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.

Ball-Milling-Enabled Reactivity of Manganese Metal*

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

Mechanochemical Syntheses of N-Containing Heterocycles with TosMIC

A mechanochemical van Leusen pyrrole synthesis with a base leads to 3,4-disubstitued pyrroles in moderate to excellent yields. The developed protocol is compatible with a range of electron-withdrawing groups and can also be applied to the synthesis of oxazoles. Attempts to mechanochemically convert the resulting pyrroles into porphyrins proved to be difficult.

1,3-Bis(carboxymethyl)imidazolium Chloride as a Sustainable, Recyclable, and Metal-Free Ionic Catalyst for the Biginelli Multicomponent Reaction in Neat Condition

A simple and novel methodology has been developed for the synthesis of 1,3-bis(carboxymethyl)imidazolium chloride [BCMIM][Cl] salt. The ionic salt [BCMIM][Cl] catalyzed the reaction among arylaldehydes; the substituted 1,3-dicarbonyl compounds and urea/thiourea at 80 °C with 5 mol % under neat condition provided the substituted dihydropyrimidin-2(1H)-one/thiones in the synthesis step with yields of up to 96%. In addition, we synthesized the commercially available drug Monastrol by employing this methodology. The new synthesis method employs the benefits of a broad substrate scope, short reaction time, and high atom economy along with low catalyst loading in neat conditions, and is devoid of chromatographic purification. The ionic salt [BCMIM][Cl] was recycled and reused up to six cycles without substantial damage of its catalytic efficiency.

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

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

Oxidative Dehydrosulfurative Cross-Coupling of 3,4-Dihydropyrimidine-2-thiones with Alkynes for Access to 2-Alkynylpyrimidines

A reaction method is described for the one-step synthesis of 2-alkynylpyrimidines from 3,4-dihydropyrimidin-1H-2-thiones (DHPMs) via dehydrosulfurative Sonogashira cross-coupling with concomitant oxidative dehydrogenation using a Pd/Cu catalytic system. Together with the ready availability of DHPMs possessing various substituents at the C4-C6 positions, this transformation offers rapid and general access to diverse 2-alkynylpyrimidine derivatives.

Metal-free mechanochemical oxidations in Ertalyte® jars

Aimed at eliminating or at least significantly reducing the use of solvents, sodium hypochlorite pentahydrate crystals (NaOCl·5H2O) in the presence of a catalytic amount of a nitrosyl radical (TEMPO or AZADO) have been successfully used to induce mechanochemical oxidative processes on several structurally different primary and secondary alcohols. The proposed redox process is safe, inexpensive and performing effectively, especially on the macroscale. Herein, an Ertalyte® jar has been successfully used, for the first time, in a mechanochemical process.

Mechanochemistry of supramolecules

The urge to use alternative energy sources has gained significant attention in the eye of chemists in recent years. Solution-based traditional syntheses are extremely useful, although they are often associated with certain disadvantages like generation of waste as by-products, use of large quantities of solvents which causes environmental hazard, etc. Contrastingly, achieving syntheses through mechanochemical methods are generally time-saving, environmentally friendly and more economical. This review is written to shed some light on supramolecular chemistry and the synthesis of various supramolecules through mechanochemistry.

Ball Milling Promoted N-Heterocycles Synthesis

In the last years, numerous protocols have been published using ball milling for organic synthesis. Compared to other methods such as microwave or ultrasound irradiation and ionic liquids, ball mill chemistry is an economical, and ecofriendly method in organic synthesis that is rather underrepresented in the knowledge of organic chemists. The aim of this review is to explore the advantages of the application of ball milling in synthesis of N-heterocyclic compounds.

Multicomponent mechanochemical synthesis

Historically, the use of mechanochemical methods in synthesis has been almost negligible, but their perception by the synthetic community has changed in recent years and they are on their way to becoming mainstream. However, the hybridization of mechanochemical synthesis with methodologies designed to increase synthetic efficiency by allowing the generation of several bonds in a single operation has taken off only recently, but it already constitutes a very promising approach to sustainable chemistry. In this context, we provide in this Perspective a critical summary and discussion of the main known synthetic methods based on mechanochemical multicomponent reactions.

Towards medicinal mechanochemistry: evolution of milling from pharmaceutical solid form screening to the synthesis of active pharmaceutical ingredients (APIs)

This overview highlights the emergent area of mechanochemical reactions for making active pharmaceutical ingredients (APIs), and covers the latest advances in the recently established area of mechanochemical screening and synthesis of pharmaceutical solid forms, specifically polymorphs, cocrystals, salts and salt cocrystals. We also provide an overview of the most recent developments in pharmaceutical uses of mechanochemistry, including real-time reaction monitoring, techniques for polymorph control and approaches for continuous manufacture using twin screw extrusion, and more. Most importantly, we show how the overlap of previously unrelated areas of mechanochemical screening for API solid forms, organic synthesis by milling, and mechanochemical screening for molecular recognition, enables the emergence of a new research discipline in which different aspects of pharmaceutical and medicinal chemistry are addressed through mechanochemistry rather than through conventional solution-based routes. The emergence of such medicinal mechanochemistry is likely to have a strong impact on future pharmaceutical and medicinal chemistry, as it offers not only access to materials and reactivity that are sometimes difficult or even impossible to access from solution, but can also provide a general answer to the demands of the pharmaceutical industry for cleaner, safer and efficient synthetic solutions.

Orthogonality in main group compounds: a direct one-step synthesis of air- and moisture-stable cyclophosphazanes by mechanochemistry

Mechanochemistry has been established to be an environmentally-friendly way of conducting reactions in a solvent-free manner.

Ball-milling and cheap reagents breathe green life into the one hundred-year-old Hofmann reaction

A very efficient mechanically activated synthesis of isocyanides directly from primary amines and without extra-solvent addition has been reported.

Mechanically induced oxidation of alcohols to aldehydes and ketones in ambient air: Revisiting TEMPO-assisted oxidations

The present work addresses the development of an eco-friendly and cost-efficient protocol for the oxidation of primary and secondary alcohols to the corresponding aldehydes and ketones by mechanical processing under air. Ball milling was shown to promote the quantitative conversion of a broad set of alcohols into carbonyl compounds with no trace of an over-oxidation to carboxylic acids. The mechanochemical reaction exhibited higher yields and rates than the classical, homogeneous, TEMPO-based oxidation.

An Efficient, Eco-friendly and Sustainable One-Pot Synthesis of 3,4-Dihydropyrimidin-2(1H)-ones Directly from Alcohols Catalyzed by Heteropolyanion-Based Ionic Liquids

Efficient, eco-friendly and sustainable access to 3,4-dihydropyrimidin-2(1H)-ones directly from alcohols under microwave and solvent-free conditions has been reported. The practical protocol involves heteropolyanion-based catalyzed oxidation of alcohols to aldehydes with NaNO₃ as the oxidant followed by cyclocondensation with dicarbonyl compounds and urea or thiourea in a two-step, one-pot manner. Compatibility with different functional groups, good to excellent yields and reusable catalysts are the main highlights. The utilization of alcohols instead of aldehydes is a valid and green alternative to the classical Biginelli reaction.

Mechanochemical synthesis of small organic molecules

With the growing interest in renewable energy and global warming, it is important to minimize the usage of hazardous chemicals in both academic and industrial research, elimination of waste, and possibly recycle them to obtain better results in greener fashion. The studies under the area of mechanochemistry which cover the grinding chemistry to ball milling, sonication, etc. are certainly of interest to the researchers working on the development of green methodologies. In this review, a collection of examples on recent developments in organic bond formation reactions like carbon–carbon (C–C), carbon–nitrogen (C–N), carbon–oxygen (C–O), carbon–halogen (C–X), etc. is documented. Mechanochemical syntheses of heterocyclic rings, multicomponent reactions and organometallic molecules including their catalytic applications are also highlighted.

Fe3O4@PEG-SO3H rod-like morphology along with the spherical nanoparticles: novel green nanocomposite design, preparation, characterization and catalytic application

A new heterogeneous nanocatalyst was successfully synthesized, completely characterized and efficiently applied in the synthesis of dihydropyrimidines.