Selective Mechanochemical Monoarylation of Unbiased Dibromoarenes by in Situ Crystallization

Mechanochemical Buchwald-Hartwig Cross-Coupling Reactions of Aromatic Primary Amines and Their Application to Two-Step One-Pot Rapid Synthesis of Unsymmetrical Triarylamines

Herein, we report the development of the first general mechanochemical protocol for Buchwald-Hartwig cross-coupling using aromatic primary amines as nucleophiles. The mechanochemical arylamination reactions were rapid and completed within 30 min for most substrates, and all the synthetic operations were carried out in air. Notably, this mechanochemical approach effectively facilitated the arylamination of poorly soluble aryl halides, which are barely reactive in solution. In addition, we demonstrated that a two-step one-pot mechanochemical arylamination sequence enables the rapid, efficient, solvent-free, time-economical, and modular synthesis of various unsymmetrical triarylamines.

Thermally accelerated Heck reaction under direct mechanocatalysis using palladium milling balls

We present a direct mechanocatalytic approach for the Mizoroki-Heck coupling of aryl iodides with olefins using palladium milling balls as catalyst under elevated temperatures in a ball milling reactor. The high chemoselectivity for C(sp2)-I bonds ensures a single product when employing multi-halogenated arenes. Additionally, a one-pot Wittig olefination/Heck cross-coupling enables direct access to stilbene derivatives from aldehydes and aryl iodides without intermediate isolation.

Mechanochemical Synthesis of Carbazole Isomer Phosphors

The 1H-benzo[f]indole (Bd[f]), a carbazole (Cz) isomer is first reported as the source of Cz-based phosphors in 2020. In this work, the novel carbazole isomers, 1H-benzo[g]indole (Bd[g]) based derivatives, are synthesized by a one-step solvent-free mechanical ball milling reaction, establishing a facile, efficient, and environmentally friendly method for the synthesis of new Cz isomer phosphorescent derivatives with high yields compared to previously reported multi-step solvent-based thermochemical synthesis routes of Bd[f] derivatives with low yields. Six Bd[g] derivatives with different substituents, namely OCH3-Bd, In-Bd, Bn-Bd, F-Bd, Cl-Bd, and Br-Bd, are synthesized, which exhibit distinctly different single-crystal structures and phosphorescent properties. After irradiation with 365 nm UV light, Bd[g] derivatives-doped poly (methyl methacrylate) (PMMA) films exhibit photoactivated green room-temperature phosphorescence with ultra-long lifetimes up to 1.65 s. Interestingly, the phosphorescence is stable in seawater along with good bactericidal properties, which also provide new candidates for indole-based marine antifoulants. This study demonstrates that mechanical ball milling is an efficient method for the synthesis of benzoindole heterocycles. Bd[g], as new members of the benzoindole family, are new building units to construct carbazole isomer phosphorescent molecules besides Bd[f].

Mechanochemical Synthesis of Molecular Chemoreceptors

The design of environmentally friendly methods for synthesizing molecular receptors is an expanding area within applied organic chemistry. This work systematically summarizes advances in the mechanochemical synthesis of molecular chemoreceptors. It discusses key achievements related to the synthesis of chemoreceptors containing azine, Schiff base, thiosemicarbazone, hydrazone, rhodamine 6G, imide, or amide moieties. Additionally, it highlights the application potential of mechanochemically synthesized molecular chemoreceptors in the recognition of ions and small molecules, along with a discussion of the mechanisms of detection processes.

Selective Oxyfunctionalization of Benzylic C-H with No Solvent

The direct selective oxyfunctionalization of C-H into C═O represents a highly useful, yet challenging, synthetic methodology. Herein, a one-step oxyfunctionalization of benzylic C-H into aryl ketone, with no overoxidation of the -OH functional group, is reported through mechanochemistry. The substrate scope is also tolerant of a wide range of different functional groups, providing a particularly sustainable yet widely adaptable route for the synthesis of aryl ketones, which represent both a classic synthetic precursor and a useful strategy for lignin monomer valorization. A series of mechanistic and spectroscopic investigations were also conducted to shed light on the unique C-H over -OH selectivity, opening up new avenues for oxidation chemistry.

Ball-Milling-Enabled Nickel-Catalyzed Reductive 1,4-Alkylarylation of 1,3-Enynes under an Air Atmosphere

A ball-mill-enabled nickel-catalyzed 1,4-alkylarylation of 1,3-enynes with organic bromides has been developed, offering a versatile method for assembling tetrasubstituted allenes. This approach, the first of ball-milling-based remote radical coupling, overcomes the limitations of traditional solution-phase methods, such as the need for air- and moisture-sensitive reagents, the use of bulk solvents, and prolonged reaction times. Given the outstanding performance of ball-milling-based radical reduction coupling reactions, we anticipate further advancements in sustainable and efficient synthetic methodologies.

Mechanochemical Activation of Aryl Diazonium Salts: Synthesis of Polycyclic (Hetero)Aromatics

Although the synthesis of polycyclic (hetero)aromatics via the [4 + 2] benzannulation process has been thoroughly explored, the restricted availability of energy sources (including thermal, light, and electrical energy) mandates the utilization of substantial quantities of organic solvents, inevitably leading to environmental pollution, resource wastage, and low reaction efficiency. Herein, we report a new method for the synthesis of polycyclic (hetero)aromatics from diazonium salts and alkynes under ball-milling conditions. This mechanochemical approach requires only substoichiometric amounts of DMSO as a liquid-assisted grinding additive and furnishes the desired product in a short time.

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.

Intermediates in Mechanochemical Reactions

Mechanochemical reactions offer methodological and environmental advantages for chemical synthesis, constantly attracting attention within the scientific community. Besides unmistakable sustainability advantages, the conditions under which mechanochemical reactions occur, namely solventless conditions, sometimes facilitate the isolation of otherwise labile or inaccessible products. Despite these advantages, limited knowledge exists regarding the mechanisms of these reactions and the types of intermediates involved. Nevertheless, in an expanding number of cases, ex situ and in situ monitoring techniques have allowed for the observation, characterization, and isolation of reaction intermediates in mechanochemical transformations. In this Minireview, we present a series of examples in which reactive intermediates have been detected in mechanochemical reactions spanning organic, organometallic, inorganic, and materials chemistry. Many of these intermediates were stabilized by non-covalent interactions, which played a pivotal role in guiding the chemical transformations. We believe that by uncovering and understanding such instances, the growing mechanochemistry community could find novel opportunities in catalysis and discover new mechanochemical reactions while achieving simplification in chemical reaction design.

Synthesis of bioactive evodiamine and rutaecarpine analogues under ball milling conditions

Mechanochemical reactions achieved by processes such as milling and grinding are promising alternatives to traditional solution-based chemistry. This approach not only eliminates the need for large amounts of solvents, thereby reducing waste generation, but also finds applications in chemical and materials synthesis. The focus of this study is on the synthesis of quinazolinone derivatives by ball milling, in particular evodiamine and rutaecarpine analogues. These compounds are of interest due to their diverse bioactivities, including potential anticancer properties. The study examines the reactions carried out under ball milling conditions, emphasizing their efficiency in terms of shorter reaction times and reduced environmental impact compared to conventional methods. The ball milling reaction of evodiamine and rutaecarpine analogues resulted in yields of 63-78% and 22-61%, respectively. In addition, these compounds were tested for their cytotoxic activity, and evodiamine exhibited an IC50 of 0.75 ± 0.04 μg mL-1 against the Ca9-22 cell line. At its core, this research represents a new means to synthesise these compounds, providing a more environmentally friendly and sustainable alternative to traditional approaches.

Mechanophotocatalysis: A Generalizable Approach to Solvent-minimized Photocatalytic Reactions for Organic Synthesis

This proof-of-concept study cements the viability and generality of mechanophotocatalysis, merging mechanochemistry and photocatalysis to enable solvent-minimized photocatalytic reactions. We demonstrate the transmutation of four archetypal solution-state photocatalysis reactions to a solvent-minimized environment driven by the combined actions of milling, light, and photocatalysts. The chlorosulfonylation of alkenes and the pinacol coupling of aldehydes and ketones were conducted under solvent-free conditions with competitive or superior efficiencies to their solution-state analogues. Furthermore, decarboxylative alkylations are shown to function efficiently under solvent-minimized conditions, while the photoinduced energy transfer promoted [2+2] cycloaddition of chalcone experiences a significant initial rate enhancement over its solution-state variant. This work serves as a platform for future discoveries in an underexplored field: validating that solvent-minimized photocatalysis is not only generalizable and competitive with solution-state photocatalysis, but can also offer valuable advantages.

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.

Allylation of C-, N-, and O-Nucleophiles via a Mechanochemically-Driven Tsuji-Trost Reaction Suitable for Late-Stage Modification of Bioactive Molecules

We present the first solvent-free, mechanochemical protocol for a palladium-catalyzed Tsuji-Trost allylation. This approach features exceptionally low catalyst loadings (0.5 mol %), short reaction times (<90 min), and a simple setup, eliminating the need for air or moisture precautions, making the process highly efficient and environmentally benign. We introduce solid, nontoxic, and easy-to-handle allyl trimethylammonium salts as valuable alternative to volatile or hazardous reagents. Our approach enables the allylation of various O-, N-, and C-nucleophiles in yields up to 99 % even for structurally complex bioactive compounds, owing to its mild conditions and exceptional functional group tolerance.

Allylation of C-, N-, and O-Nucleophiles via a Mechanochemically-Driven Tsuji-Trost Reaction Suitable for Late-Stage Modification of Bioactive Molecules

We present the first solvent-free, mechanochemical protocol for a palladium-catalyzed Tsuji-Trost allylation. This approach features exceptionally low catalyst loadings (0.5 mol %), short reaction times (<90 min), and a simple setup, eliminating the need for air or moisture precautions, making the process highly efficient and environmentally benign. We introduce solid, nontoxic, and easy-to-handle allyl trimethylammonium salts as valuable alternative to volatile or hazardous reagents. Our approach enables the allylation of various O-, N-, and C-nucleophiles in yields up to 99 % even for structurally complex bioactive compounds, owing to its mild conditions and exceptional functional group tolerance.

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.

Anionic ring-opening polymerization of functional epoxide monomers in the solid state

Despite recent advancements in mechanochemical polymerization, understanding the unique mechanochemical reactivity during the ball milling polymerization process still requires extensive investigations. Herein, solid-state anionic ring-opening polymerization is used to synthesize polyethers from various functional epoxide monomers. The critical parameters of the monomers are investigated to elucidate the unique reactivity of ball milling polymerization. The controllable syntheses of the desired polyethers are characterized via NMR, GPC, and MALDI-ToF analyses. Interestingly, bulky monomers exhibit faster conversions in the solid-state in clear contrast to that observed for solution polymerization. Particularly, a close linear correlation is observed between the conversion of the ball milling polymerization and melting point of the functional epoxide monomers, indicating melting point as a critical predictor of mechanochemical polymerization reactivity. This study provides insights into the efficient design and understanding of mechanochemical polymerization.

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.

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.

Palladium-Catalyzed Cross-Electrophile Coupling between Aryl Diazonium Salt and Aryl Iodide/Diaryliodonium Salt in H2O-EtOH

We report herein a mild highly chemoselective palladium-catalyzed cross-electrophile coupling between readily accessible aromatic diazonium salt and aryl iodide or diaryliodonium salt in water-ethanol (2:1) medium. Mechanistic studies revealed that ethanol is crucial to generate an active Pd(0) catalyst, and the counterion of the diazonium salt renders a cationic Pd(II) species that facilitates subsequent oxidative addition to aryl iodide/diaryliodonium salt. Silver(I) salt was crucial to retain the catalytic activity of palladium, removing the iodide ion as a precipitate.

The impact of the physical state and the reaction phase in the direct mechanocatalytic Suzuki-Miyaura coupling reaction

The direct mechanocatalytic Suzuki-Miyaura coupling reaction, utilizing palladium milling balls as active catalysts, was investigated regarding the physical state of the reagents and the reaction phase. The substitution patterns and functional groups of different aryl iodides were varied and different boronic acid derivates were utilized to achieve a wide range of substrate combinations. In the neat grinding experiments, liquid aryl iodides were more reactive than solid ones and a steric influence of the substituents, especially pronounced in ortho compounds, was observed. In order to overcome the general low reactivity of the solid phase, several liquid-assisted grinding experiments were conducted and the influence of substrate solubility and catalyst wettability analyzed. Among all LAG additives, EtOH showed the greatest impact on the reactivity, as it converts boronic acid derivatives into liquid and reactive esters under mechanochemical conditions, significantly speeding up the reaction.

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.

Mechanical Activation of Zero-Valent Metal Reductants for Nickel-Catalyzed Cross-Electrophile Coupling

The cross-electrophile coupling of either twisted-amides or heteroaryl halides with alkyl halides, enabled by ball-milling, is herein described. The operationally simple nickel-catalyzed process has no requirement for inert atmosphere or dry solvents and delivers the corresponding acylated or heteroarylated products across a broad range of substrates. Key to negating the necessity of inert reaction conditions is the mechanical activation of the raw metal terminal reductant: manganese in the case of twisted amides and zinc for heteroaryl halides.

Temperature‐Controlled Mechanochemistry for the Nickel‐Catalyzed Suzuki–Miyaura‐Type Coupling of Aryl Sulfamates via Ball Milling and Twin‐Screw Extrusion.**

The nickel catalyzed Suzuki-Miyaura-type coupling of aryl sulfamates and boronic acid derivatives enabled by temperature-controlled mechanochemistry via the development of a programmable PID-controlled jar heater is reported. This base-metal-catalyzed, solvent-free, all-under-air protocol was also scaled 200-fold using twin-screw extrusion technology affording decagram quantities of material.

Mechanochemistry: New Tools to Navigate the Uncharted Territory of "Impossible" Reactions

Mechanochemical transformations have made chemists enter unknown territories, forcing a different chemistry perspective. While questioning or revisiting familiar concepts belonging to solution chemistry, mechanochemistry has broken new ground, especially in the panorama of organic synthesis. Not only does it foster new "thinking outside the box", but it also has opened new reaction paths, allowing to overcome the weaknesses of traditional chemistry exactly where the use of well-established solution-based methodologies rules out progress. In this Review, the reader is introduced to an intriguing research subject not yet fully explored and waiting for improved understanding. Indeed, the study is mainly focused on organic transformations that, although impossible in solution, become possible under mechanochemical processing conditions, simultaneously entailing innovation and expanding the chemical space.

The Direct Mechanocatalytic Suzuki-Miyaura Reaction of Small Organic Molecules

The molecular Suzuki cross-coupling reaction was conducted mechanochemically, without solvents, ligands, or catalyst powders. Utilizing one catalytically active palladium milling ball, products could be formed in quantitative yield in as little as 30 min. In contrast to previous reports, the adjustment of milling parameters led to the complete elimination of abrasion from the catalyst ball, thus enabling the first reported systematic catalyst analysis. XPS, in situ XRD, and reference experiments provided evidence that the milling ball surface was the location of the catalysis, allowing a mechanism to be proposed. The versatility of the approach was demonstrated by extending the substrate scope to deactivated and even sterically hindered aryl iodides and bromides.

Palladium-catalyzed solid-state borylation of aryl halides using mechanochemistry

This study describes the solid-state palladium-catalyzed cross-coupling between aryl halides and bis(pinacolato)diboron using ball milling. The reactions were completed within 10 min for most aryl halides to afford a variety of synthetically useful arylboronates in high yields. Notably, all experimental operations could be performed in air, and did not require the use of large amounts of dry and degassed organic solvents. The utility of this method was further demonstrated by gram-scale synthesis under solvent-free, mechanochemical conditions.

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".

Manipulating Reaction Energy Coordinate Landscape of Mechanochemical Diaza-Cope Rearrangement

Chiral vicinal diamines, a unique class of optically-active building blocks, play a crucial role in material design, pharmaceutical, and catalysis. Traditionally, their syntheses are all solvent-based approaches, which make organic solvent an indispensable part of their production. As part of our program aiming to develop chemical processes with reduced carbon footprints, we recently reported a highly practical and environmentally-friendly synthetic route to chiral vicinal diamines by solvent-free mechanochemical diaza-Cope rearrangement. We herein showed that a new protocol by co-milling with common laboratory solid additives, such as silica gel, can significantly enhance the efficiency of the reaction, compared to reactions in the absence of additives. One possible explanation is the Lewis acidic nature of additives that accelerates a key Schiff base formation step. Reaction monitoring experiments tracing all the reaction species, including reactants, intermediates, and product, suggested that the reaction profile is distinctly different from ball-milling reactions without additives. Collectively, this work demonstrated that additive effect is a powerful tool to manipulate a reaction pathway in mechanochemical diazo-Cope rearrangement pathway, and this is expected to find broad interest in organic synthesis using mechanical force as an energy input.

Mechanochemical Nucleophilic Substitution of Alcohols via Isouronium Intermediates

An expansion of the solvent-free synthetic toolbox is essential for advances in the sustainable chemical industry. Mechanochemical reactions offer a superior safety profile and reduced amount of waste compared to conventional solvent-based synthesis. Herein a new mechanochemical method was developed for nucleophilic substitution of alcohols using fluoro-N,N,N',N'-tetramethylformamidinium hexafluorophosphate (TFFH) and K₂ HPO₄ as an alcohol-activating reagent and a base, respectively. Alcohol activation and reaction with a nucleophile were performed in one milling jar via reactive isouronium intermediates. Nucleophilic substitution with amines afforded alkylated amines in 31-91 % yields. The complete stereoinversion occurred for the S_N 2 reaction of (R)- and (S)-ethyl lactates. Substitution with halide anions (F^- , Br^- , I^- ) and oxygen-centered (CH₃ OH, PhO^- ) nucleophiles was also tested. Application of the method to the synthesis of active pharmaceutical ingredients has been demonstrated.

Solid-State C-N Cross-Coupling Reactions with Carbazoles as Nitrogen Nucleophiles Using Mechanochemistry

The palladium-catalyzed solid-state C-N cross-coupling of carbazoles with aryl halides via a high-temperature ball-milling technique has been reported. This reaction allowed simple, fast, and efficient synthesis of N-arylcarbazole derivatives in good to excellent yields without the use of large amounts of organic solvents in air. Importantly, the developed solid-state coupling approach enabled the cross-coupling of poorly soluble aryl halides with large polyaromatic structures that are barely reactive under conventional solution-based conditions.

Mechanochemical Organocatalysis: Do High Enantioselectivities Contradict What We Might Expect?

Ball mills input energy to samples by pulverising the contents of the jar. Each impact on the sample or wall of the jar results in an instantaneous transmission of energy in the form of a temperature and pressure increase (volume reduction). Conversely, enantioselective organocatalytic reactions proceed through perceived delicate and well-organised transition states. Does there exist a dichotomy in the idea of enantioselective mechanochemical organocatalysis? This Review provides a survey of the literature reporting the combination of organocatalytic reactions with mechanochemical ball milling conditions. Where possible, direct comparisons of stirred in solution, stirred neat and ball milled processes are drawn with a particular focus on control of stereoselectivity.

Insight into the Reactivity Profile of Solid-State Aryl Bromides in Suzuki-Miyaura Cross-Coupling Reactions Using Ball Milling

Despite recent advances in solid-state organic synthesis using ball milling, insight into the unique reactivity of solid-state substrates, which is often different from that in solution, has been poorly explored. In this study, we investigated the relationship between the reactivity and melting points of aryl halides in solid-state Suzuki-Miyaura cross-coupling reactions and the effect of reaction temperature on these processes. We found that aryl halides with high melting points showed significantly low reactivity in the solid-state cross-coupling near room temperature, but the reactions were notably accelerated by increasing the reaction temperature. Given that the reaction temperature is much lower than the melting points of these substrates, the acceleration effect is most likely ascribed to the weakening of the intermolecular interactions between the substrate molecules in the solid-state. The present study provides important perspectives for the rational design of efficient solid-state organic transformations using ball milling.

Oxidative Intramolecular C–C Bond Formation Reactions of 1,2-Diarylbenzenes: Syntheses of Highly Conjugated Double-Bridged Polycyclic Aromatic Hydrocarbons

Oxidation reactions of 1,2-diarylbenzenes induce intramolecular C–C bond formation. The substrates studied were prepared by the stepwise Suzuki–Miyaura coupling reaction that introduced 2-naphthyl, 2-anthranyl, and 2-pyrenyl groups on the ortho-positions of benzene. The subsequent oxidation reaction with FeCl3 induced an oxidative C–C bond formation reaction in the interior regions of the molecules. In marked contrast to our previous observations, two C–C bonds were formed. Theoretical calculations indicated that large spin densities at the reaction positions of the bis(cation radical) and/or cation radical species are needed for the C–C bond formation. The π-expanded molecules obtained showed bathochromic shifts in the absorption spectra.

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.

Mechanochemical Iridium(III)-Catalyzed B-Amidation of o-Carboranes with Dioxazolones

Mechanochemistry was successfully applied to the functionalization of carboranes. The mechanochemical iridium(III)-catalyzed regioselective B(3)- and B(4)-amidation of unsubstituted o-carboranes with dioxazolones was developed. In addition, the mechanochemical iridium(III)-catalyzed regioselective B(4)-amidation of substituted o-carboranes was demonstrated. Because mechanochemical B-amidation proceeds smoothly without organic solvents or external heating, the present method is regarded as a sustainable and environmentally friendly surrogate for typical solvent-based reactions.

Mechanochemical Synthesis of 1,2-Diketoindolizine Derivatives from Indolizines and Epoxides Using Piezoelectric Materials

A simple and efficient mechanochemical-induced approach for the synthesis of 1,2-diketoindolizine derivatives has been developed. BaTiO₃ was used as the piezoelectric material in this transformation. This method features no usage of solvent, simple experimental operation, scalable potential, and high conversion efficiency, which make it attractive and practical.

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.