Mechanistic Insights on the Mechanosynthesis of Phenytoin, a WHO Essential Medicine

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.

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.

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.

The Transformation of Inorganic to Organic Carbonates: Chasing for Reaction Pathways in Mechanochemistry

Mechanochemical reactions are solvent-free alternatives to solution-based syntheses enabling even conventionally impossible transformations. Their reaction pathways, however, usually remain unexplored within the heavily vibrating, dense milling vessels. Here, we showcase how the green organic solvent diethyl carbonate is synthesized mechanochemically from inorganic alkali carbonates and how the complementary combination of milling parameter studies, synchrotron X-ray diffraction real time monitoring, and quantum chemical calculations reveal the underlying reaction pathways. With this, reaction intermediates are identified, and chemical concepts of solution-chemistry are challenged or corroborated for mechanochemistry.

Piezoelectric harvesting of mechanical energy for redox chemistry

Much work has been done in the utilization of mechanical force to enable chemical processes. However, this process is limited to thermal- and deformation-driven reactions. In fact, the transfer of energy in mechanical reactors can be quite inefficient, with energy lost to heat and mechanical deformation. Although these losses diminish at larger scales, small-scale reactions (from a few milligrams to a kilogram) can suffer from unfavorable energy demands. Recent work has sought to harvest unused energy in mechanical reactors by converting it to a flow of electrons through the use of piezoelectric materials, as many economically important reactions rely on the transfer of electrons to enact chemical change. Recent work has shown that the addition of piezoelectric powders to mechanochemical reactions results in enhanced yields for reductive and oxidative chemistry. However, these materials ultimately contaminate the end product and must be removed. Additionally, impacts on a piezoelectric material produce an AC output; limiting this approach's usefulness to irreversible reactions. We have developed a cleaner approach using an external piezoelectric element to either supply or sink electrons during milling. Methylene blue was reduced to leucomethylene blue using our approach. Mechanochemical reaction rates for this reduction were determined with respect to media quantities and sizes with a maximum rate of 7.76 μM s^-1. It was found that the conversion rate is linearly dependent on the number of media and geometrically dependent on the size of the media. Our approach allows selective reduction and eliminates contamination of the products with piezoelectric material. Shuttling electrons in a mechanochemical reaction will enable difficult chemistry, such as the reduction of CO₂ or the production of low oxidation state inorganic compounds, to be achieved more easily.

From Inert to Catalytically Active Milling Media: Galvanostatic Coating for Direct Mechanocatalysis

The inert milling balls, commonly utilized in mechanochemical reactions, were coated with a layer of Pd and utilized as catalyst in the direct mechanocatalytic Suzuki reaction. With high yields (>80 %), the milling balls can be recycled multiple times in the absence of any solvents, ligands, catalyst-molecules and -powders, while utilizing as little as 0.8 mg of Pd per coated milling ball. The coating sequence, the support material, and the layer thickness were examined towards archiving high catalyst retention, low abrasion and high conversion. The approach was transferred to the coating of milling vessels revealing the interplay between catalytically available surface area and the mechanical energy impact in direct mechanocatalysis.

From amines to (form)amides: a simple and successful mechanochemical approach

Two easily accessible routes for preparing an array of formylated and acetylated amines under mechanochemical conditions are presented. The two methodologies exhibit complementary features as they enable the derivatization of aliphatic and aromatic amines.

Mechanistic Insights on the Mechanosynthesis of Phenytoin, a WHO Essential Medicine

In recent years, mechanochemistry has enriched the toolbox of synthetic chemists, enabling faster and more sustainable access to new materials and existing products, including active pharmaceutical ingredients (APIs). However, molecular‐level understanding of most mechanochemical reactions remains limited, delaying the implementation of mechanochemistry in industrial applications. Herein, we have applied in situ monitoring by Raman spectroscopy to the mechanosynthesis of phenytoin, a World Health Organization (WHO) Essential Medicine, enabling the observation, isolation, and characterization of key molecular‐migration intermediates involved in the single‐step transformation of benzil, urea, and KOH into phenytoin. This work contributes to the elucidation of a reaction mechanism that has been subjected to a number of interpretations over time and paints a clear picture of how mechanosynthesis can be applied and optimized for the preparation of added‐value molecules.