Mechanical-Force-Induced Non-spontaneous Dehalogenative Deuteration of Aromatic Iodides Enabled by Using Piezoelectric Materials as a Redox Catalyst

The development of green and efficient deuteration methods is of great significance for various fields such as organic synthesis, analytical chemistry, and medicinal chemistry. Herein, we have developed a dehalogenative deuteration strategy using piezoelectric materials as catalysts in a solid-phase system under ball-milling conditions. This non-spontaneous reaction is induced by mechanical force. D2O can serve as both a deuterium source and an electron donor in the transformation, eliminating the need for additional stoichiometric exogenous reductants. A series of (hetero)aryl iodides can be transformed into deuterated products with high deuterium incorporation. This method not only effectively overcomes existing synthetic challenges but can also be used for deuterium labelling of drug molecules and derivatives. Bioactivity experiments with deuterated drug molecule suggest that the D-ipriflavone enhances the inhibitory effects on osteoclast differentiation of BMDMs in vitro.

Greenly Synthesized CoPBA@PANI as a Proficient Electrocatalyst for Oxygen Evolution Reaction and Its Green Sustainability Assessments

Water electrolysis is a key factor to generate mobile and sustainable energy sources for H2 production. Cobalt-based Prussian Blue analogues encompassed with polymer support electrocatalysts CoPBAX@PANI (CoPBA@PANI-100, CoPBA@PANI-200, and CoPBA@PANI-300) have been synthesized and characterized. The well-designed CoPBA@PANI-200/GC shows a low overpotential (η10) of 301 mV with a small Tafel slope (56 mV dec-1), comapred to that of IrO2 (348 mV ; 98 mV dec-1) for OER. The conductivity with stability of CoPBAX@PANI have been increased due to the synergistic effect of CoPBA with PANI. PANI provides additional active sites and shows strong binding with Co ions, and the even distribution of CoPBA overcomes the sluggish kinetics. The turnover frequency (TOF) of CoPBA@PANI-200/GC (0.0212, s-1) was ∼15 times higher than IrO2 (0.0014 s-1) at 1.60 V. Furthermore, CoPBA@PANI-200/NF delivers low overpotential of 274 mV@10 mA cm-2 and exhibits a durability of >250 h with a potential loss of 4.2%. Benefiting from strong electronic interaction between polymer support and evenly distributed CoPBA, CoPBAx@PANI shows higher electrochemical active surface area (ECSA) of 53.08 mF cm-2. The solar-based water electrolysis confirmed the practical use of CoPBA@PANI-200/NF (1.57 V) for eco-benign industrial H2 production. The CoPBA@PANI-200 shows exceptional OER performances as well as favorable kinetics to resolve the sluggish water oxidation. Hence, the cost-effective CoPBA@PANI performances opens a prospective way to boost the efficiency of other cobalt-derived catalysts in renewable energy devices.

Mechanochemical Cascade Cyclization of Cyclopropyl Ketones with 1,2-Diamino Arenes for the Direct Synthesis of 1,2-Disubstituted Benzimidazoles†

A mechanochemical synthesis of 1,2-disubstituted benzimidazoles from donor-acceptor cyclopropyl ketones and 1,2-diaminoarenes under metal-free and solventless conditions is reported. The reaction does not require inert conditions and is promoted by a stoichiometric amount of 1,1,1,3,3,3-hexafluoroisopropanol. This cascade reaction involves ring-opening, cyclization, and retro-Mannich reaction of cyclopropyl ketones with aryl 1,2-diamines. Compared to its solution-phase counterpart, this mechanochemical approach shows fast reactivity (24 vs 1.5 h). Mechanistic investigations by electrospray ionization mass spectrometry helped us to propose the reaction mechanism.

Mechanochemical reduction of alkyl and aryl halides using mesoporous zinc oxide

In this study, we propose a mechanochemical approach that combines mesoporous ZnO (m-ZnO) as a mechanoredox catalyst and silane-mediated atom transfer chemistry to achieve efficient hydrodehalogenation of organic halides. The reaction can be conducted under mild conditions without the use of a large amount of organic solvent. Substrates ranging from activated alkyl halides to unactivated aryl halides were converted to the corresponding debrominated hydrogenation products in moderate to excellent isolated yields (50-95%). In addition, m-ZnO can be recycled and reused without appreciable loss of catalytic activity.

Mechanochemistry Drives Alkene Difunctionalization via Radical Ligand Transfer and Electron Catalysis

A general and modular protocol is reported for olefin difunctionalization through mechanochemistry, facilitated by cooperative radical ligand transfer (RLT) and electron catalysis. Utilizing mechanochemical force and catalytic amounts of 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO), ferric nitrate can leverage nitryl radicals, transfer nitrooxy-functional group via RLT, and mediate an electron catalysis cycle under room temperature. A diverse range of activated and unactivated alkenes exhibited chemo- and regioselective 1,2-nitronitrooxylation under solvent-free or solvent-less conditions, showcasing excellent functional group tolerance. Mechanistic studies indicated a significant impact of mechanochemistry and highlighted the radical nature of this nitrative difunctionalization process.

Mechanochemical Regioselective [3+3] Annulation of 6-Amino Uracil with Propargyl Alcohols Catalyzed by a Brønsted Acid/Hexafluoroisopropanol

A mechanochemistry approach is developed for regioselective synthesis of functionalized dihydropyrido[2,3-d]pyrimidines by milling propargylic alcohols and 6-aminouracils with HFIP/p-TsOH. In the case of tert-propargyl alcohols, this [3+3] cascade annulation proceeded through allenylation of uracil followed by a 6-endo trig cyclization. With sec-propargyl alcohols, the reaction furnished the propargylation of uracil. This atom economy ball milling reaction allows access to a broad range of dihydropyrido[2,3-d]pyrimidine derivatives in excellent yields. We demonstrated the gram scale synthesis of 3 g and post-synthetic modifications to effect the cyclization of 5 to 6.

JEDI: A versatile code for strain analysis of molecular and periodic systems under deformation

Stretching or compression can induce significant energetic, geometric, and spectroscopic changes in materials. To fully exploit these effects in the design of mechano- or piezo-chromic materials, self-healing polymers, and other mechanoresponsive devices, a detailed knowledge about the distribution of mechanical strain in the material is essential. Within the past decade, Judgement of Energy DIstribution (JEDI) analysis has emerged as a useful tool for this purpose. Based on the harmonic approximation, the strain energy in each bond length, bond angle, and dihedral angle of the deformed system is calculated using quantum chemical methods. This allows the identification of the force-bearing scaffold of the system, leading to an understanding of mechanochemical processes at the most fundamental level. Here, we present a publicly available code that generalizes the JEDI analysis, which has previously only been available for isolated molecules. Now, the code has been extended to two- and three-dimensional periodic systems, supramolecular clusters, and substructures of chemical systems under various types of deformation. Due to the implementation of JEDI into the Atomic Simulation Environment, the JEDI analysis can be interfaced with a plethora of program packages that allow the calculation of electronic energies for molecular systems and systems with periodic boundary conditions. The automated generation of a color-coded three-dimensional structure via the Visual Molecular Dynamics program allows insightful visual analyses of the force-bearing scaffold of the strained system.

Synthesis of Quinazolinones and Benzothiazoles Using α-Keto Acids under Ball Milling

Mechanochemistry refers to the initiation of chemical reactions via mechanical forces such as milling, grinding, or shearing to achieve the chemical transformations. As a manifestation of mechanocatalysis, herein, an oxidant-free and solvent-free approach for the synthesis of quinazolinones (23 derivatives) and benzothiazoles (23 derivatives) has been developed through stainless-steel-driven decarboxylative acyl radical generation from α-keto acids. A library of 2-arylquinazolinones and 2-arylbenzothiazoles has been prepared in moderate to good yields at room temperature. Moreover, control experiments and XPS studies supported the reduction (by zerovalent iron) of molecular oxygen through the moderate abrasion of balls, which promoted the generation of a superoxide radical anion via a SET process.

Mechanochemistry enabling highly efficient Birch reduction using sodium lumps and d-(+)-glucose

In this study, a mechanochemical protocol for highly efficient and ammonia-free sodium-based Birch reduction was developed, leveraging the use of cheap and easy-to-handle sodium lumps. The key to achieving this transformation is the use of d-(+)-glucose as a proton source, which solidifies the reaction mixture in bulk state, enhancing the efficiency of the in situ mechanical activation of sodium lumps through the ball-milling process. Under the developed conditions, a diverse array of aromatic and heteroaromatic compounds were selectively reduced to produce the corresponding 1,4-cyclohexadiene derivatives in high yields within 30 min. Notably, all synthetic operations can be carried out without inert gases or the need for dry or bulk organic solvents. Furthermore, a scaled-up synthesis can be conducted without any yield losses. These results suggest that the present mechanochemical approach offers a more convenient, economically attractive, and sustainable alternative to previously established Birch reduction protocols.

Solid-state mechanochemistry for the rapid and efficient synthesis of tris-cyclometalated iridium(iii) complexes

Tris-cyclometalated iridium(iii) complexes have received widespread attention as attractive prospective materials for e.g., organic light-emitting diodes (OLEDs), photoredox catalysts, and bioimaging probes. However, their preparation usually requires prolonged reaction times, significant amounts of high-boiling solvents, multistep synthesis, and inert-gas-line techniques. Unfortunately, these requirements represent major drawbacks from both a production-cost and an environmental perspective. Herein, we show that a two-step mechanochemical protocol using ball milling enables the rapid and efficient synthesis of various tris-cyclometalated iridium(iii) complexes from relatively cheap iridium(iii) chloride hydrate without the use of significant amounts of organic solvent in air. Notably, a direct one-pot procedure is also demonstrated. The present solid-state approach can be expected to inspire the development of cost-effective and timely production methods for these valuable iridium-based complexes, as well as the discovery of new phosphorescent materials, sensors, and catalysts.

Rhodium(II)-Catalyzed N-H Insertions of Carbenes under Mechanochemical Conditions

Under mechanochemical conditions in a mixer mill, Rh2(OAc)4 catalyzes the reaction between aryldiazoesters and anilines to give α-amino esters. The process proceeds under mild conditions and is insensitive to air. It is solvent-free and scalable. A broad substrate scope, short reaction times, operational simplicity, and good functional group tolerance are additional salient features of this protocol.

Direct Synthesis of Benzoselenophene and Benzothiophene Derivatives from 1,1-Diarylethenes and Biaryls by Chalcogen Cation-Mediated Successive Bond Formation

A Tf2O-mediated sequential C-Se bond-forming reaction of 1,1-diarylethenes and biaryls with methaneselenic acid has been developed. Upon demethylation workup with ethanolamine, the corresponding benzoselenophene derivatives are obtained directly. The related synthesis of benzothiophene derivatives with sodium methanesulfinate is also possible with the unique assistance of the ball milling technique. The active species is considered to be a highly electrophilic chalcogen cation, which enables successive bond formation even at room temperature.

Mechanochemical synthesis of organoselenium compounds

We disclose herein a strategy for the rapid synthesis of versatile organoselenium compounds under mild conditions. In this work, magnesium-based selenium nucleophiles are formed in situ from easily available organic halides, magnesium metal, and elemental selenium via mechanical stimulation. This process occurs under liquid-assisted grinding (LAG) conditions, requires no complicated pre-activation procedures, and operates broadly across a diverse range of aryl, heteroaryl, and alkyl substrates. In this work, symmetrical diselenides are efficiently obtained after work-up in the air, while one-pot nucleophilic addition reactions with various electrophiles allow the comprehensive synthesis of unsymmetrical monoselenides with high functional group tolerance. Notably, the method is applied to regioselective selenylation reactions of diiodoarenes and polyaromatic aryl halides that are difficult to operate via solution approaches. Besides selenium, elemental sulfur and tellurium are also competent in this process, which showcases the potential of the methodology for the facile synthesis of organochalcogen compounds.

Kinetic Phenomena in Mechanochemical Depolymerization of Poly(styrene)

Synthetic polyolefinic plastics comprise one of the largest shares of global plastic waste, which is being targeted for chemical recycling by depolymerization to monomers and small molecules. One promising method of chemical recycling is solid-state depolymerization under ambient conditions in a ball-mill reactor. In this paper, we elucidate kinetic phenomena in the mechanochemical depolymerization of poly(styrene). Styrene is produced in this process at a constant rate and selectivity alongside minor products, including oxygenates like benzaldehyde, via mechanisms analogous to those involved in thermal and oxidative pyrolysis. Continuous monomer removal during reactor operation is critical for avoiding repolymerization, and promoting effects are exhibited by iron surfaces and molecular oxygen. Kinetic independence between depolymerization and molecular weight reduction was observed, despite both processes originating from the same driving force of mechanochemical collisions. Phenomena across multiple length scales are shown to be responsible for differences in reactivity due to differences in grinding parameters and reactant composition.

Mechanochemical Scholl Reaction on Phenylated Cyclopentadiene Core: One-Step Synthesis of Fluoreno[5]helicenes

In this study, we explore feasibility of the mechanochemical approach in the synthesis of tetrabenzofluorenes (fluoreno[5]helicenes). For this, commercially available phenylated cyclopentadiene precursors are subjected to the Scholl reaction in the solid state using FeCl3 as an oxidant and sodium chloride as the solid reaction medium. This ball milling process gave access to the 5-membered ring containing-helicenes in one synthetic step in high (95-96 %) isolated yields. The solution-phase reactions, however, were found to be moderate to low yielding in this regard (10-40 %).

From non-conductive MOF to proton-conducting metal-HOFs: a new class of reversible transformations induced by solvent-free mechanochemistry

Proton-conducting materials play an important role as solid electrolytes in electrochemical devices for energy storage and conversion, including proton exchange membrane fuel cells. Metal-organic frameworks (MOFs), covalent-organic frameworks (COFs) and more recently hydrogen-bonded organic frameworks (HOFs) have emerged as useful crystalline platforms for proton transport that provide high conductivity and enable insight into conduction pathways. Here, we present two new HOFs with high conductivity, reaching 2 × 10-2 S cm-1 at 60 °C and 75% relative humidity, obtained in reactions that represent a new class of reversible transformations of solids. The reactions are induced by solvent-free mechanochemistry and involve breaking of coordination linkages in a MOF and formation of extended hydrogen-bonded networks of metal-HOFs (MHOFs). This unprecedented class of MOF-to-MHOF transformations has been demonstrated using a non-conductive MOF (JUK-1) and formamidinium or methylammonium thiocyanates as solid reactants. Structural details of the solid-state reactions are revealed by powder X-ray diffraction and Rietveld refinements for the MHOF products. None of the attempts using conventional methods were successful in obtaining the MHOFs, emphasizing a unique role of mechanochemical stimuli in the reactivity of supramolecular polymer solids, including crystalline MOFs and HOFs. The reversible nature of non-covalent interactions in such materials may be utilized for the development of healable polymer systems.

Mechanochemical Iron-Catalyzed Nitrene Transfer Reactions: Direct Synthesis of N-Acyl Sulfonimidamides from Sulfinamides and Dioxazolones

A mechanochemical synthesis of sulfonimidamides by iron(II)-catalyzed exogenous ligand-free N-acyl nitrene transfer to sulfinamides is reported. The one-step method tolerates a wide range of sulfinamides with various substituents under solvent-free ambient conditions. Compared to its solution-phase counterpart, this mechanochemical approach shows better conversion and chemoselectivity. Mechanistic investigations by ESI-MS revealed the generation of crucial nitrene iron intermediates.

Opportunities and Challenges in Applying Solid-State NMR Spectroscopy in Organic Mechanochemistry

In recent years it is shown that mechanochemical strategies can be beneficial in directed conversions of organic compounds. Finding new reactions proved difficult, and due to the lack of mechanistic understanding of mechanochemical reaction events, respective efforts have mostly remained empirical. Spectroscopic techniques are crucial in shedding light on these questions. In this overview, the opportunities and challenges of solid-state nuclear magnetic resonance (NMR) spectroscopy in the field of organic mechanochemistry are discussed. After a brief discussion of the basics of high-resolution solid-state NMR under magic-angle spinning (MAS) conditions, seven opportunities for solid-state NMR in the field of organic mechanochemistry are presented, ranging from ex situ approaches to structurally elucidated reaction products obtained by milling to the potential and limitations of in situ solid-state NMR approaches. Particular strengths of solid-state NMR, for instance in differentiating polymorphs, in NMR-crystallographic structure-determination protocols, or in detecting weak noncovalent interactions in molecular-recognition events employing proton-detected solid-state NMR experiments at fast MAS frequencies, are discussed.

Mechanochemical Pd-Catalyzed Amino- and Oxycarbonylations using FeBr2(CO)4 as a CO Source

Herein, we describe the development of mechanochemical amino- and oxycarbonylation employing FeBr2(CO)4 as a solid CO source. This Pd/XantPhos-catalyzed reaction affords a range of carboxamides and esters from aryl iodides and various amines or phenols. Both primary and secondary amines, including amino acids, can be employed as N-nucleophiles.

Tinkering with Mechanochemical Tools for Scale Up

Mechanochemistry provides an environmentally benign platform to develop more sustainable chemical processes by limiting raw materials, energy use, and waste generation while using physically smaller equipment. A continuously growing research community has steadily showcased examples of beneficial mechanochemistry applications at both the laboratory and the preparative scale. In contrast to solution-based chemistry, mechanochemical processes have not yet been standardized, and thus scaling up is still a nascent discipline. The purpose of this Minireview is to highlight similarities, differences and challenges of the various approaches that have been successfully applied for a range of chemical applications at various scales. We hope to provide a discussion starting point for those interested in further developing mechanochemical processes for commercial use and/or industrialisation.

Mechanochemical Synthesis of Corannulene: Scalable and Efficient Preparation of A Curved Polycyclic Aromatic Hydrocarbon under Ball Milling Conditions

Corannulene, a curved polycyclic aromatic hydrocarbon, is prepared in a multigram scale through mechanochemical synthesis. Initially, a mixer mill approach is examined and found to be suitable for a gram scale synthesis. For larger scales, planetary mills are used. For instance, 15 g of corannulene could be obtained in a single milling cycle with an isolated yield of 90 %. The yields are lower when the jar rotation rate is lower or higher than 400 revolutions per minute (rpm). Cumulatively, 98 g of corannulene is produced through the ball milling-based grinding techniques. These results indicate the future potential of mechanochemistry in the rational chemical synthesis of highly curved nanocarbons such as fullerenes and carbon nanotubes.

Dual Nickel(II)/Mechanoredox Catalysis: Mechanical-Force-Driven Aryl-Amination Reactions Using Ball Milling and Piezoelectric Materials

The combination of a nickel(II) catalyst and a mechanoredox catalyst under ball-milling conditions promotes mechanical-force-driven C-N cross-coupling reactions. In this nickel(II)/mechanoredox cocatalyst system, the modulation of the oxidation state of the nickel center, induced by piezoelectricity, is used to facilitate a highly efficient aryl-amination reaction, which is characterized by a broad substrate scope, an inexpensive combination of catalysts (NiBr2 and BaTiO3 ), short reaction times, and an almost negligible quantity of solvents. Moreover, this reaction can be readily up-scaled to the multi-gram scale, and all synthetic operations can be carried out under atmospheric conditions without the need for complicated reaction setups. Furthermore, this force-induced system is suitable for excitation-energy-accepting molecules and poorly soluble polyaromatic substrates that are incompatible with solution-based nickel(II)/photoredox cocatalysts.

Solid-state mechanochemical cross-coupling of insoluble substrates into insoluble products by removable solubilizing silyl groups: uniform synthesis of nonsubstituted linear oligothiophenes

Conventional solution-based organic reactions that involve insoluble substrates are challenging and inefficient. Furthermore, even if the reaction is successful, the corresponding products are insoluble in most cases, making their isolation and subsequent transformations difficult. Hence, the conversion of insoluble compounds into insoluble products remains a challenge in practical synthetic chemistry. In this study, we showcase a potential solution to address these solubility issues by combining a mechanochemical cross-coupling approach with removable solubilizing silyl groups. Our strategy involves solid-state Suzuki-Miyaura cross-coupling reactions between organoboron nucleophiles bearing a silyl group with long alkyl chains and insoluble polyaromatic halides. The silyl group on the nucleophile can act as a solubilizing group that enables product isolation via silica gel column chromatography and can be easily removed by the addition of fluoride anions to form the desired insoluble coupling products with sufficient purity. Furthermore, we demonstrate that after aromatic electrophilic bromination of the desilylated products, sequential solid-state cross-coupling of the obtained insoluble brominated substrates, followed by desilylation, afforded further π-extended functional molecules. Using this conceptually new protocol, we achieved the first uniform synthesis of the longest nonsubstituted linear insoluble 9-mer oligothiophene.

Mechanochemical Synthesis of Aryl Fluorides by Using Ball Milling and a Piezoelectric Material as the Redox Catalyst

Aryl fluorides are important structural motifs in many pharmaceuticals. Although the Balz-Schiemann reaction provides an entry to aryl fluorides from aryldiazonium tetrafluoroborates, it suffers from drawbacks such as long reaction time, high temperature, toxic solvent, toxic gas release, and low functional group tolerance. Here, we describe a general method for the synthesis of aryl fluorides from aryldiazonium tetrafluoroborates using a piezoelectric material as redox catalyst under ball milling conditions in the presence of Selectfluor. This approach effectively addresses the aforementioned limitations. Furthermore, the piezoelectric material can be recycled multiple times. Mechanistic investigations indicate that this fluorination reaction may proceed via a radical pathway, and Selectfluor plays a dual role as both a source of fluorine and a terminal reductant.

Mechanochemical Synthesis of Stimuli Responsive Microgels

Mechanochemical approaches are widely used for the efficient, solvent-free synthesis of organic molecules, however their applicability to the synthesis of functional polymers has remained underexplored. Herein, we demonstrate for the first time that mechanochemically triggered free-radical polymerization allows solvent- and initiator-free syntheses of structurally and morphologically well-defined complex functional macromolecular architectures, namely stimuliresponsive microgels. The developed mechanochemical polymerization approach is applicable to a variety of monomers and allows synthesizing microgels with tunable chemical structure, variable size, controlled number of crosslinks and reactive functional end-groups.

Direct mechanocatalysis by resonant acoustic mixing (RAM)

We demonstrate the use of a metal surface to directly catalyse copper-catalysed alkyne-azide click-coupling (CuAAC) reactions under the conditions of Resonant Acoustic Mixing (RAM) - a recently introduced and scalable mechanochemical methodology that uniquely eliminates the need for bulk solvent, as well as milling media. By using a simple copper coil as a catalyst, this work shows that direct mechanocatalysis can occur in an impact-free environment, relying solely on high-speed mixing of reagents against a metal surface, without the need for specially designed milling containers and media. By introducing an experimental setup that enables real-time Raman spectroscopy monitoring of RAM processes, we demonstrate 0th-order reaction kinetics for several selected CuAAC reactions, supporting surface-based catalysis. The herein presented RAM-based direct mechanocatalysis methodology is simple, enables the effective one-pot, two-step synthesis of triazoles via a combination of benzyl azide formation and CuAAC reactions on a wide scope of reagents, provides control over reaction stoichiometry that is herein shown to be superior to that seen in solution or by using more conventional CuCl catalyst, and is applied for simple gram-scale synthesis of the anticonvulsant drug Rufinamide.

Electrochemically Versatile Graphite Nanoplatelets Prepared by a Straightforward, Highly Efficient, and Scalable Route

Nanostructured carbon materials with tailor-made structures (e.g., morphology, topological defect, dopant, and surface area) are of significant interest for a variety of applications. However, the preparation method selected for obtaining these tailor-made structures determines the area of application, precluding their use in other technological areas of interest. Currently, there is a lack of simple and low-cost methodologies versatile enough for obtaining freestanding carbon nanostructures that can be used in either energy storage or chemical detection. Here, a novel methodology for the development of a versatile electrochemically active platform based on freestanding graphite nanoplatelets (GNP) has been developed by exploiting the interiors of hollow carbon nanofibers (CNF) comprising nanographene stacks using dry ball-milling. Even though ball-milling could be considered as a universal method for any carbonaceous material, often, it is not as simple (one step, no purification, and no solvents), efficient (just GNP without tubular structures), and quick (just 20 min) as the sustainable method developed in this work, free of surfactants and stabilizer agents. We demonstrate that the freestanding GNP developed in this work (with an average thickness of 3.2 nm), due to the selective edge functionalization with the minimal disruption of the basal plane, can act either as a supercapacitor or as a chemical sensor, showing both a dramatic improvement in the charge storage ability of more than 30 times and an enhanced detection of electrochemically active molecules such as ascorbic acid with a 236 mV potential shift with respect to CNF in both cases. As shown here, GNP stand as an excellent versatile alternative compared to the standard commercially available carbon-based materials. Overall, our approach paves the way for the discovery of new nanocarbon-based electrochemical active platforms with a wide electrochemical applicability.

Customized oxygen-rich biochar with ultrahigh microporosity for ideal solid phase microextraction of substituted benzenes

The synergistic effect of high microporosity and abundant heteroatoms is important for improving the performance of biochar in various fields. However, it is still challenging to create enough micropores for biochar, while simultaneously retaining the heteroatoms from biomass. A series of biochar with variable microstructures was successfully prepared by carbonization and following ball milling on lotus pedicel (LP), watermelon rind (WR), and litchi rind (LR). The pore structures and heteroatoms of biochar were characterized in detail. Notably, high microporosity could be realized by the carbonization of LR, and further ball milling resulted in a higher microporous surface area (1323.4 m²·g^-1) and richer oxygen. Furthermore, the obtained biochar was fabricated as solid phase microextraction (SPME) coatings with uniform morphologies and similar thicknesses to deeply investigate the relationships between the microstructures and extraction performance. The best performance was demonstrated by the LR800BM, with enrichment factors from 1780 to 155,217. Finally, it was coupled with gas chromatography-mass spectrometry (GC-MS) to develop an analytical method with a wide linear range (1-50,000 ng·L^-1), low limits of detection (0.10-1.4 ng·L^-1), good repeatability (0.83 %-7.5 %) and reproducibility (4.2 %-8.9 %). This work provides valuable insights into the structure-performance relationship of biochar, which is important for the design of high-performance biochar-based adsorbents and their applications in the environment.

Lithium-Mediated Mechanochemical Cyclodehydrogenation

Cyclodehydrogenation is an essential synthetic method for the preparation of polycyclic aromatic hydrocarbons, polycyclic heteroaromatic compounds, and nanographenes. Among the many examples, anionic cyclodehydrogenation using potassium(0) has attracted synthetic chemists because of its irreplaceable reactivity and utility in obtaining rylene structures from binaphthyl derivatives. However, existing methods are difficult to use in terms of practicality, pyrophoricity, and lack of scalability and applicability. Herein, we report the development of a lithium(0)-mediated mechanochemical anionic cyclodehydrogenation reaction for the first time. This reaction could be easily performed using a conventional and easy-to-handle lithium(0) wire at room temperature, even under air, and the reaction of 1,1'-binaphthyl is complete within 30 min to afford perylene in 94% yield. Using this novel and user-friendly protocol, we investigated substrate scope, reaction mechanism, and gram-scale synthesis. As a result, remarkable applicability and practicality over previous methods, as well as limitations, were comprehensively studied by computational studies and nuclear magnetic resonance analysis. Furthermore, we demonstrated two-, three-, and five-fold cyclodehydrogenations for the synthesis of novel nanographenes. In particular, quinterrylene ([5]rylene or pentarylene), the longest nonsubstituted molecular rylene, was synthesized for the first time.

Mechanochemistry-Directed Ligand Design: Development of a High-Performance Phosphine Ligand for Palladium-Catalyzed Mechanochemical Organoboron Cross-Coupling

Mechanochemical synthesis that uses transition-metal catalysts has attracted significant attention due to its numerous advantages, including low solvent waste, short reaction times, and the avoidance of problems associated with the low solubility of starting materials. However, even though the mechanochemical reaction environment is largely different from that of homogeneous solution systems, transition-metal catalysts, which were originally developed for use in solution, have been used directly in mechanochemical reactions without any molecular-level modifications to ensure their suitability for mechanochemistry. Alas, this has limited the development of more efficient mechanochemical cross-coupling processes. Here, we report a conceptually distinct approach, whereby a mechanochemistry-directed design is used to develop ligands for mechanochemical Suzuki-Miyaura cross-coupling reactions. The ligand development was guided by the experimental observation of catalyst deactivation via the aggregation of palladium species, a problem that is particularly prominent in solid-state reactions. By embedding the ligand into a poly(ethylene glycol) (PEG) polymer, we found that phosphine-ligated palladium(0) species could be immobilized in the fluid phase created by the PEG chains, preventing the physical mixing of the catalyst into the crystalline solid phase and thus undesired catalyst deactivation. This catalytic system showed high catalytic activity in reactions of polyaromatic substrates close to room temperature. These substrates usually require elevated temperatures to be reactive in the presence of catalyst systems with conventional ligands such as SPhos. The present study hence provides important insights for the design of high-performance catalysts for solid-state reactions and has the potential to inspire the development of industrially attractive, almost solvent-free mechanochemical cross-coupling technologies.

A green protocol ball milling synthesis of dihydropyrano[2,3-c]pyrazole using nano-silica/aminoethylpiperazine as a metal-free catalyst

BACKGROUND

Ball mill is an effective, and green method for the synthesis of heterocyclic compounds in very good yields. This method is a simple, economical, and environmentally friendly process. In this work, an efficient approach for the synthesis of pyranopyrazoles (PPzs) using ball milling and metal-free nano-catalyst (Nano-silica/aminoethylpiperazine), under solvent-free conditions was reported.

RESULTS

The new nano-catalyst silica/aminoethylpiperazine was prepared by immobilization of 1-(2-aminoethyl)piperazine on nano-silica chloride. The structure of the prepared nano-catalyst was identified by FT-IR, FESEM, TGA, EDX, EDS-map, XRD, and pH techniques. This novel nano-catalyst was used for the synthesis of dihydropyrano[2,3-c]pyrazole derivatives under ball milling and solvent-free conditions.

CONCLUSIONS

Unlike other pyranopyrazoles synthesis reactions, this method has advantages including short reaction time (5-20 min), room temperature, and relatively high efficiency, which makes this protocol very attractive for the synthesis of pyranopyrazoles derivatives.

Disentangling the Effect of Pressure and Mixing on a Mechanochemical Bromination Reaction by Solid-State NMR Spectroscopy

Mechanical forces, including compressive stresses, have a significant impact on chemical reactions. Besides the preparative opportunities, mechanochemical conditions benefit from the absence of any organic solvent, the possibility of a significant synthetic acceleration and unique reaction pathways. Together with an accurate characterization of ball-milling products, the development of a deeper mechanistic understanding of the occurring transformations at a molecular level is critical for fully grasping the potential of organic mechanosynthesis. We herein studied a bromination of a cyclic sulfoximine in a mixer mill and used solid-state nuclear magnetic resonance (NMR) spectroscopy for structural characterization of the reaction products. Magic-angle spinning (MAS) was applied for elucidating the product mixtures taken from the milling jar without introducing any further post-processing on the sample. Ex situ ¹³ C-detected NMR spectra of ball-milling products showed the formation of a crystalline solid phase with the regioselective bromination of the S-aryl group of the heterocycle in position 4. Completion is reached in less than 30 minutes as deduced from the NMR spectra. The bromination can also be achieved by magnetic stirring, but then, a longer reaction time is required. Mixing the solid educts in the NMR rotor allows to get in situ insights into the reaction and enables the detection of a reaction intermediate. The pressure alone induced in the rotor by MAS is not sufficient to lead to full conversion and the reaction occurs on slower time scales than in the ball mill, which is crucial for analysing mixtures taken from the milling jar by solid-state NMR. Our data suggest that on top of centrifugal forces, an efficient mixing of the starting materials is required for reaching a complete reaction.

Recent applications of mechanochemistry in synthetic organic chemistry

The promotion of chemical reactions by an unconventional energy source, mechanical energy (mechanochemistry) has increasing number of applications in organic synthesis. The advantages of mechanochemistry are versatile, from reduction of solvent use, increase of reaction efficiency to better environmental sustainability. This paper gives a short review on the recent developments in the fast growing field of organic mechanochemistry which are illustrated by selected examples.

Mechanosynthesis of Triazolyl-bis(indolyl)methane Pharmacophores via Gold Catalysis: A Prelude to Their Molecular Electronic Properties and Biological Potency

Chemical structures possessing both 1,2,3-triazole and bis(indolyl)methane fragments gained considerable interest in drug synthesis owing to their established biological efficacies. However, 1,2,3-triazoles linked at the bridging position of bis(indolyl)methane is a logical and unexplored design approach. In this regard, nine new triazolyl-bis(indolyl)methane conjugates under AuCl catalyzed ball-milling conditions were accomplished. Comparative evaluation on absorptive and emissive properties of the synthesized dyads were also analyzed. To unravel the influence of different peripheral substituents on the electronic structure and π-orbital properties, theoretical investigations were performed. Screening of molecules for free radical scavenging, anti-inflammatory and antidiabetic showed comparable potency against reference drugs. In particular, compounds 7 h, 7 d and 7 a displayed good efficiency of α-amylase inhibition. The DNA gyrase inhibitory potential of all compounds were assessed in silico which revealed high binding affinity (ΔG=-8.99 Kcal/mol) for 7 i followed by 7 h (ΔG=-7.80 Kcal/mol) with the targeted protein.

Mechanochemical protocol facilitates the generation of arylmanganese nucleophiles from unactivated manganese metal

The direct synthesis of organomanganese reagents from organic halides and manganese metal remains a challenge. Current solution-based approaches require the preparation of activated manganese (Rieke manganese) or the use of multiple metal additives to promote the insertion of manganese metal into a carbon-halogen bond. Here, we show that a mechanochemical ball-milling protocol facilitates the generation of various arylmanganese nucleophiles from aryl halides and commercially available, unactivated manganese metal without the need for complicated pre-activation processes and metal additives. These manganese-based carbon nucleophiles can be used directly for one-pot addition reactions with various electrophiles and palladium-catalyzed cross-coupling reactions under bulk-solvent-free mechanochemical conditions. Importantly, all experimental operations can be conducted under atmospheric conditions.

Shaking Things from the Ground-Up: A Systematic Overview of the Mechanochemistry of Hard and High-Melting Inorganic Materials

We provide a systematic overview of the mechanochemical reactions of inorganic solids, notably simple binary compounds, such as oxides, nitrides, carbides, sulphides, phosphides, hydrides, borides, borane derivatives, and related systems. Whereas the solid state has been traditionally considered to be of little synthetic value by the broader community of synthetic chemists, the solid-state community, and in particular researchers focusing on the reactions of inorganic materials, have thrived in building a rich and dynamic research field based on mechanically-driven transformations of inorganic substances typically seen as inert and high-melting. This review provides an insight into the chemical richness of such mechanochemical reactions and, at the same time, offers their tentative categorisation based on transformation type, resulting in seven distinct groupings: (i) the formation of adducts, (ii) the reactions of dehydration; (iii) oxidation-reduction (redox) reactions; (iv) metathesis (or exchange) reactions; (v) doping and structural rearrangements, including reactions involving the reaction vessel (the milling jar); (vi) acid-base reactions, and (vii) other, mixed type reactions. At the same time, we offer a parallel description of inorganic mechanochemical reactions depending on the reaction conditions, as those that: (i) take place under mild conditions (e.g., manual grinding using a mortar and a pestle); (ii) proceed gradually under mechanical milling; (iii) are self-sustained and initiated by mechanical milling, i.e., mechanically induced self-propagating reactions (MSRs); and (iv) proceed only via harsh grinding and are a result of chemical reactivity under strongly non-equilibrium conditions. By elaborating on typical examples and general principles in the mechanochemistry of hard and high-melting substances, this review provides a suitable complement to the existing literature, focusing on the properties and mechanochemical reactions of inorganic solids, such as nanomaterials and catalysts.

Reshuffle Bonds by Ball Milling: A Mechanochemical Protocol for Charge-Accelerated Aza-Claisen Rearrangements

This study presents the development of a mechanochemical protocol for a charge-accelerated aza-Claisen rearrangement. The protocol waives the use of commonly applied transition metals, ligands, or pyrophoric Lewis acids, e.g., AlMe₃. Based on (heterocyclic) tertiary allylamines and acyl chlorides, the desired tertiary amides were prepared in yields ranging from 17% to 84%. Moreover, the same protocol was applied for a Belluš-Claisen-type rearrangement resulting in the synthesis of a 9-membered lactam without further optimization.

Electro-mechanochemical approach towards the chloro sulfoximidations of allenes under solvent-free conditions in a ball mill

An electro-mechanochemical protocol for the synthesis of vinylic sulfoximines has been developed. Utilising mechanochemically strained BaTiO₃ nanoparticles, the catalytic active system is generated in situ by the reduction of copper(II) chloride. Various combinations of electron-donating and -withdrawing groups are tolerated, and the approach leads to products with difunctionalised double bonds in good to excellent yields. Attempts to add a sulfoximidoyl chloride to an alkyne proved difficult. Additions of a sulfonyl iodide to allenes and alkynes proceeded smoothly in the presence of silica gel without the need for activation by a piezoelectric material.

Spiers Memorial Lecture: Mechanochemistry, tribochemistry, mechanical alloying - retrospect, achievements and challenges

The paper presents a view on the achievements, challenges and prospects of mechanochemistry. The extensive reference list can serve as a good entry point to a plethora of mechanochemical literature.

Solid-state cross-coupling reactions of insoluble aryl halides under polymer-assisted grinding conditions

In this study, polymer-assisted grinding (POLAG), a ball-milling technique based on the use of polymer additives, was applied to mechanochemical solid-state Suzuki-Miyaura cross-coupling reactions of insoluble aryl halides. We found that the efficiency of this challenging solid-state cross-coupling was improved by the addition of polytetrafluoroethylene (PTFE) as a POLAG additive under high-temperature ball-milling conditions. Our results suggest that POLAG is a promising approach for controlling the reactivity of insoluble substrates that are barely reactive under conventional solution-based conditions.

The "η-sweet-spot" (ηmax) in liquid-assisted mechanochemistry: polymorph control and the role of a liquid additive as either a catalyst or an inhibitor in resonant acoustic mixing (RAM)

Resonant acoustic mixing (RAM) offers a simple, efficient route for mechanochemical synthesis in the absence of milling media or bulk solvents. Here, we show the use of RAM to conduct the copper-catalysed coupling of sulfonamides and carbodiimides. This coupling was previously reported to take place only by mechanochemical ball milling, while in conventional solution environments it is not efficient, or does not take place at all. The results demonstrate RAM as a suitable methodology to conduct reactions previously accessed only by ball milling and provide a detailed, systematic overview of how the amount of liquid additive, measured by the ratio of liquid volume to weight of reactants (η, in μL mg^-1), can affect the course of a mechanochemical reaction and the polymorphic composition of its product. Switching from ball milling to RAM allowed for the discovery of a new polymorph of the model sulfonylguanidine obtained by catalytic coupling of di(cyclohexyl)carbodiimide (DCC) and p-toluenesulfonamide, and the ability to control reaction temperature in RAM enabled in situ control of the polymorphic behaviour of this nascent product. We show that the reaction conversion for a given reaction time does not change monotonically but, instead, achieves a maximum for a well-defined η-value. This "η-sweet-spot" of conversion is herein designated η_max. The herein explored reactions demonstrate sensitivity to η on the order of 0.01 μL mg^-1, which corresponds to an amount of liquid additive below 5 mol% compared to the reactants, and is at least one to two orders of magnitude lower than the η-value typically considered in the design of liquid-assisted ball milling mechanochemical reactions. Such sensitivity suggests that strategies to optimise liquid-assisted mechanochemical reactions should systematically evaluate η-values at increments of 0.01 μL mg^-1, or even finer. At η-values other than η_max the reaction conversion drops off, demonstrating that the same liquid additive can act either as a catalyst or an inhibitor of a mechanochemical reaction, depending on the amount.

Polymorphs and solid solutions: materials with new luminescent properties obtained through mechanochemical transformation of dicyanoaurate(I) salts

We report the use of mechano- and thermochemical methods to create new solid-state luminescent materials from well-known inorganic salts, potassium dicyanoaurate(I) KAu(CN)₂, and potassium dicyanocuprate(I) KCu(CN)₂. In particular, manual grinding or ball milling of commercial samples of KAu(CN)₂ led to the formation of a novel polymorph of the salt, herein termed m-KAu(CN)₂, evident by a significant change in color of the fluorescence emission of the solid material from orange to violet. The formation of m-KAu(CN)₂ is reversible upon addition of small amounts of solvents, and powder X-ray diffraction analysis indicates that the structure of m-KAu(CN)₂ might be related to that of pristine KAu(CN)₂ through a change in ordering of Au(CN)₂^- ions in a layered structure. Thermal treatment of KAu(CN)₂ led to the discovery of another polymorph of this well-known salt, herein termed t-KAu(CN)₂, making KAu(CN)₂ a rare example of a system in which thermochemical and mechanochemical treatments lead to the formation of different, in each case previously not reported, polymorphic forms. The thermally-induced transformation from KAu(CN)₂ to t-KAu(CN)₂ takes place around 250 °C and proceeds in a crystal-to-crystal fashion, which enabled the preliminary structural characterisation through single crystal X-ray diffraction, revealing the retention of the layered structure and a change in ordering of Au(CN)₂^- ions. Milling of the simple salt KAu(CN)₂ in the presence of equimolar amounts or less of its copper(I)-based analogue coordination polymer KCu(CN)₂ leads to the formation of a series of solid solution materials, isostructural to m-KAu(CN)₂ and with visible fluorescence emission distinct from KCu(CN)₂ or any herein investigated forms of KAu(CN)₂.

Advancing mechanochemical synthesis by combining milling with different energy sources

Owing to its efficiency and unique reactivity, mechanochemical processing of bulk solids has developed into a powerful tool for the synthesis and transformation of various classes of materials. Nevertheless, mechanochemistry is primarily based on simple techniques, such as milling in comminution devices. Recently, mechanochemical reactivity has started being combined with other energy sources commonly used in solution-based chemistry. Milling under controlled temperature, light irradiation, sound agitation or electrical impulses in newly developed experimental setups has led to reactions not achievable by conventional mechanochemical processing. This Perspective describes these unique reactivities and the advances in equipment tailored to synthetic mechanochemistry. These techniques - thermo-mechanochemistry, sono-mechanochemistry, electro-mechanochemistry and photo-mechanochemistry - represent a notable advance in modern mechanochemistry and herald a new level of solid-state reactivity: mechanochemistry 2.0.

Mechanochemical Approach towards Multi-Functionalized 1,2,3-Triazoles and Anti-Seizure Drug Rufinamide Analogs Using Copper Beads

Highly regiospecific, copper-salt-free and neat conditions have been demonstrated for the 1,3-dipolar azide-alkyne cycloaddition (AAC) reactions under mechanochemical conditions. A group of structurally challenging alkynes and heterocyclic derivatives was efficiently implemented to achieve highly functionalized 1,4-disubstituted-1,2,3-triazoles in good to excellent yield by using the Cu beads without generation of unwanted byproducts. Furthermore, the high-speed ball milling (HSBM) strategy has also been extended to the synthesis of the commercially available pharmaceutical agent, Rufinamide, an antiepileptic drug (AED) and its analogues. The same strategy was also applied for the synthesis of the Cl-derivative of Rufinamide. Analysis of the single crystal XRD data of the triazole was also performed for the final structural confirmation. The Cu beads are easily recoverable from the reaction mixture and used for the further reactions without any special treatment.

Mechanochemical synthesis of unsymmetrical salens for the preparation of Co–salen complexes and their evaluation as catalysts for the synthesis of α-aryloxy alcohols via asymmetric phenolic kinetic resolution of terminal epoxides

In this paper, we report the mechanochemical synthesis of unsymmetrical salens using grinding and ball milling technologies, respectively, both of which were afforded in good yield. The chelating effect of the unsymmetrical salens with zinc, copper, and cobalt was studied and the chiral Co–salen complex 2f was obtained in 98% yield. Hydrolytic kinetic resolution (HKR) of epichlorohydrin with water catalyzed by complex 2f (0.5 mol %) was explored and resulted in 98% ee, suggesting complex 2f could serve as an enantioselective catalyst for the asymmetric ring opening of terminal epoxides by phenols. A library of α-aryloxy alcohols 3 was thereafter synthesized in good yield and high ee using 2f via the phenolic KR of epichlorohydrin.

A greener approach towards the synthesis of N-heterocyclic thiones and selones using the mechanochemical technique

This manuscript describes the synthesis of N-heterocyclic thiones and selones of a variety of imidazolium salts involving an eco-friendly and solventless ball-milling technique. The products have been isolated in almost quantitative yield, involving a minimum quantity of solvents only for the isolation of products by column chromatography, and in some cases for purification purposes. Both mono- and bisimidazolium salts afforded N-heterocyclic thiones and selones. The methodology is found to be superior in terms of reaction time, yield and energy efficiency as compared to conventional solution-state reactions.

Enabling Sustainable Chemistry with Ionic Liquids and Deep Eutectic Solvents: A Fad or the Future?

Ionic liquids (ILs) and deep eutectic solvents (DESs) debuted with a promise of a superior sustainability footprint due to their low vapor pressure. However, their toxicity and high cost compromise this footprint, impeding their real-world applications. Fortunately, their property tunability through a rational selection of precursors, including bioderived ones, provides a strategy to ameliorate toxicity, lower cost, and endow new functions. This Review discusses whether ILs and DESs are sustainable solvents and how they contribute to sustainable chemical processes.