From Synthesis of Amino Acids and Peptides to Enzymatic Catalysis: A Bottom-Up Approach in Mechanochemistry

Chemical and Enzymatic Mechanosynthesis of Organocatalytic Peptide Materials Based on Proline and Phenylalanine

In recent years, mechanosynthesis of peptides through either chemical or enzymatic routes has been accomplished. In part, this advancement has been driven due to the organocatalytic properties of peptide-based biomaterials. In this work, we report the merging of chemical and enzymatic protocols under mechanochemical conditions to synthesize peptide materials based on L-proline and L-phenylalanine. Compared to traditional step-by-step peptide synthesis in solution, our mechanochemical approach combining peptide coupling reagents with the proteolytic enzyme papain offers a more sustainable route by reducing the number of synthetic steps, shortening reaction times, increasing chemical yields, and minimizing waste production. Notably, the mechanosynthesized peptides exhibited organocatalytic activity in the asymmetric aldol reaction between cyclohexanone and 4-nitrobenzaldehyde.

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.

Mechanochemical Synthesis of Boroxine-linked Covalent Organic Frameworks

We report a rapid, room-temperature mechanochemical synthesis of 2- and 3-dimensional boroxine covalent organic frameworks (COFs), enabled by using trimethylboroxine as a dehydrating additive to overcome the hydrolytic sensitivity of boroxine-based COFs. The resulting COFs display high porosity and crystallinity, with COF-102 being the first example of a mechanochemically prepared 3D COF, exhibiting a surface area of ca. 2,500 m2 g-1. Mechanochemistry enabled a>20-fold reduction in solvent use and ~100-fold reduction in reaction time compared with solvothermal methods, providing target COFs quantitatively with no additional work-up besides vacuum drying. Real-time Raman spectroscopy permitted the first quantitative kinetic analysis of COF mechanosynthesis, while transferring the reaction design to Resonant Acoustic Mixing (RAM) enabled synthesis of multi-gram amounts of the target COFs (tested up to 10 g).

Preparation, design, identification and application of self-assembly peptides from seafood: A review

Hydrogels formed by self-assembling peptides with low toxicity and high biocompatibility have been widely used in food and biomedical fields. Seafood contains rich protein resources and is also one of the important sources of natural bioactive peptides. The self-assembled peptides in seafood have good functional activity and are very beneficial to human health. In this review, the sequence of seafood self-assembly peptide was introduced, and the preparation, screening, identification and characterization. The rule of self-assembled peptides was elucidated from amino acid sequence composition, amino acid properties (hydrophilic, hydrophobic and electric), secondary structure, interaction and peptide properties (hydrophilic and hydrophobic). It was introduced that the application of hydrogels formed by self-assembled peptides, which lays a theoretical foundation for the development of seafood self-assembled peptides in functional foods and the application of biological materials.

Molecular Recognition in Mechanochemistry: Insights from Solid-State NMR Spectroscopy

Molecular-recognition events are highly relevant in biology and chemistry. In the present study, we investigated such processes in the solid state under mechanochemical conditions using the formation of racemic phases upon reacting enantiopure entities as example. As test systems, α-(trifluoromethyl)lactic acid (TFLA) and the amino acids serine and alanine were used. The effects of ball-milling and resonant acoustic mixing (RAM) on the formation of racemic phases were probed by using solid-state Nuclear Magnetic Resonance (NMR) spectroscopy. In a mixer mill, a highly efficient and fast racemic phase formation occurred for both TFLA and the two amino acids. RAM led to the racemic phase for TFLA also, and this process was facilitated upon employing pre-milled enantiopure entities. In contrast, under comparable conditions RAM did not result in the formation of racemic phases for serine and alanine.

Mechanochemistry in Glycation Research

Mechanochemistry by milling has recently attracted considerable interest for its ability to drive solvent-free chemical transformations exclusively through mechanical energy and at ambient temperatures. Despite its popularity and expanding applications in different fields of chemistry, its impact on Food Science remains limited. This review aims to demonstrate the specific benefits that mechanochemistry can provide in performing controlled glycation, and in "activating" sugar and amino acid mixtures, thereby allowing for continued generation of colors and aromas even after termination of milling. The generated mechanical energy can be tuned under specific conditions either to form only the corresponding Schiff bases and Amadori compounds or to generate their degradation products, as a function of the frequency of the oscillations in combination with the reactivity of the selected substrates. Similarly, its ability to initiate the Strecker degradation and generate pyrazines and Strecker aldehydes was also demonstrated when proteogenic amino acids were milled with glyoxal.

Mechanochemical Synthesis of Corannulene Flanked N-heterocyclic Carbene (NHC) Precursors and Preparation of Their Metal Complexes

The synthesis of new compounds is an important pillar for the advancement of the field of chemistry and adjacent fields. In this regard, over the last decades huge efforts have been made to not only develop new molecular entities but also more efficient sustainable synthetic methodologies due to the increasing concerns over environmental sustainability. In this context, we have developed synthetic routes to novel corannulene flanked imidazolium bromide NHC precursors both in the solid-state and solution phases. Our work presents a comprehensive comparative study of mechanochemical routes and conventional solution-based methods. Green metrics and energy consumption comparison were performed for both routes revealing ball-milling generation of these compounds to be an environmentally greener technique to produce such precursors compared to conventional solvent-based methods. In addition, we have demonstrated proof-of-concept of the herein reported corannulene flanked NHCs to be robust ligands for transition metals and their ligand substitution reactions.

Calcium Carbide (CaC2) as a C2-Synthon by Mechanochemistry

Mechanochemical reactions by ball milling have opened new avenues in chemical synthesis. Particularly, mechanochemistry has facilitated the reaction of insoluble materials to simplify stablished synthetic protocols and develop new ones. One notable application involves the use of calcium carbide (CaC2) as a C2-synthon through mechanochemistry, which has offered a more practical alternative to incorporate C2-units compared to the conventional use of highly flammable gaseous acetylene. For example, by ball milling, the acetylenic anions [C2]2- found in CaC2 have been harnessed for the synthesis of diverse functional carbon materials as well as discrete organic molecules. This Concept aims to contribute to the conceptualization of this innovative approach while highlighting both its advantages and the challenges inherent in the use of CaC2.

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

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

Organic Reactions Enabled by Mechanical Force-Induced Single Electron Transfer

Mechanochemical reactions, achieved through milling, grinding, or other mechanical actions, have emerged as a solvent-free alternative to traditional solution-based chemistry. Mechanochemistry not only provides the opportunity to eliminate bulk solvent use, reducing waste generation, but also unveils a new reaction strategy which enables the realization of reactions previously inaccessible in solution. While the majority of organic reactions facilitated by mechanical force traditionally follow two-electron transfer pathways similar to their solution-based counterparts, the field of mechanochemically induced single-electron transfer (SET) reactions has witnessed rapid development. This review outlines examples of mechanochemical reactions facilitated by the SET process, focusing on the reagents that initiate SET, thereby positioning mechanochemistry as a burgeoning field within the realm of single-electron chemistry.

Solvent-Free Mechanosynthesis of Oligopeptides by Coupling Peptide Segments of Different Lengths - Elucidating the Role of Cesium Carbonate in Ball Mill Processes

We report an idea for the synthesis of oligopeptides using a solvent-free ball milling approach. Our concept is inspired by block play, in which it is possible to construct different objects using segments (blocks) of different sizes and lengths. We prove that by having a library of short peptides and employing the ball mill mechanosynthesis (BMMS) method, peptides can be easily coupled to form different oligopeptides with the desired functional and biological properties. Optimizing the BMMS process we found that the best yields we obtained when TBTU and cesium carbonate were used as reagents. The role of Cs2CO3 in the coupling mechanism was followed on each stage of synthesis by 1H, 13C and 133Cs NMR employing Magic Angle Spinning (MAS) techniques. It was found that cesium carbonate acts not only as a base but is also responsible for the activation of substrates and intermediates. The unique information about the BMMS mechanism is based on the analysis of 2D NMR data. The power of BMMS is proved by the example of different peptide combinations, 2+2, 3+2, 4+2, 5+2 and 4+4. The tetra-, penta-, hexa-, hepta- and octapeptides obtained under this project were fully characterized by MS and NMR techniques.

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.

Process Mass Intensity (PMI): A Holistic Analysis of Current Peptide Manufacturing Processes Informs Sustainability in Peptide Synthesis

Small molecule therapeutics represent the majority of the FDA-approved drugs. Yet, many attractive targets are poorly tractable by small molecules, generating a need for new therapeutic modalities. Due to their biocompatibility profile and structural versatility, peptide-based therapeutics are a possible solution. Additionally, in the past two decades, advances in peptide design, delivery, formulation, and devices have occurred, making therapeutic peptides an attractive modality. However, peptide manufacturing is often limited to solid-phase peptide synthesis (SPPS), liquid phase peptide synthesis (LPPS), and to a lesser extent hybrid SPPS/LPPS, with SPPS emerging as a predominant platform technology for peptide synthesis. SPPS involves the use of excess solvents and reagents which negatively impact the environment, thus highlighting the need for newer technologies to reduce the environmental footprint. Herein, fourteen American Chemical Society Green Chemistry Institute Pharmaceutical Roundtable (ACS GCIPR) member companies with peptide-based therapeutics in their portfolio have compiled Process Mass Intensity (PMI) metrics to help inform the sustainability efforts in peptide synthesis. This includes PMI assessment on 40 synthetic peptide processes at various development stages in pharma, classified according to the development phase. This is the most comprehensive assessment of synthetic peptide environmental metrics to date. The synthetic peptide manufacturing process was divided into stages (synthesis, purification, isolation) to determine their respective PMI. On average, solid-phase peptide synthesis (SPPS) (PMI ≈ 13,000) does not compare favorably with other modalities such as small molecules (PMI median 168-308) and biopharmaceuticals (PMI ≈ 8300). Thus, the high PMI for peptide synthesis warrants more environmentally friendly processes in peptide manufacturing.

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

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

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 %).

Controllable Synthesis, Formation Mechanism, and Photocatalytic Activity of Tellurium with Various Nanostructures

Telluriums (Te) with various nanostructures, including particles, wires, and sheets, are controllably synthesized by adjusting the content of polyvinylpyrrolidone (PVP) in a facile solvothermal reaction. Te nanostructures all have complete grain sizes with excellent crystallinity and mesopore structures. Further, the formation mechanisms of Te nanostructures are proposed to be that the primary nuclei of Te are released from the reduction of TeO32- using N2H4·H2O, and then grow into various nanostructures depending on the different content of PVP. These nanostructures of Te all exhibit the photocatalytic activities for the degradation of MB and H2 production under visible light irradiation, especially Te nanosheets, which have the highest efficiencies of degradation (99.8%) and mineralization (65.5%) at 120 min. In addition, compared with pure Te nanosheets, the rate of H2 production increases from 412 to 795 μmol∙h-1∙g-1 after the introduction of Pt, which increases the output by nearly two times. The above investigations indicate that Te with various nanostructures is a potential photocatalyst in the field of degradation of organic pollutants and H2 fuel cells.

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

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

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

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

Mechanochemical 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.

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.

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.

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.

Supramolecular Peptide Nanostructures Regulate Catalytic Efficiency and Selectivity

We report three constitutionally isomeric tetrapeptides, each comprising one glutamic acid (E) residue, one histidine (H) residue, and two lysine (KS ) residues functionalized with side-chain hydrophobic S-aroylthiooxime (SATO) groups. Depending on the order of amino acids, these amphiphilic peptides self-assembled in aqueous solution into different nanostructures:nanoribbons, a mixture of nanotoroids and nanoribbons, or nanocoils. Each nanostructure catalyzed hydrolysis of a model substrate, with the nanocoils exhibiting the greatest rate enhancement and the highest enzymatic efficiency. Coarse-grained molecular dynamics simulations, analyzed with unsupervised machine learning, revealed clusters of H residues in hydrophobic pockets along the outer edge of the nanocoils, providing insight for the observed catalytic rate enhancement. Finally, all three supramolecular nanostructures catalyzed hydrolysis of the l-substrate only when a pair of enantiomeric Boc-l/d-Phe-ONp substrates were tested. This study highlights how subtle molecular-level changes can influence supramolecular nanostructures, and ultimately affect catalytic efficiency.

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.

Mechanochemical solid state single electron transfer from reduced organic hydrocarbon for catalytic aryl-halide bond activation

Solid-state radical generation is an attractive but underutilized methodology in the catalytic strong bond activation process, such as the aryl-halide bond. Traditionally, such a process of strong bond activation relied upon the use of transition metal complexes or strongly reducing photocatalysts in organic solvents. The generation of the aryl radical from aryl halides in the absence of transition-metal or external stimuli, such as light or cathodic current, remains an elusive process. In this study, we describe a reduced organic hydrocarbon, which can act as a super reductant in the solid state to activate strong bonds by solid-state single electron transfer (SSSET) under the influence of mechanical energy leading to a catalytic strategy based on the mechano-SSSET or mechanoredox process. Here, we investigate the solid-state synthesis of the super electron donor phenalenyl anion in a ball mill and its application as an active catalyst in strong bond (aryl halide) activation. Aryl radicals generated from aryl halides by employing this strategy are competent for various carbon-carbon bond-forming reactions under solvent-free and transition metal-free conditions. We illustrate this approach for partially soluble or insoluble polyaromatic arenes in accomplishing solid-solid C-C cross-coupling catalysis, which is otherwise difficult to achieve by traditional methods using solvents.

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.

Complementary mechanochemical and biphasic approaches for the synthesis of organic thiocyanates using hexacyanoferrates as non-toxic cyanide sources

Herein, we describe two complementary approaches towards various organic thiocyanates that are affordable, reliable and follow the principles of sustainable chemistry, starting from commercially available thiols or disulfides. Additionally, the application of this mild method to the first total synthesis of psammaplin B is demonstrated. Non-toxic and inexpensive ferricyanide is used as the cyanide source, which can be activated either in a mechanochemical, solvent-free approach, or in a biphasic solvent system allowing easier work-up. A total of 27 examples is demonstrated, with up to quantitative yields.

Solid-State Enzymatic Hydrolysis of Mixed PET/Cotton Textiles

Waste polyester textiles are not recycled due to separation challenges and partial structural degradation during use and recycling. Chemical recycling of polyethylene terephthalate (PET) textiles through depolymerization can provide a feedstock of recycled monomers to make "as-new" polymers. While enzymatic PET recycling is a more selective and more sustainable approach, methods in development, however, have thus far been limited to clean, high-quality PET feedstocks, and require an energy-intensive melt-amorphization step ahead of enzymatic treatment. Here, high-crystallinity PET in mixed PET/cotton textiles could be directly and selectively depolymerized to terephthalic acid (TPA) by using a commercial cutinase from Humicola insolens under moist-solid reaction conditions, affording up to 30±2 % yield of TPA. The process was readily combined with cotton depolymerization through simultaneous or sequential application of the cellulase enzymes CTec2®, providing up to 83±4 % yield of glucose without any negative influence on the TPA yield.

Mechanoenzymatic reactions for the hydrolysis of PET†

Recent advances in the enzymatic degradation of poly(ethylene terphthalate) (PET) have led to a number of PET hydrolytic enzymes and mutants being developed. With the amount of PET building up in the natural world, there is a pressing need to develop scalable methods of breaking down the polymer into its monomers for recycling or other uses. Mechanoenzymatic reactions have gained traction recently as a green and efficient alternative to traditional biocatalytic reactions. For the first time we report increased yields of PET degradation by whole cell PETase enzymes by up to 27-fold by utilising ball milling cycles of reactive aging, when compared with typical solution-based reactions. This methodology leads to up to a 2600-fold decrease in the solvent required when compared with other leading degradation reactions in the field and a 30-fold decrease in comparison to reported industrial scale PET hydrolysis reactions.

Mechanoenzymatic reactions are described for the degradation of different PET materials using whole cell PETases.

Lipase-Mediated Mechanoenzymatic Synthesis of Sugar Esters in Dissolved Unconventional and Neat Reaction Systems

Mechanochemical and biocatalytic approaches in modern research are two major assets to develop greener processes. In the present study, these modular tools of sustainability are pointed toward the production of versatile and daily employed compounds such as surfactants. Toward this aim, glycolipids, a class of nonionic surfactants composed of ubiquitous and primary metabolites such as sugar and fatty acid moieties, represent a promising alternative to petroleum-derived surface-active agents. Therefore, the combination of biocatalysis with mechanochemistry aiming at glycolipid synthesis seemed a logical step that was taken in this study for the first time. The monoacylated model compound glucose-6-O-decanoate was synthesized with the help of a bead mill apparatus using two different unconventional dissolved reaction systems, namely, menthol-based hydrophobic deep eutectic solvents and 2-methyl-2-butanol, thus reaching up to 12% yield in the latter based on the conversion of vinyl decanoate, after only 90 min of reaction. In addition, a neat reaction system using an excess of vinylated fatty ester as an adjuvant allowed a 27 mM/h space-time yield. The overall significant increase in productivities, up to 6 times, compared to standard heating and shaking methods, shows the tremendous potential of mechanoenzymatic synthesis.

Iron-Catalyzed Intramolecular Arene C(sp2 )-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.

Green Strategies for the Preparation of Enantiomeric 5-8-Membered Carbocyclic β-Amino Acid Derivatives through CALB-Catalyzed Hydrolysis

Candida antarctica lipase B-catalyzed hydrolysis of carbocyclic 5−8-membered cis β-amino esters was carried out in green organic media, under solvent-free and ball-milling conditions. In accordance with the high enantioselectivity factor (E > 200) observed in organic media, the preparative-scale resolutions of β-amino esters were performed in tBuOMe at 65 °C. The unreacted β-amino ester enantiomers (1R,2S) and product β-amino acid enantiomers (1S,2R) were obtained with modest to excellent enantiomeric excess (ee) values (ees > 62% and eep > 96%) and in good chemical yields (>25%) in one or two steps. The enantiomers were easily separated by organic solvent/H2O extraction.

Mechanoenzymatic Reactions Involving Polymeric Substrates or Products

Mechanoenzymology is an emerging field in which mechanical mixing is used to sustain enzymatic reactions in low-solvent or solvent-free mixtures. Many enzymes have been reported that thrive under such conditions. Considering the central role of biopolymers and synthetic polymers in our society, this minireview highlights the use of mechanoenzymology for the synthesis or depolymerization of oligomeric or polymeric materials. In contrast to traditional in-solution reactions, solvent-free mechanoenzymology has the advantages of avoiding solubility issues, proceeding in a minimal volume, and reducing solvent waste while potentially improving the reaction efficiency and accessing new reactivity. It is expected that this strategy will continue to gain popularity and find more applications.

Towards Predicting the Sequential Appearance of Zeolitic Imidazolate Frameworks Synthesized by Mechanochemistry

We use computational materials methods to study the sequential appearance of zinc-based zeolitic imidazolate frameworks (ZIFs) generated in the mechanochemical conversion process. We consider nine ZIF topologies, namely RHO, ANA, QTZ, SOD, KAT, DIA, NEB, CAG and GIS, combined with the two ligands 2-methylimidazolate and 2-ethylimidazolate. Of the 18 combinations obtained, only six (three for each ligand) were actually observed during the mechanosynthesis process. Energy and porosity calculations based on density functional theory, in combination with the Ostwald rule of stages, were found to be insufficient to distinguish the experimentally observed ZIFs. We then show, using classical molecular dynamics, that only ZIFs withstanding quasi-hydrostatic pressure P ≥ 0.3 GPa without being destroyed were observed in the laboratory. This finding, along with the requirement that successive ZIFs be generated with decreasing porosity and/or energy, provides heuristic rules for predicting the sequences of mechanically generated ZIFs for the two ligands considered.

A Unique Mechanochemical Redox Reaction Yielding Nanostructured Double Perovskite Sr2FeMoO6 With an Extraordinarily High Degree of Anti-Site Disorder

Strontium ferromolybdate, Sr2FeMoO6, is an important member of the family of double perovskites with the possible technological applications in the field of spintronics and solid oxide fuel cells. Its preparation via a multi-step ceramic route or various wet chemistry-based routes is notoriously difficult. The present work demonstrates that Sr2FeMoO6 can be mechanosynthesized at ambient temperature in air directly from its precursors (SrO, α-Fe, MoO3) in the form of nanostructured powders, without the need for solvents and/or calcination under controlled oxygen fugacity. The mechanically induced evolution of the Sr2FeMoO6 phase and the far-from-equilibrium structural state of the reaction product are systematically monitored with XRD and a variety of spectroscopic techniques including Raman spectroscopy, 57Fe Mössbauer spectroscopy, and X-ray photoelectron spectroscopy. The unique extensive oxidation of iron species (Fe0 → Fe3+) with simultaneous reduction of Mo cations (Mo6+ → Mo5+), occuring during the mechanosynthesis of Sr2FeMoO6, is attributed to the mechanically triggered formation of tiny metallic iron nanoparticles in superparamagnetic state with a large reaction surface and a high oxidation affinity, whose steady presence in the reaction mixture of the milled educts initiates/promotes the swift redox reaction. High-resolution transmission electron microscopy observations reveal that the mechanosynthesized Sr2FeMoO6, even after its moderate thermal treatment at 923 K for 30 min in air, exhibits the nanostructured nature with the average particle size of 21(4) nm. At the short-range scale, the nanostructure of the as-prepared Sr2FeMoO6 is characterized by both, the strongly distorted geometry of the constituent FeO6 octahedra and the extraordinarily high degree of anti-site disorder. The degree of anti-site disorder ASD = 0.5, derived independently from the present experimental XRD, Mössbauer, and SQUID magnetization data, corresponds to the completely random distribution of Fe3+ and Mo5+ cations over the sites of octahedral coordination provided by the double perovskite structure. Moreover, the fully anti-site disordered Sr2FeMoO6 nanoparticles exhibit superparamagnetism with the blocking temperature T B = 240 K and the deteriorated effective magnetic moment μ = 0.055 μ B per formula unit.

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 (CO2 ) 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.

A guide to direct mechanocatalysis

Direct mechanocatalysis (DM) describes solvent-free catalytic reactions that are initiated by mechanical forces in mechanochemical reactors such as ball mills. The distinctive feature of DM is that the milling materials, e.g. the milling balls themselves are the catalyst of the reaction. In this article we follow the historical evolution of this novel concept and give a guide to this emerging, powerful synthesis tool. Within this perspective we seek to highlight the impact of the relevant milling parameters, the nature of the catalyst and potential additives, the scope of reactions that are currently accessible by this method, and the thus far raised hypotheses on the underlying mechanisms of direct mechanochemical transformations.

Mechanochemical Synthesis of Organic Dyes and Fluorophores

The syntheses of dyes and fluorophores have significant commercial importance. In recent years, mechanochemistry has emerged as a green and sustainable alternative for the synthesis of conventional dyes, new fluorophores, and also synthetic modification of known dyes for their use as chemosensors. The dyestuffs based on BODIPYs, rhodamine, fluorescein, perylenedimides, coumarins, benzothiazoles, etc. were synthesized or derivatized by grinding or milling. The synopsis aims to pay key attention to their synthesis and the applications as chemosensors will be briefly covered.

Mechanochemical Synthesis of Primary Amides

Ball milling of aromatic, heteroaromatic, vinylic, and aliphatic esters with ethanol and calcium nitride afforded the corresponding primary amides in a transformation that was compatible with a variety of functional groups and maintained the integrity of a stereocenter α to carbonyl. This methodology was applied to α-amino esters and N-BOC dipeptide esters and also to the synthesis of rufinamide, an antiepileptic drug.

Mechanochemical transformation of planar polyarenes to curved fused-ring systems

The transformation of planar aromatic molecules into π-extended non-planar structures is a challenging task and has not been realized by mechanochemistry before. Here we report that mechanochemical forces can successfully transform a planar polyarene into a curved geometry by creating new C-C bonds along the rim of the molecular structure. In doing so, mechanochemistry does not require inert conditions or organic solvents and provide better yields within shorter reaction times. This is illustrated in a 20-minute synthesis of corannulene, a fragment of fullerene C60, in 66% yield through ball milling of planar tetrabromomethylfluoranthene precursor under ambient conditions. Traditional solution and gas-phase synthetic pathways do not compete with the practicality and efficiency offered by the mechanochemical synthesis, which now opens up a new reaction space for inducing curvature at a molecular level.

Mechanoenzymology: State of the Art and Challenges towards Highly Sustainable Biocatalysis

Global awareness of the importance of developing environmentally friendlier and more sustainable methods for the synthesis of valuable chemical compounds has led to the design of novel synthetic strategies, involving bio- and organocatalysis as well as the application of novel efficient and ground-breaking technologies such as present-day solvent-free mechanochemistry. In this regard, the evaluation of biocatalytic protocols mediated by the combination of mechanical activation and enzymatic catalysis has recently attracted the attention of the chemical community. Such mechanoenzymatic strategy represents an innovative and promising "green" approach in chemical synthesis that poses nevertheless new paradigms regarding the relative resilience of biomolecules to the mechanochemical stress and to the apparent high energy, at least in so-called hot-spots, during the milling process. Herein, relevant comments on the conceptualization of such mechanoenzymatic approach as a sustainable option in chemical synthesis, recent progress in the area, and associated challenges are discussed.