Force-Triggered Self-Destructive Hydrogels

Self-destructive polymers (SDPs) are defined as a class of smart polymers that autonomously degrade upon experiencing an external trigger, such as a chemical cue or optical excitation. Because SDPs release the materials trapped inside the network upon degradation, they have potential applications in drug delivery and analytical sensing. However, no known SDPs that respond to external mechanical forces have been reported, as it is fundamentally challenging to create mechano-sensitivity in general and especially so for force levels below those required for classical force-induced bond scission. To address this challenge, the development of force-triggered SDPs composed of DNA crosslinked hydrogels doped with nucleases is described here. Externally applied piconewton forces selectively expose enzymatic cleavage sites within the DNA crosslinks, resulting in rapid polymer self-degradation. The synthesis and the chemical and mechanical characterization of DNA crosslinked hydrogels, as well as the kinetics of force-triggered hydrolysis, are described. As a proof-of-concept, force-triggered and time-dependent rheological changes in the polymer as well as encapsulated nanoparticle release are demonstrated. Finally, that the kinetics of self-destruction are shown to be tuned as a function of nuclease concentration, incubation time, and thermodynamic stability of DNA crosslinkers.

Biasing the Formation of Solution-Unstable Intermediates in Coordination Self-Assembly by Mechanochemistry

Due to the reversible nature of coordination bonds and solvation effect, coordination self-assembly pathways are often difficult to elucidate experimentally in solution, as intermediates and products are in constant equilibration. The present study shows that some of these transient and high-energy self-assembly intermediates can be accessed by means of ball-milling approaches. Among them, highly aqueous-unstable Pd3 L11 and Pd6 L14 open-cage intermediates of the framed Fujita Pd6 L14 cage and Pd2 L22 , Pd3 L21 and Pd4 L22 intermediates of Mukherjee Pd6 L24 capsule are successfully trapped in solid-state, where Pd=tmedaPd2+ , L1=2,4,6-tris(4-pyridyl)-1,3,5-triazine and L2=1,3,5-tris(1-imidazolyl)benzene). Their structures are assigned by a combination of solution-based characterization tools such as standard NMR spectroscopy, DOSY NMR, ESI-MS and X-ray diffraction. Collectively, these results highlight the opportunity of using mechanochemistry to access unique chemical space with vastly different reactivity compared to conventional solution-based supramolecular self-assembly reactions.

Synthesis without solvent: consequences for mechanochemical reactivity

Solvents are so nearly omnipresent in synthetic chemistry that a classic question for their use has been: "What is the best solvent for this reaction?" The increasing use of mechanochemical approaches to synthesis-by grinding, milling, extrusion, or other means-and usually with no, or only limited, amounts of solvent, has raised an alternative question for the synthetic chemist: "What happens if there is no solvent?" This review focuses on a three-part answer to that question: when there is little change ("solvent-optional" reactions); when solvent needs to be present in some form, even if only in the amounts provided by liquid-assisted (LAG) or solvate-assisted grinding; and those cases in which mechanochemistry allows access to compounds that cannot be obtained from solution-based routes. The emphasis here is on inorganic and organometallic systems, including selected examples of mechanosynthesis and mechanocatalysis. Issues of mechanochemical depictions and the adequacy of LAG descriptions are also reviewed.

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.

Crystal engineering: from promise to delivery

Twenty years ago, I wrote a Chem. Commun. feature article entitled "Crystal Engineering: where from? Where to?": an update is in order. In this Highlight I argue that molecular crystal engineering, one of the areas of fast development of the field, has definitely reached the stage of "delivering the goods": new functional materials assembled via non-covalent interactions and/or improved properties of existing materials. As a proof of concept, the crystal engineering approach to tackle two contemporary emergencies, namely, urea fertilizer degradation and development of antimicrobial resistance by pathogens, is discussed and application-driven examples are provided.

Ball milling synthesis of S-quinolyl xanthates via coupling of haloquinolines with potassium O-alkyl xanthates

An environmentally benign protocol that provides various S-quinolyl xanthates via a ball milling enabled cross coupling reaction of haloquinolines and readily available potassium O-alkyl xanthates is first reported. The reaction proceeded well under mild, transition metal- and solvent-free conditions, making it an attractive method for the introduction of xanthates into the quinoline scaffold.

Milling Medium-Free Suzuki Coupling by Direct Mechanocatalysis: From Mixer Mills to Resonant Acoustic Mixers

Here we describe the development of a sustainable and cost-effective approach for catalytic cross-coupling reactions in mechanochemistry. It is found that the substrate's impact with the vessel wall alone is sufficient to initiate the reaction, thus indicating that milling balls function primarily as a mixing agent for direct mechanocatalytic Suzuki coupling. The absence of milling balls can be offset by adjusting the rheology using liquid-assisted grinding (LAG). The LAG sweet spot of 0.25 μL mg-1 is confirmed for both resonance acoustic mixers (RAMs) and ball-free mixer mills, and is higher than in the presence of milling balls. RAMs exhibit excellent performance in the Suzuki reaction, achieving yields of 90 % after 60 min and complete conversion after 90 min. The longevity of the milling vessel is significantly improved in a RAM, allowing for at least 20 reactions without deterioration.

Supramolecular intermediates in thermo-mechanochemical direct amidations

We present a solvent-free thermo-mechanochemical approach for the direct coupling of carboxylic acids and amines, which avoids activators and additives. Detailed analysis of the reactions by ex situ and in situ monitoring methods led to the observation, isolation, and characterisation of multicomponent crystalline intermediates that precede the formation of amides. We applied our methodology for the quantitative synthesis of the active pharmaceutical ingredient moclobemide.

Illuminating milling mechanochemistry by tandem real-time fluorescence emission and Raman spectroscopy monitoring

In pursuit of accessible and interpretable methods for direct and real-time observation of mechanochemical reactions, we demonstrate a tandem spectroscopic method for monitoring of ball-milling transformations combining fluorescence emission and Raman spectroscopy, accompanied by high-level molecular and periodic density-functional theory (DFT) calculations, including periodic time-dependent (TD-DFT) modelling of solid-state fluorescence spectra. This proof-of-principle report presents this readily accessible dual-spectroscopy technique as capable of observing changes to the supramolecular structure of the model pharmaceutical system indometacin during mechanochemical polymorph transformation and cocrystallisation. The observed time-resolved in situ spectroscopic and kinetic data are supported by ex situ X-ray diffraction and solid-state nuclear magnetic resonance spectroscopy measurements. The application of first principles (ab initio) calculations enabled the elucidation of how changes in crystalline environment, that result from mechanochemical reactions, affect vibrational and electronic excited states of molecules. The herein explored interpretation of both real-time and ex situ spectroscopic data through ab initio calculations provides an entry into developing a detailed mechanistic understanding of mechanochemical milling processes and highlights the challenges of using real-time spectroscopy.

Structural Variants and Ultralow Detection Ability for Tryptamine in Two Polymorphs of a Zincophosphite Framework

Two organic-inorganic hybrid zinc phosphites incorporating 1,2,4,5-tetrakis(imidazol-1-ylmethyl)benzene (TIMB) molecules were synthesized under hydro(solvo)thermal methods and structurally characterized by single-crystal X-ray diffraction (SCXD). Interestingly, the solvent ratio of water to dimethylformamide induced the formation of a new compound of Zn2(TIMB)0.5(HPO3)2·3H2O (1) and our previously reported structure of Zn2(TIMB)0.5(HPO3)2·H2O (2). Additionally, their dehydrated crystals (1a and 2a) were prepared through heat treatment at 150 °C. SCXD and powder X-ray diffraction showed that all four compounds share the same framework formula of Zn2(TIMB)0.5(HPO3)2 but exhibit a huge difference in their inorganic components and final structures. In 1 and 1a, the inorganic units formed two-dimensional zincophosphite layers, while in 2 and 2a, they formed one-dimensional chains. The inorganic parts of 1 (1a) and 2 (2a) were bridged with TIMB linkers, resulting in 3D structures with rectangular and tubular windows, respectively. Furthermore, 1 was coated on the screen-printed carbon electron as a hybrid material, displaying excellent performance while having a linear relationship with an R2 value of 0.99 within the concentration range of 10-10 to 10-6 mol/L for detecting tryptamine (Try) molecules. Moreover, the results showed that 1 exhibits an ultralow limit of detection of 5.43 × 10-11 mol/L and high specificity toward Try over histamine, ascorbic acid, uric acid, and glucose. The synthesis, structural diversity, stability, and sensing ability are also discussed.

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.

Preparation of Fe/Cu bimetals by ball milling iron powder and copper sulfate for trichloroethylene degradation: Combined effect of FeSx and Fe/Cu alloy

The addition of a secondary metal (such as Cu, Co, Ni and Pd) to form iron-based bimetallic particles could enhance the reactivity of zero valent iron (ZVI). This study proposed a new synthesis method for preparing Cu-Fe bimetals (Cu-Febm (CuSO4)) by ball milling mZVI and CuSO4. During ball-milling process, 40% of Cu2+ can be reduced to Cu0, which formed galvanic couple with Fe0 in a way of Fe/Cu alloy structure. Part Cu2+ was only reduced to Cu+ (corresponding to Cu2O), while 29% of SO42- was reduced to Sx2- (corresponding to FeSx). The appearance of Cu2O was not conducive to the activity of Cu-Febm (CuSO4) particles, the formation of Fe0/FeSx structure compensated for the partial loss of Fe/Cu alloy. H•abs was identified as the main active species for TCE degradation by Cu-Febm (CuSO4) bimetals. The Cu-Febm (CuSO4) bimetals has great potential for the removal of chlorinated hydrocarbons in water.

Mechanoluminescent Materials Enable Mechanochemically Controlled Atom Transfer Radical Polymerization and Polymer Mechanotransduction

Organic mechanophores have been widely adopted for polymer mechanotransduction. However, most examples of polymer mechanotransduction inevitably experience macromolecular chain rupture, and few of them mimic mussel's mechanochemical regeneration, a mechanically mediated process from functional units to functional materials in a controlled manner. In this paper, inorganic mechanoluminescent (ML) materials composed of CaZnOS-ZnS-SrZnOS: Mn2+ were used as a mechanotransducer since it features both piezoelectricity and mechanolunimescence. The utilization of ML materials in polymerization enables both mechanochemically controlled radical polymerization and the synthesis of ML polymer composites. This procedure features a mechanochemically controlled manner for the design and synthesis of diverse mechanoresponsive polymer composites.

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.

Mechanochemistry-Amended Barbier Reaction as an Expedient Alternative to Grignard Synthesis

Organomagnesium halides (Grignard reagents) are essential carbanionic building blocks widely used in carbon-carbon and carbon-heteroatom bond-forming reactions with various electrophiles. In the Barbier variant of the Grignard synthesis, the generation of air- and moisture-sensitive Grignard reagents occurs concurrently with their reaction with an electrophile. Although operationally simpler, the classic Barbier approach suffers from low yields due to multiple side reactions, thereby limiting the scope of its application. Here, we report a mechanochemical adaptation of the Mg-mediated Barbier reaction, which overcomes these limitations and facilitates the coupling of versatile organic halides (e.g., allylic, vinylic, aromatic, aliphatic) with a diverse range of electrophilic substrates (e.g., aromatic aldehydes, ketones, esters, amides, O-benzoyl hydroxylamine, chlorosilane, borate ester) to assemble C-C, C-N, C-Si, and C-B bonds. The mechanochemical approach has the advantage of being essentially solvent-free, operationally simple, immune to air, and surprisingly tolerant to water and some weak Brønsted acids. Notably, solid ammonium chloride was found to improve yields in the reactions of ketones. Mechanistic studies have clarified the role of mechanochemistry in the process, indicating the generation of transient organometallics facilitated by improved mass transfer and activation of the surface of magnesium metal.

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.

Mechanoredox/Nickel Co-Catalyzed Cross Electrophile Coupling of Benzotriazinones with Alkyl (Pseudo)halides

An air-tolerant mechanoredox/nickel cocatalyzed cross electrophile coupling of benzotriazinones with alkyl (pseudo)halides is developed by liquid-assisting grinding in the presence of manganese powders and strontium titanate as a reductant and a cocatalyst, respectively. Mechanical activation of metal surfaces via ball milling eliminates the chemical activator for manganese, while mechanoredox cocatalysis of strontium titanate remarkably improves the aryl/alkyl cross electrophile coupling via piezoelectricity-mediated radical generation from alkyl halides. Both benzotriazinones and alkyl (pseudo)halides display reactivities in the mechanoredox/nickel cocatalysis different from those of conventional thermal chemistry in solution. The scope of the reaction is demonstrated with 26 examples, showing a high chemoselectivity of bromides vs chlorides.

Induction-heated ball-milling: a promising asset for mechanochemical reactions

While ball-milling is becoming one of the common tools used by synthetic chemists, an increasing number of studies highlight that it is possible to further expand the nature and number of products which can be synthesized, by heating the reaction media during mechanochemical reactions. Hence, developing set-ups enabling heating and milling to be combined is an important target, which has been looked into in both academic and industrial laboratories. Here, we report a new approach for heating up reaction media during ball-milling reactions, using induction heating (referred to as i-BM). Our set-up is attractive not only because it enables a very fast heating of the milling medium (reaching ≈80 °C in just 15 s), and that it is directly adaptable to commercially-available milling equipment, but also because it enables heating either the walls of the milling jars or the beads themselves, depending on the choice of the materials which compose them. Importantly, the possibility to heat a milling medium "from the inside" (when using for example a PMMA jar and stainless steel beads) is a unique feature compared to previously proposed systems. Through numerical simulations, we then show that it is possible to finely tune the properties of this heating system (e.g. heating rate and maximum temperature reached), by playing with the characteristics of the milling system and/or the induction heating conditions used. Lastly, examples of applications of i-BM are given, showing how it can be used to help elucidate reaction mechanisms in ball-milling, to synthesize new molecules, and to control the physical nature of milling media.

Review of covalent organic frameworks for enzyme immobilization: Strategies, applications, and prospects

Efficient enzyme immobilization systems offer a promising approach for improving enzyme stability and recyclability, reducing enzyme contamination in products, and expanding the applications of enzymes in the biomedical field. Covalent organic frameworks (COFs) possess high surface areas, ordered channels, optional building blocks, highly tunable porosity, stable mechanical properties, and abundant functional groups, making them ideal candidates for enzyme immobilization. Various COF-enzyme composites have been successfully synthesized, with performances that surpass those of free enzymes in numerous ways. This review aims to provide an overview of current enzyme immobilization strategies using COFs, highlighting the characteristics of each method and recent research applications. The future opportunities and challenges of enzyme immobilization technology using COFs are also discussed.

Hydrogenation of different carbon substrates into light hydrocarbons by ball milling

The conversion of carbon-based solids, like non-recyclable plastics, biomass, and coal, into small molecules appears attractive from different points of view. However, the strong carbon-carbon bonds in these substances pose a severe obstacle, and thus-if such reactions are possible at all-high temperatures are required1-5. The Bergius process for coal conversion to hydrocarbons requires temperatures above 450 °C6, pyrolysis of different polymers to pyrolysis oil is also typically carried out at similar temperatures7,8. We have now discovered that efficient hydrogenation of different solid substrates with the carbon-based backbone to light hydrocarbons can be achieved at room temperature by ball milling. This mechanocatalytic method is surprisingly effective for a broad range of different carbon substrates, including even diamond. The reaction is found to proceed via a radical mechanism, as demonstrated by reactions in the presence of radical scavengers. This finding also adds to the currently limited knowledge in understanding mechanisms of reactions induced by ball milling. The results, guided by the insight into the mechanism, could induce more extended exploration to broaden the application scope and help to address the problem of plastic waste by a mechanocatalytic approach.

Mechanochemical Mediated Coexistence of B←N Coordination and Hydrogen Bonding

Mechanochemistry afforded a photoactive cocrystal via coexisting (B)O-H⋅⋅⋅N hydrogen bonds and B←N coordination. Specifically, solvent-free mechanochemical ball mill grinding and liquid-assisted grinding of a boronic acid and an alkene resulted in mixtures of hydrogen-bonded and coordinated complexes akin to mixtures of noncovalent complexes that can be obtained in solution in equilibria processes. The alkenes of the hydrogen-bonded assembly undergo an intermolecular [2+2] photodimerization in quantitative conversion, effectively reporting the outcome of the self-assembly processes. Our results suggest that interplay involving noncovalent bonds subjected to mechanochemical conditions can lead to functional solids where, in the current case, the structure composed of the weaker hydrogen bonding interactions predominates.

Mechanochemical Phosphorylation of Acetylides Using Condensed Phosphates: A Sustainable Route to Alkynyl Phosphonates

In pursuit of a more sustainable route to phosphorus-carbon (P-C) bond-containing chemicals, we herein report that phosphonates can be prepared by mechanochemical phosphorylation of acetylides using polyphosphates in a single step, redox-neutral process, bypassing white phosphorus (P4) and other high-energy, environmentally hazardous intermediates. Using sodium triphosphate (Na5P3O10) and acetylides, alkynyl phosphonates 1 can be isolated in yields of up to 32%, while reaction of sodium pyrophosphate (Na4P2O7) and sodium carbide (Na2C2) engendered, in an optimized yield of 63%, ethynyl phosphonate 2, an easily isolable compound that can be readily converted to useful organophosphorus chemicals. Highly condensed phosphates like Graham's salt and bioproduced polyphosphate were also found to be compatible after reducing the chain length by grinding with orthophosphate. These results demonstrate the possibility of accessing organophosphorus chemicals directly from condensed phosphates and may offer an opportunity to move toward a "greener" phosphorus industry.

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.

Enhancing Ultrasound-Assisted Iodine-Mediated Reversible-Deactivation Radical Polymerization by Piezoelectric Nanoparticles

The development of mechanochemical tools for regulating the polymerization process has received an increasing amount of attention in recent years. Herein, we report the example of the mechanically controlled iodine-mediated reversible-deactivation radical polymerization (mechano-RDRP) using piezoelectric tetragonal BaTiO3 nanoparticles (T-BTO) as mechanoredox catalyst and alkyl iodide as the initiator. We demonstrated a more efficient mechanochemical initiation and reversible deactivation process than sonochemical activation via a mechanoredox-mediated alkyl iodide cleavage reaction. The mechanochemical activation of the C-I bond was verified by density functional theory (DFT) calculations. Theoretical calculations together with experimental results confirmed the more efficient initiation and polymerization than the traditional sonochemical approach. The influence of BaTiO3, initiator, and solvent was further examined to reveal the mechanism of the mechano-RDRP. The results showed good controllability over molecular weight and capacity for a one-pot chain extension. This work expands the scope of mechanically controlled polymerization and shows good potential in the construction of adaptive materials.

Mechanochemical asymmetric three-component Mannich reaction involving unreactive arylamines

We report here a mechanochemical protocol for an asymmetric three-component Mannich reaction involving unreactive arylamines with simple cyclic ketones and arylaldehydes catalyzed by (S)-proline with a chiral diol. In this mechanochemical protocol, ball milling enables reaction acceleration and enantioselectivity control. The reported asymmetric three-component Mannich reactions usually involve reactive arylamines such as p-anisidine and phenylamine, while the catalytic asymmetric Mannich reactions involving unreactive arylamines in solution did not proceed smoothly or gave low yields and enantioselectivities. However, the use of ball-milling techniques overcomes the deficiency of the batch systems in solution and avoids the use of toxic organic solvents. The desired products were obtained in moderate-to-good yields (49%-80%) with good-to-high enantioselectivities (up to 99% ee). This is the first example of a mechanochemically activated catalytic asymmetric three-component Mannich reaction involving unreactive arylamines.

Mechanochemical Duff Reaction in Solid Phase for Easy Access to Mono- and Di-formyl Electron-Rich Arenes

A sustainable alternative to the century-old Duff reaction was developed by adopting a solid-phase mechanochemical route. A series of mono-formyl electron-rich arenes were prepared in high yields in silica as the solid reaction media using a combination of hexamethylenetetramine (HMTA) as the formyl source and a small amount of H2SO4 in a mixer mill. The use of toxic, costly, and low-boiling trifluoroacetic acid was avoided in the new mold of the mechanochemical Duff reaction. The mono-formyl phenols were obtained with exclusive ortho-selectivity, whereas unprecedented para-formylation was observed for other electron-rich aromatics. By controlling the stoichiometry of HMTA, the method offers easy access to di-formylated phenols as well. The scalability of the reaction was validated with selected substrates at the gram-scale level. In a case study, a mechanochemical tandem reaction was explored in the synthesis of a rhodol derivative. The solvent-free, metal-free mild method of formylation, with the absence of tedious work-up steps and shorter reaction times using an inexpensive mineral acid, is a sustainable alternative to the available methods for aromatic formylation.

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.

Biasing Divergent Polycyclic Aromatic Hydrocarbon Oxidation Pathway by Solvent-Free Mechanochemistry

Precise control in reaction selectivity is the goal in modern organic synthesis, and it has been widely studied throughout the synthetic community. In comparison, control of divergent reactivity of a given reagent under different reaction conditions is relatively less explored aspect of chemical selectivity. We herein report an unusual reaction between polycyclic aromatic hydrocarbons and periodic acid H₅IO₆ (1), where the product outcome is dictated by the choice of reaction conditions. That is, reactions under solution-based condition give preferentially C-H iodination products, while reactions under solvent-free mechanochemical condition provide C-H oxidation quinone products. Control experiments further indicated that the iodination product is not a reaction intermediate toward the oxidation product and vice versa. Mechanistic studies unveiled an in situ crystalline-to-crystalline phase change in 2 during ball-milling treatment, where we assigned it as a polymeric hydrogen-bond network of 1. We believe that this polymeric crystalline phase shields the more embedded electrophilic I═O group of 1 from C-H iodination and bias a divergent C-H oxidation pathway (with I═O) in the solid state. Collectively, this work demonstrates that mechanochemistry can be employed to completely switch a reaction pathway and unmask hidden reactivity of chemical reagents.

Diastereoselective Rotaxane Synthesis with Pillar[5]arenes via Co-crystallization and Solid-State Mechanochemical Processes

Chiral rotaxanes have attracted much attention in recent decades for their unique chirality based on their interlocked structures. Thus, selective synthesis methods of chiral rotaxanes have been developed. The introduction of substituents with chiral centers to produce diastereomers is a powerful strategy for the construction of chiral rotaxanes. However, in case of a small energy difference between the diastereomers, diastereoselective synthesis is extremely difficult. Herein, we report a new diastereoselective rotaxane synthesis method using solid-phase diastereoselective [3]pseudorotaxane formation and mechanochemical solid-phase end-capping reactions of the [3]pseudorotaxanes. By co-crystallization of stereodynamic planar chiral pillar[5]arene with stereogenic carbons at both rims and axles with suitable end groups and lengths, the [3]pseudorotaxane with a high diastereomeric excess (ca. 92% de) was generated in the solid state because of higher effective molarity with aid by packing effects and significant energy differences between [3]pseudorotaxane diastereomers. In contrast, the de of the pillar[5]arene was low in solution (ca. 10% de) because of a small energy difference between diastereomers. Subsequent end-capping reactions of the polycrystalline [3]pseudorotaxane with high de in solvent-free conditions successfully yielded rotaxanes while maintaining the high de generated by the co-crystallization.

Mechanochemical Oxidative Degradation of Thienopyridine Containing Drugs: Toward a Simple Tool for the Prediction of Drug Stability

The long-term stability of an active-pharmaceutical ingredient and its drug products plays an important role in the licensing process of new pharmaceuticals and for the application of the drug at the patient. It is, however, difficult to predict degradation profiles at early stages of the development of new drugs, making the entire process very time-consuming and costly. Forced mechanochemical degradation under controlled conditions can be used to realistically model long-term degradation processes naturally occurring in drug products, avoiding the use of solvents, thus excluding irrelevant solution-based degradation pathways. We present the forced mechanochemical oxidative degradation of three platelet inhibitor drug products, where the drug products contain thienopyridine. Model studies using clopidogrel hydrogen sulfate (CLP) and its drug formulation Plavix show that the controlled addition of excipients does not affect the nature of the main degradants. Experiments using drug products Ticlopidin-neuraxpharm and Efient show that significant degradation occurs after short reaction times of only 15 min. These results highlight the potential of mechanochemistry for the study of degradation processes of small molecules relevant to the prediction of degradation profiles during the development of new drugs. Furthermore, these data provide exciting insights into the role of mechanochemistry for chemical synthesis in general.

Rationalizing the formation of porosity in mechanochemically-synthesized polymers

In this study, we present a matrix of 144 mechanochemically-synthesized polymers. All polymers were constructed by the solvent-free Friedel-Crafts polymerization approach, employing 16 aryl-containing monomers and 9 halide-containing linkers, which were processed in a high-speed ball mill. This Polymer Matrix was utilized to investigate the origin of porosity in Friedel-Crafts polymerizations in detail. By examining the physical state, molecular size, geometry, flexibility, and electronic structure of the utilized monomers and linkers, we identified the most important factors influencing the formation of porous polymers. We analyzed the significance of these factors for both monomers and linkers based on the yield and specific surface area of the generated polymers. Our in-depth evaluation serves as a benchmark study for future targeted design of porous polymers by the facile and sustainable concept of mechanochemistry.

Green solvents, materials, and lead-free semiconductors for sustainable fabrication of perovskite solar cells

Perovskite materials research has received unprecedented recognition due to its applications in photovoltaics, LEDs, and other large area low-cost electronics. The exceptional improvement in the photovoltaic conversion efficiency of Perovskite solar cells (PSCs) achieved over the last decade has prompted efforts to develop and optimize device fabrication technologies for the industrial and commercial space. However, unstable operation in outdoor environments and toxicity of the employed materials and solvents have hindered this proposition. While their optoelectronic properties are extensively studied, the environmental impacts of the materials and manufacturing methods require further attention. This review summarizes and discusses green and environment-friendly methods for fabricating PSCs, particularly non-toxic solvents, and lead-free alternatives. Greener solvent choices are surveyed for all the solar cell films, (i.e. electron and hole transport, semiconductor, and electrode layers) and their impact on thin film quality, morphology and device performance is explored. We also discuss lead content in perovskites, its environmental impact and sequestration routes, and progress in replacing lead with greener alternatives. This review provides an analysis of sustainable green routes in perovskite solar cell fabrication, discussing the impact of each layer in the device stack, via life cycle analysis.

Acceleration of Diels-Alder reactions by mechanical distortion

Challenges in quantifying how force affects bond formation have hindered the widespread adoption of mechanochemistry. We used parallel tip-based methods to determine reaction rates, activation energies, and activation volumes of force-accelerated [4+2] Diels-Alder cycloadditions between surface-immobilized anthracene and four dienophiles that differ in electronic and steric demand. The rate dependences on pressure were unexpectedly strong, and substantial differences were observed between the dienophiles. Multiscale modeling demonstrated that in proximity to a surface, mechanochemical trajectories ensued that were distinct from those observed solvothermally or under hydrostatic pressure. These results provide a framework for anticipating how experimental geometry, molecular confinement, and directed force contribute to mechanochemical kinetics.

The first solid-state route to luminescent Au(I)-glutathionate and its pH-controlled transformation into ultrasmall oligomeric Au10-12(SG)10-12 nanoclusters for application in cancer radiotheraphy

There is still a need for synthetic approaches that are much faster, easier to scale up, more robust and efficient for generating gold(I)-thiolates that can be easily converted into gold-thiolate nanoclusters. Mechanochemical methods can offer significantly reduced reaction times, increased yields and straightforward recovery of the product, compared to the solution-based reactions. For the first time, a new simple, rapid and efficient mechanochemical redox method in a ball-mill was developed to produce the highly luminescent, pH-responsive Au(I)-glutathionate, [Au(SG)]_n. The efficient productivity of the mechanochemical redox reaction afforded orange luminescent [Au(SG)]_n in isolable amounts (mg scale), usually not achieved by more conventional methods in solution. Then, ultrasmall oligomeric Au_10-12(SG)_10-12 nanoclusters were prepared by pH-triggered dissociation of [Au(SG)]_n. The pH-stimulated dissociation of the Au(I)-glutathionate complex provides a time-efficient synthesis of oligomeric Au_10-12(SG)_10-12 nanoclusters, it avoids high-temperature heating or the addition of harmful reducing agent (e.g., carbon monoxide). Therefore, we present herein a new and eco-friendly methodology to access oligomeric glutathione-based gold nanoclusters, already finding applications in biomedical field as efficient radiosensitizers in cancer radiotherapy.

In situ study of Au nanoparticle formation in a mechanochemical-aging-based method

As we strive to perform chemical transformations in a more sustainable fashion, enabling solid-state reactions through mechanochemistry has emerged as a highly successful approach. Due to the wide-ranging applications of gold nanoparticles (AuNPs), mechanochemical strategies have already been employed for their synthesis. However, the underlying processes surrounding gold salt reduction, nucleation and growth of AuNPs in the solid state are yet to be understood. Here, we present a mechanically activated aging synthesis of AuNPs, through a solid-state Turkevich reaction. Solid reactants are only briefly exposed to input of mechanical energy before being aged statically over a period of six weeks at different temperatures. This system offers an excellent opportunity for an in situ analysis of both reduction and nanoparticle formation processes. During the aging period, the reaction was monitored using a combination of X-ray photoelectron spectroscopy, diffuse reflectance spectroscopy, powder X-ray diffraction and transmission electron microscopy, to gain meaningful insights into the mechanisms of solid-state formation of gold nanoparticles. The acquired data allowed for the establishment of the first kinetic model for solid-state nanoparticle formation.

Insight into mechanochemical destruction of PFOA by BaTiO3: An electron-dominated reduction process

Perfluorooctanoic acid (PFOA) is a representative persistent organic pollutant and its disposal by mechanochemical (MC) technology emerges in recent years. However, degradation mechanism of PFOA especially rupture of C-F bonds during MC process is still unclear. Therefore, we innovatively employed barium titanate as co-milling reagent in MC system to disclose an electron-dominated reduction process. By stimulating piezoelectric effect of BaTiO₃ under MC impact, free electrons were generated. The results implied more than 95.00% degradation and 60.00% defluorination efficiency were obtained after 6 h' ball milling. DPPH• was used as probe to confirm the existence of piezo-excited electrons, which were further verified to be major reactive species by atmosphere experiments. Thus, PFOA destruction was dominated by reduction process, characterizing by breakage of C-F bonds induced by electrons. Accordingly, the fate of organic fluorides was explored and BaF₂ was identified as final product. The cleavage of carboxyl group initiated PFOA decomposition, following by successive removal of CF₂ groups and elimination of F^-. Moreover, the practical experiments and reusable trials implied promising application of this method. Overall, this paper provides a novel perspective for reductive decomposition of PFOA by MC technology and reveals the major role of electrons during reaction process.

TM-Free and TM-Catalyzed Mechanosynthesis of Functional Polymers

Mechanochemically induced methods are commonly used for the depolymerization of polymers, including plastic and agricultural wastes. So far, these methods have rarely been used for polymer synthesis. Compared to conventional polymerization in solutions, mechanochemical polymerization offers numerous advantages such as less or no solvent consumption, the accessibility of novel structures, the inclusion of co-polymers and post-modified polymers, and, most importantly, the avoidance of problems posed by low monomer/oligomer solubility and fast precipitation during polymerization. Consequently, the development of new functional polymers and materials, including those based on mechanochemically synthesized polymers, has drawn much interest, particularly from the perspective of green chemistry. In this review, we tried to highlight the most representative examples of transition-metal (TM)-free and TM-catalyzed mechanosynthesis of some functional polymers, such as semiconductive polymers, porous polymeric materials, sensory materials, materials for photovoltaics, etc.

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.

Mn(I)-catalyzed sigmatropic rearrangement of β, γ-unsaturated alcohols

Sigmatropic rearrangement provides a versatile strategy to site-selectively reorganize carbon-skeleton with high atom- and step-economy. Herein, we disclose a Mn(I)-catalyzed sigmatropic rearrangement of β, γ-unsaturated alcohols via C-C σ bond activation. A variety of α-aryl-allylic alcohols and α-aryl-propargyl alcohols could undergo in-situ 1,2- or 1,3- sigmatropic rearrangements to allow for converting to complex structural arylethyl- and arylvinyl- carbonyl compounds under a simple catalytic system. More importantly, this catalysis model can be further applied to assemble macrocyclic ketones through bimolecular [2n + 4] coupling-cyclization and monomolecular [n + 1] ring-extension. The presented skeleton rearrangement would be a useful tool complementary to the traditional molecular rearrangement.

An Approach to Functionally Embellished o-Alkynylbenzoates or Furan-3(2H)-ones from Diynones and DMAD: Controlled Divergence and Product Selectivity

The discovery of reaction regime controlled product diversification in a one-pot reaction between diynones and dimethyl-1,3-acetonedicarboxylate (DMAD) to selectively furnish either functionally unique pentasubstituted o-alkynylbenzoates or fully substituted furan-3(2H)-ones is delineated. The potential of these two versatile platforms to enter new utilitarian chemical space has also been explored.

Amidation by reactive extrusion for the synthesis of active pharmaceutical ingredients teriflunomide and moclobemide

The solventless synthesis of an amide was performed in a twin-screw extruder in the presence of a coupling agent, providing a high yielding and productive method. The reaction conditions were optimized to prepare APIs, teriflunomide and moclobemide.

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.

Design and synthesis of novel arylurea derivatives of aryloxy(1-phenylpropyl) alicyclic diamines with antimicrobial activity against multidrug-resistant Gram-positive bacteria

The alarming increase in the resistance of bacteria to the currently available antibiotics necessitates the development of new effective antimicrobial agents that are active against bacterial pathogens causing major public health problems. For this purpose, our in-house libraries were screened against a wide panel of clinically relevant Gram-positive and Gram-negative bacteria, based on which compound I was selected for further optimization. Synthetic efforts in a group of arylurea derivatives of aryloxy(1-phenylpropyl) alicyclic diamines, followed with an in vitro evaluation of the activity against multidrug-resistant strains identified compound 44 (1-(3-chlorophenyl)-3-(1-{3-phenyl-3-[3-(trifluoromethyl)phenoxy] propyl}piperidin-4-yl)urea). Compound 44 showed antibacterial activity against Gram-positive bacteria including fatal drug-resistant strains i.e., Staphylococcus aureus (methicillin-resistant, MRSA; vancomycin-intermediate, VISA) and Enterococcus faecium (vancomycin-resistant, VREfm) at low concentrations (0.78-3.125 μg/mL) comparable to last resort antibiotics (i.e., vancomycin and linezolid). It is also potent against biofilm-forming S. aureus and Staphylococcus epidermidis (including linezolid-resistant, LRSE) strains, but with no activity against Gram-negative bacteria (Escherichia coli, Klebsiella pneumoniae and Pseudomonas aeruginosa). Compound 44 showed strong bactericidal properties against susceptible and drug-resistant Gram-positive bacteria. Depolarization of the bacterial cytoplasmic membrane induced by compound 44 suggests a dissipation of the bacterial membrane potential as its mechanism of antibacterial action. The high antimicrobial activity of compound 44, along with its selectivity over mammalian cells (lung MCR-5 and skin BJ fibroblast cell lines) and no hemolytic properties toward horse erythrocytes, proposes arylurea derivatives of aryloxy(1-phenylpropyl) alicyclic diamines for development of novel antibacterial agents.

Mechanochemical Access to Catechol-Derived Metal-Organic Frameworks

Mechanochemistry, a resurging synthetic approach, has been developed into an effective and controllable method to access a family of crystalline porous catechol-derived metal-organic frameworks (MOFs) for the first time. We have identified that the obtained crystalline phase is readily tunable by precursors and the addition of solvents or drying agents. The described mechanochemistry allows us to synthesize these materials in a highly sustainable manner. Thus, mechanochemistry is expected to pave a promising avenue to access a broader class of MOF materials, in addition to those based on the motifs of carboxylic acid or imidazole.

Phase Purity Regulated by Mechano-Chemical Synthesis of Metal-Organic Frameworks for the Electrocatalytic Oxygen Evolution Reaction

Three new metal organic frameworks (ZnTIA-1mc, CuTIA-1mc, and CoTIA-1mc) were synthesized by the mechanochemical grinding (mc) method in the unadulterated form. They compared with their solvothermally synthesized (st) counterparts, where the mixtures of isomeric forms have been isolated. Kinetics study with the function of grinding time during the mechanosynthesis process revealed the formation of new metastable phases. Less crystallinity and short of mechanical defects in the structure of synthesized mc metal organic frameworks showed enhanced electrocatalytic activity toward oxygen evolution reaction (OER). Among all, CoTIA-1mc showed high OER activity with 289 mV overpotential, 10 mA cm^-2 current density, and 55.4 mV dec^-1 Tafel slope in 1 M KOH which is close to the commercially used RuO₂.

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.

Mixed valence Sn doped (CH3NH3)3Bi2Br9 produced by mechanochemical synthesis

Bismuth halides with formula A₃Bi₂X₉, where A is an inorganic or organic cation, show desirable properties as solar absorbers and luminescent materials. Control of structural and electronic dimensionality of these compounds is important to yield materials with good light absorption and charge transport. Here we report mechanochemical reaction of (CH₃NH₃)₃Bi₂Br₉ with SnBr₂ at room temperature in air, yielding a material with strong absorption across the visible region. We attribute this to mixed valence doping of Sn(II) and Sn(IV) on the Bi site. X-Ray diffraction shows no secondary phases, even after heating at 200 °C to improve crystallinity. X-Ray photoelectron spectroscopy suggests the presence of Sn(II) and Sn(IV) states. A similar approach to dope Sn into the iodide analogue (CH₃NH₃)₃Bi₂I₉ was unsuccessful.