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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 Synthesis and Three-Dimensional Electron Diffraction Structure Solution of a Novel Cu-Based Protocatechuate Metal-Organic Framework

Mechanochemical synthesis is a powerful approach to obtain new materials, limiting costs, and times. However, defected and submicrometrical-sized crystal products make critical their characterization through classical single-crystal X-ray diffraction. A valid alternative is represented by three-dimensional (3D) electron diffraction, in which a transmission electron microscope is used, like a diffractometer. This work matches a green water-based mechanochemical synthesis and 3D electron diffraction to obtain and characterize a Cu-based protocatechuate metal-organic framework (PC-MOF). Its structure has been fully refined through dynamical diffraction theory, and free water molecules could be detected in the channels of the framework. Thermal characterization, focused on the dehydration profile determination, leads to the formation of a novel high-temperature 2D coordination polymer, fully solved with 3D electron diffraction data. At last, the strong activity of the PC-MOF against cationic dyes like methylene blue has been reported.

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 %).

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.

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.

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.

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.

Iron‐Catalyzed Intramolecular Arene C(sp 2 )−H Amidations under Mechanochemical Conditions

In a ball mill, FeBr₃‐catalyzed intramolecular amidations lead to 3,4‐dihydro‐2(1H)‐quinolinones in good to almost quantitative yields. The reactions do not require a solvent and are easy to perform. No additional ligand is needed for the iron catalyst. Both 4‐substituted aryl and β‐substituted dioxazolones provide products with high selectivity. Mechanistically, an electrophilic spirocyclization followed by C−C migration explains the formation of rearranged products.

The Mechanochemical Synthesis and Activation of Carbon-Rich π-Conjugated Materials

Mechanochemistry uses mechanical force to break, form, and manipulate chemical bonds to achieve functional transformations and syntheses. Over the last years, many innovative applications of mechanochemistry have been developed. Specifically for the synthesis and activation of carbon-rich π-conjugated materials, mechanochemistry offers reaction pathways that either are inaccessible with other stimuli, such as light and heat, or improve reaction yields, energy consumption, and substrate scope. Therefore, this review summarizes the recent advances in this research field combining the viewpoints of polymer and trituration mechanochemistry. The highlighted mechanochemical transformations include π-conjugated materials as optical force probes, the force-induced release of small dye molecules, and the mechanochemical synthesis of polyacetylene, carbon allotropes, and other π-conjugated materials.

Mechanochemical Access to a Short-Lived Cyclic Dimer Pd2 L2 : An Elusive Kinetic Species En Route to Molecular Triangle Pd3 L3 and Molecular Square Pd4 L4

Pd-based molecular square Pd₄ L₄ and triangle Pd₃ L₃ represent the molecular ancestors of metal-coordination polyhedra that have been an integral part of the field for the last 30 years. Conventional solution-based reactions between cis-protected Pd ions and 2,2'-bipyridine exclusively give Pd₄ L₄ and/or Pd₃ L₃ as the sole products. We herein show that, under solvent-free mechanochemical conditions, the self-assembly energy landscape can be thermodynamically manipulated to form an elusive cyclic dimer Pd₂ L₂ for the first time. In the absence of solvent, Pd₂ L₂ is indefinitely stable in the solid-state, but converts rapidly to its thermodynamic products Pd₃ L₃ and Pd₄ L₄ in solution, confirming Pd₂ L₂ as a short-lived kinetic species in the solution-based self-assembly process. Our results highlight how mechanochemistry grants access to a vastly different chemical space than available under conventional solution conditions. This provides a unique opportunity to isolate elusive species in self-assembly processes that are too reactive to both "see" and "capture".

Metal-Catalyzed Organic Reactions by Resonant Acoustic Mixing

We demonstrate catalytic organic synthesis by Resonant Acoustic Mixing (RAM): a mechanochemical methodology that does not require bulk solvent or milling media. Using as model reactions ruthenium-catalyzed ring-closing metathesis and copper-catalyzed sulfonamide-isocyanate coupling, RAM mechanosynthesis is shown to be faster, operationally simpler than conventional ball-milling, while also providing the first example of a mechanochemical strategy for ruthenium-catalyzed ene-yne metathesis. Reactions by RAM are readily and directly scaled-up without any significant changes in reaction conditions, as shown by the straightforward 200-fold scaling-up of the synthesis of the antidiabetic drug Tolbutamide, from hundreds of milligrams directly to 30 grams.

Mechanochemical Grignard Reactions with Gaseous CO2 and Sodium Methyl Carbonate

A one-pot three-step protocol for the preparation of Grignard reagents from organobromides in a ball mill and their subsequent reactions with gaseous carbon dioxide (CO 2 ) or sodium methyl carbonate providing aryl and alkyl carboxylic acids in up to 82% yield is reported. Noteworthy are the short reaction times and the significantly reduced solvent amounts [2.0 equiv. for liquid assisted grinding (LAG) conditions]. Unexpectedly, aryl bromides with methoxy substituents lead to symmetric ketones as major products.

Mechanochemical Solvent-Free Suzuki-Miyaura Cross-Coupling of Amides via Highly Chemoselective N-C Cleavage

Although cross-coupling reactions of amides by selective N-C cleavage are one of the most powerful and burgeoning areas in organic synthesis due to the ubiquity of amide bonds, the development of mechanochemical, solid-state methods remains a major challenge. Herein, we report the first mechanochemical strategy for highly chemoselective, solvent-free palladium-catalyzed cross-coupling of amides by N-C bond activation. The method is conducted in the absence of external heating, for short reaction time and shows excellent chemoselectivity for σ N-C bond activation. The reaction shows excellent functional group tolerance and can be applied to late-stage functionalization of complex APIs and sequential orthogonal cross-couplings exploiting double solventless solid-state methods. The results extend mechanochemical reaction environments to advance the chemical repertoire of N-C bond interconversions to solid-state environmentally friendly mechanochemical methods.

Ball-Milling-Enabled Reactivity of Manganese Metal*

Efforts to generate organomanganese reagents under ball-milling conditions have led to the serendipitous discovery that manganese metal can mediate the reductive dimerization of arylidene malonates. The newly uncovered process has been optimized and its mechanism explored using CV measurements, radical trapping experiments, EPR spectroscopy, and solution control reactions. This unique reactivity can also be translated to solution whereupon pre-milling of the manganese is required.

Introduction of a Luminophore into Generic Polymers via Mechanoradical Coupling with a Prefluorescent Reagent

Herein, we report a novel strategy for introducing a luminophore into generic polymers facilitated by mechanical stimulation. In this study, polymeric mechanoradicals were formed in situ under ball-milling conditions to undergo radical-radical coupling with a prefluorescent nitroxide-based reagent in order to incorporate a luminophore into the polymer main chains via a covalent bond. This method allowed the direct and conceptually simple preparation of luminescent polymeric materials from a wide range of generic polymers such as polystyrene, polymethyl methacrylate, and polyethylene. These results indicate that the present mechanoradical coupling strategy may help to transform existing commodity polymers into more valuable functional materials.

Mechanoredox Chemistry as an Emerging Strategy in Synthesis

Recent research endeavors have established that the mechanochemical activation of piezoelectric materials can open new avenues in redox chemistry. Impact forces, such as those imparted by a ball mill, have been shown to transform piezoelectric materials such as barium titanate (BaTiO₃) into a highly polarized state, which can then donate an electron to a suitable oxidant and receive an electron from a suitable reductant, mimicking established photoredox catalytic cycles. Proof‐of‐concept studies have elucidated that mechanoredox chemistry holds great potential in sustainable and efficient radical‐based synthesis.

Sustainability Assessment of Mechanochemistry by Using the Twelve Principles of Green Chemistry

In recent years, mechanochemistry has been growing into a widely accepted alternative for chemical synthesis. In addition to their efficiency and practicality, mechanochemical reactions are also recognized for their sustainability. The association between mechanochemistry and Green Chemistry often originates from the solvent-free nature of most mechanochemical protocols, which can reduce waste production. However, mechanochemistry satisfies more than one of the Principles of Green Chemistry. In this Review we will present a series of examples that will clearly illustrate how mechanochemistry can significantly contribute to the fulfillment of Green Chemistry in a more holistic manner.

Mechanochemical Synthesis of Catalytic Materials

The mechanochemical synthesis of nanomaterials for catalytic applications is a growing research field due to its simplicity, scalability, and eco-friendliness. Besides, it provides materials with distinct features, such as nanocrystallinity, high defect concentration, and close interaction of the components in a system, which are, in most cases, unattainable by conventional routes. Consequently, this research field has recently become highly popular, particularly for the preparation of catalytic materials for various applications, ranging from chemical production over energy conversion catalysis to environmental protection. In this Review, recent studies on mechanochemistry for the synthesis of catalytic materials are discussed. Emphasis is placed on the straightforwardness of the mechanochemical route-in contrast to more conventional synthesis-in fabricating the materials, which otherwise often require harsh conditions. Distinct material properties achieved by mechanochemistry are related to their improved catalytic performance.

Mechanochemistry of fluoride solids: from mechanical activation to mechanically stimulated synthesis

Abstract

This lecture text is focused on the comparatively young field of mechanochemistry of fluoride solids, considering both their mechanical activation and their mechanochemical synthesis. Beside a literature survey, the mechanochemical synthesis of binary fluorides MF2, MF3, of complex fluorides MMgF4, of solid solutions MaxMb1−xF2 or M1−xLnxF2+x (Ln: Y, Eu) and of fluorine-containing coordination polymers is presented. Owing to their interesting potential applications in the field of fluoride ion conductivity or luminescence properties when doped, most of the given examples are alkaline earth metal compounds. A short historical survey, remarks on peculiarities and consequences of mechanical activation as well as the necessary technical equipment for mechanochemical reactions precede the section.

Graphic abstract

Mechanochemical Solvent-Free Catalytic C-H Methylation

The mechanochemical, solvent-free, highly regioselective, rhodium-catalyzed C-H methylation of (hetero)arenes is reported. The reaction shows excellent functional-group compatibility and is demonstrated to work for the late-stage C-H methylation of biologically active compounds. The method requires no external heating and benefits from considerably shorter reaction times than previous solution-based C-H methylation protocols. Additionally, the mechanochemical approach is shown to enable the efficient synthesis of organometallic complexes that are difficult to generate conventionally.

Direct Mechanocatalysis: Using Milling Balls as Catalysts

Direct mechanocatalysis describes catalytic reactions under the involvement of mechanical energy with the distinct feature of milling equipment itself being the catalyst. This novel type of catalysis features no solubility challenges of the catalysts nor the substrate and on top offering most facile way of separation.

Catalytic Room-Temperature C-N Coupling of Amides and Isocyanates by Using Mechanochemistry

A mechanochemical route is developed for room-temperature and solvent-free derivatization of different types of amides into carbamoyl isatins (up to 96 % conversion or yield), benzamides (up to 81 % yield), and imides (up to 92 % yield). In solution, this copper-catalyzed coupling either does not take place or requires high temperatures at which it may also be competing with alternative thermal reactivity, highlighting the beneficial role of mechanochemistry for this reaction. Such behavior resembles the previously investigated coupling with sulfonamide substrates, suggesting that this type of C-N coupling is an example of a mechanochemically favored reaction, for which mechanochemistry appears to be a favored environment over solution.

Exploration of New Biomass-Derived Solvents: Application to Carboxylation Reactions

A range of hitherto unexplored biomass-derived chemicals have been evaluated as new sustainable solvents for a large variety of CO₂ -based carboxylation reactions. Known biomass-derived solvents (biosolvents) are also included in the study and the results are compared with commonly used solvents for the reactions. Biosolvents can be efficiently applied in a variety of carboxylation reactions, such as Cu-catalyzed carboxylation of organoboranes and organoboronates, metal-catalyzed hydrocarboxylation, borocarboxylation, and other related reactions. For many of these reactions, the use of biosolvents provides comparable or better yields than the commonly used solvents. The best biosolvents identified are the so far unexplored candidates isosorbide dimethyl ether, acetaldehyde diethyl acetal, rose oxide, and eucalyptol, alongside the known biosolvent 2-methyltetrahydrofuran. This strategy was used for the synthesis of the commercial drugs Fenoprofen and Flurbiprofen.

Mechanochemical Palladium-Catalyzed Carbonylative Reactions Using Mo(CO)6

Esters and amides were mechanochemically prepared by palladium-catalyzed carbonylative reactions of aryl iodides by using molybdenum hexacarbonyl as a convenient solid carbonyl source and avoiding a direct handling of gaseous carbon monoxide. Real-time monitoring of the mechanochemical reaction by in situ pressure sensing revealed that CO is rapidly transferred from Mo(CO)6 to the active catalytic system without significant release of molecular carbon monoxide.