C−H Bond Functionalization by Mechanochemistry

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.

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

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

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.

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.

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.

Exploring the role of solvent polarity in mechanochemical Knoevenagel condensation: in situ investigation and isolation of reaction intermediates

Mechanochemistry has proven to be a highly effective method for the synthesis of organic compounds. We studied the kinetics of the catalyst-free Knoevenagel reaction between 4-nitrobenzaldehyde and malononitrile, activated and driven by ball milling. The reaction was investigated in the absence of solvents (neat grinding) and in the presence of solvents with different polarities (liquid-assisted grinding). The reaction was monitored using time-resolved in situ Raman spectroscopy and powder X-ray diffraction (PXRD). Our results indicate a direct relationship between solvent polarity and reaction kinetics, with higher solvent polarity leading to faster product (2-(4-nitrobenzylidone)malononitrile) formation. For the first time, we were able to isolate and determine the structure of an intermediate 2-(hydroxy(4-nitrophenyl)methyl)malononitrile based on PXRD data.

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.

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.

Manganese-catalyzed hydroarylation of multiple bonds

Transition metal-catalyzed C-H activation has become a promising strategy in organic synthesis due to its improved atom-, step- and resource economy. Considering the Earth's abundance, economic benefits, and low toxicity, 3d metal catalysts for C-H activation have received a significant focus. In particular, organometallic manganese-catalyzed C-H activation has proven to be versatile and suitable for a wide range of transformations such as C-H addition to π-components, arylation, alkylation, alkynylation, amination, and many more. Among them, manganese-catalyzed C-H addition to C-C and C-heteroatom multiple bonds exhibited unique and promising reactivity to construct a wide range of complex organic molecules. In this review, we highlight the developments in the field of manganese-catalyzed hydroarylation of multiple bonds via C-H activation with a range of applications until August 2022.

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

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

Advancing mechanochemical synthesis by combining milling with different energy sources

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

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.

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 halogenation of unsymmetrically substituted azobenzenes

The direct and selective mechanochemical halogenation of C–H bonds in unsymmetrically substituted azobenzenes using N-halosuccinimides as the halogen source under neat grinding or liquid-assisted grinding conditions in a ball mill has been described. Depending on the azobenzene substrate used, halogenation of the C–H bonds occurs in the absence or only in the presence of Pd^II catalysts. Insight into the reaction dynamics and characterization of the products was achieved by in situ Raman and ex situ NMR spectroscopy and PXRD analysis. A strong influence of the different 4,4’-substituents of azobenzene on the halogenation time and mechanism was found.

Cocrystal Formation Precedes the Mechanochemically Acetate-Assisted C-H Activation with [Cp*RhCl2 ]2

This work reports the experimentally studied mechanochemical formation of rhodacycles by ball milling pyridine- and quinoline-derived substrates and [Cp*RhCl₂ ]₂ in the presence of NaOAc. Ex-situ analysis of the mechanochemical reactions using powder X-ray diffraction (PXRD), solid-state UV-vis spectroscopy and ATR-FTIR spectroscopy revealed the formation of unexpected cocrystals between the substrates and the rhodium dimer prior to the C-H activation step. This sequence of events differs from the generally accepted steps in solution in which cleavage of [Cp*RhCl₂ ]₂ is initiated by acetate ions. Additionally, the mechanochemical approach enabled the synthesis of the six-membered rhodacycle [Cp*Rh(2-benzilpyridine)Cl], a metal complex repeatedly reported as inaccessible in solution. Altogether, the results of this investigation clarify some of the fundamental aspects of mechanochemical cyclometallations.

C−H Methylation Using Sustainable Approaches

C−H methylation of sp2 and sp3 carbon centers is significant in many biological processes. Methylated drug candidates show unique properties due to the change in solubility, conformation and metabolic activities. Several photo-catalyzed, electrochemical, mechanochemical and metal-free techniques that are widely utilized strategies in medicinal chemistry for methylation of arenes and heteroarenes have been covered in this review.

Mechanochemical Ruthenium-Catalyzed Ortho-Alkenylation of N-Heteroaryl Arenes with Alkynes under Ball-Milling Conditions

The mechanochemical, solvent-free Ru(II)-catalyzed alkenylation of N-heteroaryl arenes with alkynes has been successfully described. A wide spectrum of arenes bearing N-heteroaryl moieties such as imidazo[1,2-a]pyridine, imidazo[1,2-a]pyrimidine, benzo[d]imidazo[2,1-b]thiazole, imidazo[2,1-b]thiazole, 2H-indazole, 1H-indazole, 1H-pyrazole, and 1,2,4-oxadiazol-5(4H)-one as a directing group reacted with various substituted alkynes under ball milling in the presence of [Ru(p-cymene)Cl₂]₂, affording dialkenylated products in moderate to good yields. The reaction of 2,3-dihydrophthalazine-1,4-dione with 1-phenyl-1-propyne afforded a monoalkenylated product. Similarly, reaction of 2-phenylimidazo[1,2-a]pyridine with aliphatic terminal alkynes produced a monoalkenylated derivative as the major product along with minor amount of dialkenylated product. The developed method exhibited excellent functional group compatibility, broad substrate scope, shorter reaction times, and no external heating. Moreover, the method can be readily scaled-up as demonstrated by gram-scale synthesis of 2-(2,6-bis((E)1-phenylprop-1-en-2-yl)phenyl)imidazo[1,2-a]pyridine.

Efficient access to materials-oriented aromatic alkynes via the mechanochemical Sonogashira coupling of solid aryl halides with large polycyclic conjugated systems

Sonogashira coupling represents an indispensable tool for the preparation of organic materials that contain C(sp)-C(sp2) bonds. Improving the efficiency and generality of this methodology has long been an important research subject in materials science. Here, we show that a high-temperature ball-milling technique enables the highly efficient palladium-catalyzed Sonogashira coupling of solid aryl halides that bear large polyaromatic structures including sparingly soluble substrates and unactivated aryl chlorides. In fact, this new protocol provides various materials-oriented polyaromatic alkynes in excellent yield within short reaction times in the absence of bulk reaction solvents. Notably, we synthesized a new luminescent material via the mechanochemical Sonogashira coupling of poorly soluble Vat Red 1 in a much higher yield compared to those obtained using solution-based conditions. The utility of this method was further demonstrated by the rapid synthesis of a fluorescent metal-organic framework (MOF) precursor via two sequential mechanochemical Sonogashira cross-coupling reactions. The present study illustrates the great potential of Sonogashira coupling using ball milling for the preparation of materials-oriented alkynes and for the discovery of novel functional materials.

Mechanochemistry for the synthesis of non-classical N-metalated palladium(II) pincer complexes

The peculiarities of cyclopalladation of a series of non-classical pincer-type ligands based on monothiooxalyl amides bearing ancillary N- or S-donor groups in the amide units have been scrutinized both under conditions of conventional solution-based synthesis and in the absence of a solvent according to a solid-phase methodology including mechanochemical activation. Grinding the functionalized monothiooxamides with PdCl₂(NCPh)₂ in a mortar or vibration ball mill is shown to serve as an efficient and green alternative to the synthesis of these complex metal-organic systems in solution that can offer such advantages as the absence of any auxiliary and significant rate and yield enhancement, especially for the challenging ligands. The realization of S,N,N- or S,N,S-monoanionic tridentate coordination in the resulting pincer complexes has been confirmed by multinuclear NMR (including 2D NMR) and IR spectroscopy and, in some cases, X-ray diffraction. The course and outcome of the solid-phase reactions have been studied by a combination of different spectroscopic methods as well as SEM/EDS analysis. The preliminary evaluation of cytotoxic activity against several human cancer cell lines has revealed the high potency of some of the cyclopalladated derivatives obtained, rendering further development of solvent-free synthetic routes to this type of complexes very urgent.

Solvent-Minimized Synthesis of 4CzIPN and Related Organic Fluorophores via Ball Milling

The mechanochemical synthesis of 2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile and related organic fluorophores/photocatalysts via a solvent-minimized four-fold S_NAr pathway is herein described. Employing sodium tert-butoxide as base, and negating the need for any air/moisture-sensitive reaction set-ups, a selection of organic dyes was synthesized in just 1 h using this ball-milling technique. Furthermore, the transformation was then showcased on a multigram scale.

Synthesis of Organosulfur and Related Heterocycles under Mechanochemical Conditions

In the last few decades, ball-milling has received tremendous attention as a "green tool" for conducting various challenging organic transformations under transition-metal-free and solvent-free conditions. Organosulfur and related heterocycles are ubiquitous in numerous biologically active molecules with potential applications, and those molecules could be synthesized from readily available starting materials under mechanochemical conditions without using any hazardous chemical or solvent. This synopsis highlights the green strategies developed in recent times to synthesize organosulfur and related heterocycles under ball-milling conditions.

A Ball-Milling-Enabled Cross-Electrophile Coupling

The nickel-catalyzed cross-electrophile coupling of aryl halides and alkyl halides enabled by ball-milling is herein described. Under a mechanochemical manifold, the reductive C-C bond formation was achieved in the absence of bulk solvent and air/moisture sensitive setups, in reaction times of 2 h. The mechanical action provided by ball milling permits the use of a range of zinc sources to turnover the nickel catalytic cycle, enabling the synthesis of 28 cross-electrophile coupled products.

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.

Mechanochemistry: A Green Approach in the Preparation of Pharmaceutical Cocrystals

Mechanochemistry is considered an alternative attractive greener approach to prepare diverse molecular compounds and has become an important synthetic tool in different fields (e.g., physics, chemistry, and material science) since is considered an ecofriendly procedure that can be carried out under solvent free conditions or in the presence of minimal quantities of solvent (catalytic amounts). Being able to substitute, in many cases, classical solution reactions often requiring significant amounts of solvents. These sustainable methods have had an enormous impact on a great variety of chemistry fields, including catalysis, organic synthesis, metal complexes formation, preparation of multicomponent pharmaceutical solid forms, etc. In this sense, we are interested in highlighting the advantages of mechanochemical methods on the obtaining of pharmaceutical cocrystals. Hence, in this review, we describe and discuss the relevance of mechanochemical procedures in the formation of multicomponent solid forms focusing on pharmaceutical cocrystals. Additionally, at the end of this paper, we collect a chronological survey of the most representative scientific papers reporting the mechanochemical synthesis of cocrystals.

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 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.

Mechanical metal activation for Ni-catalyzed, Mn-mediated cross-electrophile coupling between aryl and alkyl bromides

Liquid-assisted grinding enables nickel-catalyzed, manganese-mediated cross-electrophile coupling between aryl and alkyl bromides under chemical activator-free and non-anhydrous conditions.

When phosphoryl azide meets mechanochemistry: clean, rapid, and efficient synthesis of phosphoryl amides under B(C6F5)3 catalysis in a ball mill

We described herein the first example associated with B(C6F5)3-catalyzed preparation of phosphoryl amides under mechanochemical conditions.

Mechanochemical Synthesis of 1,2,6-Thiadiazine 1-Oxides from Sulfonimidamides and the Fluorescence Properties of the Products

A solvent-free mechanochemical synthesis for 1,2,6-thiadiazine 1-oxides starting from NH-sulfonimidamides and propargyl ketones has been developed. Lewis acids affect these one-pot aza-Michael-addition/cyclization/dehydration reaction sequences. The photophysical properties of the resulting heterocyclic sulfonimidamide derivatives were characterized.

Mechanochemical Preparation of Active Pharmaceutical Ingredients Monitored by In Situ Raman Spectroscopy

The mechanochemical preparation of silver sulfadiazine and dantrolene, two marketed active pharmaceutical ingredients, was investigated by in situ Raman spectroscopy. For the first time, the mechanochemical transformations involving highly fluorescent compounds could be studied in situ with a high-resolution Raman system combined with a unique suitable Raman probe. Moreover, the kinetic features of the mechanochemical process were examined by a mathematical model allowing to describe the chemical changes under mechanical stress. This approach is promising both to broaden the scope of Raman in situ investigations that would otherwise be impossible and for process optimization at any scale.