers, with article processing charges (APC) paid by authors or their institutions.
High Visibility: indexed within ESCI (Web of Science), Scopus, EBSCO, and other databases.
Rapid Publication: manuscripts are peer-reviewed and a first decision is provided to authors approximately 18.8 days after submission; acceptance to publication is undertaken in 3.5 days (median values for papers published in this journal in the second half of 2025).
Recognition of Reviewers: APC discount vouchers, optional signed peer review, and reviewer names published annually in the journal.
Journal Cluster of Chemical Reactions and Catalysis: Catalysts, Chemistry, Electrochem, Inorganics, Molecules, Organics, Oxygen, Photochem, Reactions, Sustainable Chemistry.

Impact Factor: 2.2 (2024); 5-Year Impact Factor: 2.2 (2024)

Imprint Information Journal Flyer Open Access ISSN: 2624-781X

Latest Articles

20 pages, 3274 KB

Open AccessReview

Incorporation of Spin Labels and Paramagnetic Tags for Magnetic Resonance Studies Using Cycloaddition Reactions as a Tool

by Amarendra Nath Maity, Amiya Kumar Medda and Shyue-Chu Ke

Reactions 2026, 7(1), 12; https://doi.org/10.3390/reactions7010012 - 6 Feb 2026

The cycloaddition reaction is one of the most common reactions in organic chemistry. It has been applied in various fields. Herein, we focus on the application of cycloaddition reactions in investigating biological molecules and materials using magnetic resonance techniques. To facilitate magnetic resonance [...] Read more.

The cycloaddition reaction is one of the most common reactions in organic chemistry. It has been applied in various fields. Herein, we focus on the application of cycloaddition reactions in investigating biological molecules and materials using magnetic resonance techniques. To facilitate magnetic resonance studies such as electron paramagnetic resonance (EPR) spectroscopy and paramagnetic nuclear magnetic resonance (NMR) spectroscopy, there is often a requirement to attach spin labels and paramagnetic tags to the system of interest. The cycloaddition reaction is one of the ways to tether these spin labels and paramagnetic tags. In this review, we highlight the applications of various cycloaddition reactions such as the Cu(I)-catalyzed azide–alkyne cycloaddition (CuAAC) reaction, the strain-promoted azide–alkyne cycloaddition (SPAAC) reaction and the Diels–Alder reaction in the interdisciplinary field of magnetic resonance studies of biomolecules, including proteins, nucleic acids, carbohydrates, lipids and glycans, as well as materials. Full article

(This article belongs to the Special Issue Cycloaddition Reactions at the Beginning of the Third Millennium, 2nd Edition)

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25 pages, 2708 KB

Open AccessArticle

Biotechnological Production of Fumaric Acid by Rhizopus arrhizus Using Apple Pomace as Substrate

by Ulf Prüße, Laslo Eidt and Anja Kuenz

Reactions 2026, 7(1), 11; https://doi.org/10.3390/reactions7010011 - 2 Feb 2026

Fumaric acid is one of the most important bio-based chemicals, with applications in the food, feed, polymer, pulp, and pharmaceutical industries. To overcome the limitations of the current petrochemical production process, alternative methods are being developed. Biotechnological production using wild-type fungi like Rhizopus [...] Read more.

Fumaric acid is one of the most important bio-based chemicals, with applications in the food, feed, polymer, pulp, and pharmaceutical industries. To overcome the limitations of the current petrochemical production process, alternative methods are being developed. Biotechnological production using wild-type fungi like Rhizopus sp. is a promising alternative. In this study, apple pomace was used as a carbohydrate source for fumaric acid production using Rhizopus arrhizus NRRL 1526. Our focus was on the use of free, non-structurally bound carbohydrates present in high amounts in apple pomace originating from direct apple juice processing. Three processes were compared: pressing, extraction, and a combination of both. Two cultivation strategies were applied: pre-culture and separate upstream biomass production. Using the pre-culture approach, a fumaric acid titer of 68.3 g/L was achieved with a yield of 0.53 g/g and a productivity of 0.29 g/(L·h) from synthetic apple pomace juice. Separate biomass production enabled growth-decoupled fumaric acid production, yielding 50.2 g/L and 79.3 g/L with yields of 0.82 g/g and 0.54 g/g and productivities of 0.17 g/(L·h) and 0.27 g/(L·h) from synthetic and real apple pomace juice, respectively. Thus, the efficient use of apple pomace for the fermentative production of fumaric acid is shown. Full article

(This article belongs to the Special Issue Waste Biorefinery Technologies for Accelerating Sustainable Energy Processes, 2nd Edition)

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Graphical abstract

11 pages, 1493 KB

Open AccessArticle

A Synergistic Dual-Function Silver–Manganese Dioxide–CNTs Ternary Composite Electrocatalyst for Solid-State Zinc–Air Batteries

by Guoqing Zhang and Peng Zhang

Reactions 2026, 7(1), 10; https://doi.org/10.3390/reactions7010010 - 1 Feb 2026

Exploring effective oxygen reduction reaction (ORR) electrocatalysts is essential for advancing solid-state alkaline zinc–air batteries (ZABs). This paper presents the synthesis of silver–manganese dioxide–carbon nanotubes (SMC) ternary composites as an electrocatalyst for air electrodes, achieved through one-step pyrolysis of silver permanganate under microwave [...] Read more.

Exploring effective oxygen reduction reaction (ORR) electrocatalysts is essential for advancing solid-state alkaline zinc–air batteries (ZABs). This paper presents the synthesis of silver–manganese dioxide–carbon nanotubes (SMC) ternary composites as an electrocatalyst for air electrodes, achieved through one-step pyrolysis of silver permanganate under microwave irradiation. Characterization techniques such as scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy dispersion spectrometer (EDS) consistently confirmed the composition of SMC, comprising silver and alpha-manganese dioxide anchored on the surface of carbon nanotubes (CNTs). Electrochemical tests including polarization and chronoamperometry curves demonstrated the superior electrocatalytic activity of SMC for ORR compared to chemically produced electrocatalysts in alkaline conditions. Furthermore, the performance of a solid-state zinc–air cell with SMC as the electrocatalyst was evaluated, showing a long discharge voltage plateau and a capacity of 60.03 mAh at 30 mA·cm

^{- 2}

. The study also delves into the mechanism behind the enhanced electrocatalytic activity, concluding that the strategy and electrocatalyst developed in this research offer a promising approach for creating efficient oxygen reduction catalysts for solid-state zinc–air batteries. Full article

16 pages, 4644 KB

Open AccessArticle

A Quantum Dynamics Simulation of the Adsorption of Water/Oxygen on Perovskite Material

by Jingyang Wu, Jinniu Miao, Yue Wang, Liqian Zhao, Jiaji Liang and Peng Du

Reactions 2026, 7(1), 9; https://doi.org/10.3390/reactions7010009 - 1 Feb 2026

The stability of perovskite materials in humid conditions significantly hinders their practical deployment. This study employed ab initio molecular dynamics (AIMD) simulations based on the Car–Parrinello approach to elucidate the adsorption mechanisms within two systems: CH₃NH₃PbI₃-15O₂ [...] Read more.

The stability of perovskite materials in humid conditions significantly hinders their practical deployment. This study employed ab initio molecular dynamics (AIMD) simulations based on the Car–Parrinello approach to elucidate the adsorption mechanisms within two systems: CH₃NH₃PbI₃-15O₂-2H₂O and CH₃NH₃PbI₃-15O₂-5H₂O. The findings indicate that in the system with a higher water content (5H₂O), the degradation of the perovskite skeleton is more severe. Additionally, the adsorption energy of oxygen molecules significantly increases, along with more pronounced charge transfer between the oxygen and the perovskite material. The study also reveals that although water molecules contribute to the damage of the perovskite skeleton, oxygen molecules are the primary culprits. These insights not only clarify the specific impacts of various components in a mixed-gas environment on perovskite stability but also provide an essential theoretical basis for future modifications and optimizations of perovskite materials. Full article

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10 pages, 678 KB

Open AccessArticle

Using the Response Surface Method for the Optimization of Gas-Phase Hydrogenation of Carbon Dioxide on Nickel-Based Catalysts—A Large Laboratory-Scale Process

by Mirosław Krzysztof Szukiewicz, Erwin Górka and Elżbieta Chmiel-Szukiewicz

Reactions 2026, 7(1), 8; https://doi.org/10.3390/reactions7010008 - 23 Jan 2026

In this study, the response surface method (RSM) was used to determine the best reaction conditions for the gas-phase hydrogenation of carbon dioxide on a commercial nickel-based catalyst. The RSM was applied in our previous study to find the optimal conditions for the [...] Read more.

In this study, the response surface method (RSM) was used to determine the best reaction conditions for the gas-phase hydrogenation of carbon dioxide on a commercial nickel-based catalyst. The RSM was applied in our previous study to find the optimal conditions for the same process carried out in laboratory-scale tubular reactors. The main benefits observed were fast detection of optimal conditions and the high precision of the optimum detected (which was experimentally confirmed). These advantages were due to the small number of experiments conducted and the simplicity of the models employed; only linear and quadratic models were developed. The successful result encouraged us to carry out experiments in a larger-scale reactor—an intermediate between a laboratory plant and a pilot plant. This approach helped us to fix some problems resulting from the larger scale of the process conducted. Despite the difficulties described in the main part of this article, we can recommend using the RSM as a tool for supporting experimentation and substantially speeding up the analysis of results and their introduction into practice. At the process scale considered, maximum carbon dioxide conversion was obtained at a temperature of 354 °C and a ratio of molar fluxes of H₂ to CO₂ equal to 3.9. It should be emphasized that this result was confirmed experimentally. Full article

(This article belongs to the Special Issue Hydrogen Production and Storage, 3rd Edition)

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13 pages, 2006 KB

Open AccessArticle

Hydrodeoxygenation of Black Liquor HTL Oil Model Compounds in Supercritical Water

by Sari Rautiainen, Tyko Viertiö, Niko Vuorio, Felix Hyppönen, Luděk Meca, Pavel Kukula and Juha Lehtonen

Reactions 2026, 7(1), 7; https://doi.org/10.3390/reactions7010007 - 20 Jan 2026

Black liquor, the side stream from Kraft pulping, is a promising feedstock for the production of renewable fuels via hydrothermal liquefaction (HTL). However, further upgrading of the black liquor HTL oil is required to reduce the oxygen content for fuel use. In this [...] Read more.

Black liquor, the side stream from Kraft pulping, is a promising feedstock for the production of renewable fuels via hydrothermal liquefaction (HTL). However, further upgrading of the black liquor HTL oil is required to reduce the oxygen content for fuel use. In this work, the hydrodeoxygenation (HDO) of black liquor HTL oil model compounds was investigated to enhance the understanding of catalyst activity and selectivity under hydrothermal conditions. The study focused on isoeugenol and 4-methylcatechol as model compounds, representing different functionalities in black liquor-derived HTL-oil. Sulfided NiMo catalysts supported on titania, zirconia, activated carbon, and α-alumina were evaluated in batch mode at subcritical and supercritical upgrading using hydrogen gas. The results show that isoeugenol was fully converted in all experiments, while 4-methylcatechol conversion varied depending on the catalyst and reaction conditions. Phenols were obtained as the main products and the maximum degree of deoxygenation achieved was around 40%. This research provides insights into the potential of hydrothermal HDO for upgrading BL-derived biocrudes, emphasising the importance of catalyst selection and reaction conditions in hydrothermal conditions. Full article

(This article belongs to the Special Issue Feature Papers in Reactions in 2025)

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Graphical abstract

33 pages, 10526 KB

Open AccessReview

Recent Developments in the Catalytic Enantioselective Sakurai Reaction

by Hélène Pellissier

Reactions 2026, 7(1), 6; https://doi.org/10.3390/reactions7010006 - 10 Jan 2026

The Sakurai reaction constitutes a valuable tool for carbon–carbon bond formation. The use of nontoxic allylic reagents as well as the atom economy of the global process has prompted the development of enantioselective (aza)-variants based on the use of chiral organo- and metal [...] Read more.

The Sakurai reaction constitutes a valuable tool for carbon–carbon bond formation. The use of nontoxic allylic reagents as well as the atom economy of the global process has prompted the development of enantioselective (aza)-variants based on the use of chiral organo- and metal catalysts. This review collects the recent developments in catalytic enantioselective Sakurai reactions published since the beginning of 2011, including methodologies based on the use of chiral organocatalysts, metal/boron catalysts and multicatalyst systems. It is divided into three parts, dealing successively with enantioselective organocatalytic (aza)-Sakurai reactions, enantioselective metal/boron-catalyzed Sakurai reactions and enantioselective multicatalyzed (aza)-Sakurai reactions. It shows that, although still widely developed with aromatic aldehydes, the enantioselective catalytic Sakurai reaction has considerably matured in the last decade. Full article

(This article belongs to the Special Issue Feature Papers in Reactions in 2025)

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18 pages, 4662 KB

Open AccessArticle

Effect of Acetic Acid on Morphology, Structure, Optical Properties, and Photocatalytic Activity of TiO₂ Obtained by Sol–Gel

by Sofía Estrada-Flores, Tirso E. Flores-Guia, Catalina M. Pérez-Berumen, Luis A. García-Cerda, Aurora Robledo-Cabrera, Elsa N. Aguilera-González and Antonia Martínez-Luévanos

Reactions 2026, 7(1), 5; https://doi.org/10.3390/reactions7010005 - 10 Jan 2026

Titanium oxide (TiO₂) is of great interest in solar cell manufacturing, hydrogen production, and organic compound photodegradation. The synthesis variables and methodology affect the morphology, texture, crystalline structure, and phase mixtures of TiO₂, which, in turn, affect the optical [...] Read more.

Titanium oxide (TiO₂) is of great interest in solar cell manufacturing, hydrogen production, and organic compound photodegradation. The synthesis variables and methodology affect the morphology, texture, crystalline structure, and phase mixtures of TiO₂, which, in turn, affect the optical and catalytic properties of TiO₂. In this work, the effect of acetic acid as a catalyst and chelating agent on the morphology, texture, crystal structure, optical properties, and photocatalytic activity of TiO₂ samples obtained using the sol–gel method with sodium dodecyl sulfate (SDS) as a template was investigated. The results indicated that acetic acid not only catalyzes the hydrolysis of the TiO₂ precursor but also acts as a chelating agent, causing a decrease in crystallite size from 18.643 nm (T7 sample, pH = 6.8, without addition of acetic acid) to 16.536 nm (T2 sample, pH = 2). At pH 2 and 3, only the anatase phase was formed (T2 and T3 samples), whereas at pH 5 and 6.8, in addition to the anatase phase, the brookite phase (11.4% and 15.61% for samples T5 and T7, respectively) was formed. The band-gap value of TiO₂ decreased with decreasing pH during synthesis. Although the T2 sample had the highest specific surface area and pore volume (232.02 m²g⁻¹ and 0.46 gcm⁻³, respectively), the T3 sample had better efficiency in methylene blue dye photodegradation because its bird-nest-like morphology improved photon absorption, promoting better photocatalytic performance. Full article

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32 pages, 3554 KB

Open AccessReview

Synthetic Strategies for Nitramines: From Energetic Materials to Atmospheric Byproducts

by Simen Gjelseth Antonsen, Claus Jørgen Nielsen, Hans Olav Hovtun Palm and Yngve Henning Stenstrøm

Reactions 2026, 7(1), 4; https://doi.org/10.3390/reactions7010004 - 7 Jan 2026

Nitramines are nitrogen-containing organic compounds with the formula R¹R²N–NO₂. They are well-known as explosives and have been produced industrially for more than a century. A few nitramine-containing natural products have also been identified in recent years. Nitramines [...] Read more.

Nitramines are nitrogen-containing organic compounds with the formula R¹R²N–NO₂. They are well-known as explosives and have been produced industrially for more than a century. A few nitramine-containing natural products have also been identified in recent years. Nitramines have also found their way into specific synthetic procedures, usually as intermediates, and for the last decades, the implementation of amine-based carbon capture and storage (CCS) technologies to mitigate CO₂ emissions from fossil fuel combustion is of particular concern since small amounts are produced. Both environmental and health implications are of particular interest, and little is known today. The need for efficient and safe synthetic procedures is, therefore, vital for further research in the field. The present review gives a detailed summary of published methods and research post-millennium. Many new as well as well-established methods are presented. Representative examples with basic conditions and yields are given. Finally, indications for future research are discussed. Full article

(This article belongs to the Special Issue Feature Papers in Reactions in 2025)

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Graphical abstract

36 pages, 6309 KB

Open AccessReview

The Kabachnik–Fields Reaction: A Key Transformation in Organophosphorus Chemistry

by Giovanni Ghigo, Sara Nicoletti and Stefano Dughera

Reactions 2026, 7(1), 3; https://doi.org/10.3390/reactions7010003 - 4 Jan 2026

The Kabachnik–Fields (KF) reaction is a versatile three-component method for the condensation of amines, carbonyl compounds, and P–H reagents, enabling efficient synthesis of α-aminophosphonates—key bioactive and functional molecules. This review critically examines the literature from the last 25 years. However, with regard to [...] Read more.

The Kabachnik–Fields (KF) reaction is a versatile three-component method for the condensation of amines, carbonyl compounds, and P–H reagents, enabling efficient synthesis of α-aminophosphonates—key bioactive and functional molecules. This review critically examines the literature from the last 25 years. However, with regard to mechanistic aspects, selected earlier seminal studies are also considered when necessary to provide a coherent and comprehensive mechanistic framework. Advances in catalyst-free methodologies, sustainable synthetic approaches, and Lewis and Brønsted acid catalysis are discussed, alongside developments in enantioselective KF reactions in the presence of chiral metal complexes or organocatalysts. Full article

(This article belongs to the Special Issue Feature Papers in Reactions in 2025)

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Graphical abstract

23 pages, 9458 KB

Open AccessArticle

Experimental Study on the Co-Combustion Characteristics of Brown Gas (HHO) and Bituminous Coal/Anthracite with Different Injection Modes in a One-Dimensional Furnace

by Kaihong Huo, Yunlong Cai, Yong He, Shiyan Liu, Chaoqun Xu, Siyu Liu, Wubin Weng, Yanqun Zhu and Zhihua Wang

Reactions 2026, 7(1), 2; https://doi.org/10.3390/reactions7010002 - 2 Jan 2026

As the energy structure evolves, low-load operation of coal-fired boilers is becoming common, posing challenges to combustion stability. This study explored the co-combustion of brown gas (HHO) with bituminous coal and anthracite in a one-dimensional furnace. Results indicate that introducing HHO significantly elevated [...] Read more.

As the energy structure evolves, low-load operation of coal-fired boilers is becoming common, posing challenges to combustion stability. This study explored the co-combustion of brown gas (HHO) with bituminous coal and anthracite in a one-dimensional furnace. Results indicate that introducing HHO significantly elevated combustion temperatures, with maximum increases of 158 °C and 207 °C, respectively. In the premixed mode, the flame front shifted upstream, indicating advanced ignition timing. Moreover, HHO co-combustion notably enhanced the combustion stability of anthracite, as reflected in stabilized furnace temperatures. With increasing HHO flow rate, CO concentrations from both bituminous coal and anthracite were reduced by over 80%. The combustion efficiency of bituminous coal reached 98%, while the combustion efficiency of anthracite increased by 19% (premixed) and 13% (staged), confirming the premixed mode’s superiority in promoting complete combustion. HHO co-combustion increased SO₂ emissions but had a complex effect on NO_X emissions due to the competition between NO_X reduction caused by HHO and NO_X formation caused by the increased combustion temperature. HHO co-combustion changed the melting point of fly ash, increased the content of Al₂O₃, and reduced the content of Na₂O, K₂O, and MgO, influencing the slagging behavior of the boiler and the subsequent management of fly ash. Full article

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22 pages, 938 KB

Open AccessReview

Topology Meets Reactivity: Rationalizing Electron Rearrangements in Cycloadditions Through Thom’s Polynomials and Bonding Evolution Theory

by Leandro Ayarde-Henríquez, Cristian J. Guerra, Hans Lenes, Elizabeth Rincón and Eduardo Chamorro

Reactions 2026, 7(1), 1; https://doi.org/10.3390/reactions7010001 - 1 Jan 2026

Cited by 1

This mini-review discusses recent advances in the rigorous application of Bonding Evolution Theory (BET) to elucidate electron rearrangements in cycloaddition reactions occurring in both ground and electronically excited states. Computational studies reveal that describing bond formation and cleavage through parametric polynomials derived from [...] Read more.

This mini-review discusses recent advances in the rigorous application of Bonding Evolution Theory (BET) to elucidate electron rearrangements in cycloaddition reactions occurring in both ground and electronically excited states. Computational studies reveal that describing bond formation and cleavage through parametric polynomials derived from the Catastrophe Theory (CT) provides a deeper and more coherent understanding of chemical bonding and reactivity. However, several existing BET applications have adopted CT concepts without fully incorporating the mathematical rigor on which BET is based, resulting in conceptual ambiguities and inaccurate interpretations. A proper implementation of BET requires evaluating the Hessian matrix at potentially degenerate critical points (CPs) of the Electron Localization Function (ELF) and assessing their relative evolution along the reaction coordinate. This systematic protocol integrates key CT principles within BET’s original framework, restoring its formal consistency. The resulting analyses have revealed correlations between electron-density symmetry and CT polynomials, relationships between these polynomials and the homolytic or heterolytic character of bond dissociation, and the development of a CT-based model for scaling bond polarity. These findings demonstrate that incorporating CT-derived functions into BET is not merely a formal refinement but a fundamental step toward achieving a more rigorous and predictive understanding of electron rearrangements in cycloadditions. Full article

(This article belongs to the Special Issue Cycloaddition Reactions at the Beginning of the Third Millennium, 2nd Edition)

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14 pages, 2641 KB

Open AccessArticle

Continuous Deoxygenation of Palmitic Acid, Methyl Palmitate, and Licuri Biodiesel over Granular Pd/C Catalysts in a Trickle-Bed Reactor

by Pedro H. M. Araujó, Connor Thompson, Taylor C. Schulz and H. Henry Lamb

Reactions 2025, 6(4), 73; https://doi.org/10.3390/reactions6040073 - 13 Dec 2025

Biofuels offer potential to mitigate climate change, increase energy security, and economically support farmers around the world. Licuri (Syagrus coronata) could be an important biofuel feedstock because its kernel (edible seed) has high energy content. This research investigates the optimal reaction [...] Read more.

Biofuels offer potential to mitigate climate change, increase energy security, and economically support farmers around the world. Licuri (Syagrus coronata) could be an important biofuel feedstock because its kernel (edible seed) has high energy content. This research investigates the optimal reaction conditions to convert fatty acids (FAs) and fatty acid methyl esters (FAMEs) (including licuri biodiesel) to hydrocarbons via deoxygenation in a trickle-bed reactor over granular Pd/C catalysts. Our results indicate that a 20 wt.% palmitic acid (PA) feed is optimum for continuous deoxygenation at 300 °C and 15 bar in 5% H₂/He because of decarboxylation inhibition at higher concentrations. Deoxygenation rates are higher for PA than for methyl palmitate (MP) because of the slow initial hydrogenolysis of the methoxy bond over Pd/C. The hydrocarbon product distributions from deoxygenation of licuri biodiesel were fully consistent with FA decarboxylation and decarbonylation. A lab-prepared 5 wt.% Pd/C catalyst with higher metal dispersion provided modestly higher hydrocarbon yields from licuri biodiesel than a commercial 1 wt.% Pd/C catalyst. Full article

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26 pages, 4424 KB

Open AccessReview

C–H Annulation in Azines to Obtain 6,5-Fused-Bicyclic Heteroaromatic Cores for Drug Discovery

by Maria Carolina Theisen, Isis Apolo Silveira de Borba, Angélica Rocha Joaquim and Fernando Fumagalli

Reactions 2025, 6(4), 72; https://doi.org/10.3390/reactions6040072 - 10 Dec 2025

Fused-bicyclic heteroaromatic cores are a common framework in drugs and other biologically active compounds. Those containing azine rings are widely used in drug discovery campaigns. Although these cores are very common, C–H functionalization of their azine moieties remains challenging, especially in annulation reactions. [...] Read more.

Fused-bicyclic heteroaromatic cores are a common framework in drugs and other biologically active compounds. Those containing azine rings are widely used in drug discovery campaigns. Although these cores are very common, C–H functionalization of their azine moieties remains challenging, especially in annulation reactions. Therefore, this review highlights the progress made over the years in C–H annulation reactions that have produced these essential 6,5-fused bicyclic heteroaromatic cores for drug discovery. For that, the review was divided according to the five-membered rings moiety (pyrrole, pyrazole, imidazole, furan, thiophen, and thiazole) fused to different azines (pyridine, pyrazine, pyridazine, pyrimidine, and triazine). Although some important advances have been made over the years, there remains a need for research in synthetic methodology to expand the use of these heteroaromatic cores in biologically active compounds. Full article

(This article belongs to the Special Issue Advances in Organic Synthesis for Drug Discovery and Development)

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15 pages, 3004 KB

Open AccessArticle

Trivalent Chromium Electroplating Baths—The Inner-Sphere Complex Hypothesis

by Julio C. Avalos, Axel S. Martínez, Eugenia Aldeco-Pérez, Julieta Torres-González and German Orozco

Reactions 2025, 6(4), 71; https://doi.org/10.3390/reactions6040071 - 8 Dec 2025

There are 880 studies focused on trivalent chrome baths, and several studies suggest the formation of

{[C r (I I I) L (H_{2} O)_{5}]}^{2 +}

, where

L

is an additive such as oxalate. The literature [...] Read more.

There are 880 studies focused on trivalent chrome baths, and several studies suggest the formation of

{[C r (I I I) L (H_{2} O)_{5}]}^{2 +}

, where

L

is an additive such as oxalate. The literature suggests that this compound decreases the energy needed in the electrodeposition process. We call this approach the inner-sphere complex hypothesis because these complexes are suggested, such as principal intermediate compounds. There are several disadvantages of this postulate, which are numbered in our study. This hypothesis was tested via Fourier transform infrared spectroscopy performed in attenuated total reflectance (ATR) mode. In addition, the potassium bis(oxalato) diaqua chromate (III) dihydrate (

K [C r {(C_{2} O_{4})}_{2} {(O H_{2})}_{2}] \cdot 2 H_{2} O

) compound was selected as a probe molecule because it contains bridging

C - O - C r

bonds, which are supposedly the largest number of bonds in the inner-sphere complexes in bath solutions. There is strong evidence of numerous bridging

C - O - C r

bonds in the solid sample; conversely, in solution,

C r (I I I)

prefers to form terminal bonds (

C r - O

). These results suggest that the concentration of the inner-sphere complex is lower in solution. In solutions containing chromium (III) sulfate and oxalate anions, the concentrations of these complexes are much lower. Although some inner-sphere complexes are formed, their concentration does not seem to be relevant to the electrodeposition process. Otherwise, at high ionic strengths, the formation of ion pairs and hydrogen bonds between

C r (I I I)

and additives is probable. Our research highlights the importance of vibrational spectroscopy in resolving the mechanics of the trivalent chrome electrodeposition process. This is the first study reporting a band of

C r - O

bonds in trivalent chrome baths. Full article

(This article belongs to the Special Issue Feature Papers in Reactions in 2025)

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12 pages, 2390 KB

Open AccessArticle

Computational Investigation of Mechanism and Selectivity in (3+2) Cycloaddition Reactions Involving Azaoxyallyl Cations

by Wei Zhou, Lei Zhang, Guixian Liu, Xiaosi Ma and Xiangtai Meng

Reactions 2025, 6(4), 70; https://doi.org/10.3390/reactions6040070 - 8 Dec 2025

Azaoxyallyl cations, as novel and versatile three-atom components, have been widely utilized in cycloaddition reactions, with the competition between O- and N-cyclization pathways remaining a key research focus. This study investigates the mechanism and site selectivity of (3+2) cycloaddition between azaoxyallyl cations and [...] Read more.

Azaoxyallyl cations, as novel and versatile three-atom components, have been widely utilized in cycloaddition reactions, with the competition between O- and N-cyclization pathways remaining a key research focus. This study investigates the mechanism and site selectivity of (3+2) cycloaddition between azaoxyallyl cations and 1,2-benzisoxazoles using density functional theory calculations. The results reveal a stepwise (3+2) addition to the C=N double bond, followed by base-assisted N-O bond cleavage and isoxazole ring-opening, leading to oxazoline (via O-cyclization) or imidazolone (via N-cyclization) derivatives. When unsubstituted 1,2-benzisoxazole is used as the substrate, O-cyclization dominates as a kinetically controlled process due to lower activation barriers, while N-cyclization, as a thermodynamically controlled process, is minor. The presence of a methyl group at the C(3) position in 1,2-benzisoxazoles completely blocks N-O bond cleavage, forcing exclusive (3+2) cycloaddition to yield less stable tricyclic products via N-cyclization rather than O-cyclization. These findings align with experimental observations and provide new mechanistic insights into the site selectivity of azaoxyallyl cation cycloadditions. Full article

(This article belongs to the Special Issue Cycloaddition Reactions at the Beginning of the Third Millennium, 2nd Edition)

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15 pages, 4389 KB

Open AccessArticle

Optimization and Kinetic Study of Palmitic Acid Esterification with Subcritical Methanol via Response Surface Methodology

by Jie Luo, Zhigang Que, Ke Zhang, Yinxuan Fu, Xiaodi Cheng, Rong Huang, Jinming Shi, Haiwei Jiang, Xianbin Ai, Tonghui Deng, Xianhua Qiu and Chunbao Xu

Reactions 2025, 6(4), 69; https://doi.org/10.3390/reactions6040069 - 5 Dec 2025

Biodiesel is a green, low-carbon, and renewable fuel with the potential to substitute fossil fuels. The effects of reaction temperature (175–290 °C), residence time (5–75 min), and molar ratio of methanol to palmitic acid (6:1–35:1) on the non-catalytic esterification of palmitic acid with [...] Read more.

Biodiesel is a green, low-carbon, and renewable fuel with the potential to substitute fossil fuels. The effects of reaction temperature (175–290 °C), residence time (5–75 min), and molar ratio of methanol to palmitic acid (6:1–35:1) on the non-catalytic esterification of palmitic acid with methanol to produce biodiesel were investigated by using a batch reactor. Moreover, the reaction parameters were optimized by using the response surface methodology (RSM), and the reaction kinetics were analyzed. The results showed that the conversion rate of palmitic acid to methyl palmitate increased to 100% as the reaction temperature rose from 175 °C to 225 °C, slightly changed until 275 °C, and then decreased to 94.81% at 290 °C. The conversion rate increased with residence time and reached the maximum value of 94.93% at 60 min. With increasing the molar ratio, the conversion rate rose to a maximum value of 85.46% at 15:1 and subsequently decreased. RSM results indicated the relative influence of factors on the conversion rate as reaction temperature > residence time > molar ratio. The optimal reaction parameters were 224 °C, 26 min, and a molar ratio of 16:1, affording a palmitic acid conversion rate of 99.30%. The esterification reaction between methanol and palmitic acid follows the first-order kinetics model. Full article

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19 pages, 3843 KB

Open AccessArticle

Degradation and Nitrogen Transfer of 4-Aminophenol by Cavitation Induced by a Composite Hydrodynamic Cavitator

by Baoe Wang, Rihong Zhang, Zipeng She and Yiyong Li

Reactions 2025, 6(4), 68; https://doi.org/10.3390/reactions6040068 - 4 Dec 2025

The treatment of refractory nitrogenous organic matter in industrial wastewater management poses challenges in the removal of organic matter and nitrogen. To address these issues, this study utilized a novel composite hydrodynamic cavitator, mainly consisting of spiral pipes and a step drain, which [...] Read more.

The treatment of refractory nitrogenous organic matter in industrial wastewater management poses challenges in the removal of organic matter and nitrogen. To address these issues, this study utilized a novel composite hydrodynamic cavitator, mainly consisting of spiral pipes and a step drain, which could generate cavitation twice per pass at the throat of the spiral pipe and the step drain of the cavitation cavity, thereby distinguishing it from other existing cavitators that produce cavitation only once per pass. The composite hydrodynamic cavitator, optimized using ANSYS 19.2 simulation software, offers significant advantages in energy utilization and mass transfer efficiency. Moreover, it generates a high concentration of hydroxyl free radicals, which are crucial for organic matter degradation. Batch experiments demonstrated the effective treatment of 4-aminophenol. Within 120 min, 4-aminophenol degradation efficiency reached 74.7% and total nitrogen concentration decreased slightly from 1.28 mg/L to 1.06 mg/L, while ammonia nitrogen concentration initially increased before decreasing from its peak value of 0.82 mg/L to 0.77 mg/L. During the cavitation treatment of 4-aminophenol, intermediate products, such as benzoquinone, were generated. Under the strong oxidizing action of hydroxyl radicals, nitrogen undergoes deamination to form ammonium ions, which were likely removed predominantly as nitrogen gas. The experimental results are anticipated to establish a foundation for the application of hydrodynamic cavitation technology in the treatment of refractory organic wastewater degradation and to support denitrification processes. Full article

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44 pages, 7733 KB

Open AccessReview

Furo[3,2-b]pyrrole-5-carboxylate as a Rich Source of Fused Heterocycles: Study of Synthesis, Reactions, Biological Activity and Applications

by Renata Gašparová

Reactions 2025, 6(4), 67; https://doi.org/10.3390/reactions6040067 - 2 Dec 2025

Furo[3,2-b]pyrroles (FPs) are important members of the heteropentalene family. In particular, furo[3,2-b]pyrrole-5-carboxylates (FPcs) are commonly used as versatile building blocks for the synthesis of a large library of FP derivatives. Their structure with five potential reaction centres and an [...] Read more.

Furo[3,2-b]pyrroles (FPs) are important members of the heteropentalene family. In particular, furo[3,2-b]pyrrole-5-carboxylates (FPcs) are commonly used as versatile building blocks for the synthesis of a large library of FP derivatives. Their structure with five potential reaction centres and an electron-rich character enables a wide range of transformations, from simple substitutions to multi-step reactions, yielding complex compounds with a furo[3,2-b]pyrrole scaffold. Many furo[3,2-b]pyrrole derivatives exhibit promising biological activity, while others have been employed in the construction of π-conjugated fused systems for optoelectronics. Efficient synthetic routes to furo[3,2-b]pyrrole derivatives are therefore of considerable interest. This review focuses on the synthetic methods leading to furo[3,2-b]pyrrole-5-carboxylates (FPcs), from the first successful attempts in the 1970s to recent approaches. Various methodologies are reported for the construction of complex molecules built from furo[3,2-b]pyrrole-5-carboxylates, emphasising their utility in the synthesis of fused heterocycles. This review also covers recent advances in screening for biological activity and applications such as fluorescent dyes. Full article

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17 pages, 3157 KB

Open AccessReview

Green Strategies for the Synthesis of Heterocyclic Derivatives with Potential Against Neglected Tropical Diseases

by Vinícius Augusto Campos Péret and Renata Barbosa de Oliveira

Reactions 2025, 6(4), 66; https://doi.org/10.3390/reactions6040066 - 2 Dec 2025

Neglected tropical diseases (NTDs) remain a significant global health burden, exacerbated by the ongoing climate emergency, which alters disease distribution and increases vulnerability in affected populations. The urgent need for novel therapeutics demands innovative approaches in drug discovery, with heterocyclic compounds serving as [...] Read more.

Neglected tropical diseases (NTDs) remain a significant global health burden, exacerbated by the ongoing climate emergency, which alters disease distribution and increases vulnerability in affected populations. The urgent need for novel therapeutics demands innovative approaches in drug discovery, with heterocyclic compounds serving as versatile scaffolds due to their diverse electronic and structural properties that enable potent biological activity. This review highlights how green chemistry principles have been applied to the construction of bioactive heterocyclic cores relevant to NTD drug development. Key sustainable methodologies are discussed, including microwave-assisted solvent-free and green-solvent reactions, ultrasound-assisted synthesis, mechanochemical one-pot multistep strategies, and the use of ionic liquids and deep eutectic solvents as environmentally benign catalysts and reaction media. By focusing on these approaches, the review emphasizes how green synthetic strategies can accelerate the development of pharmacologically relevant heterocycles while minimizing environmental impact, resource consumption, and hazardous waste generation. Full article

(This article belongs to the Special Issue Advances in Organic Synthesis for Drug Discovery and Development)

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