Switching Chemoselectivity: Using Mechanochemistry to Alter Reaction Kinetics

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.

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 %).

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.

Mechanochemistry Frees Thiourea Dioxide (TDO) from the 'Veils' of Solvent, Exposing All Its Reactivity

The synthesis of nitrogen-based heterocycles has always been considered essential in developing pharmaceuticals in medicine and agriculture. This explains why various synthetic approaches have been proposed in recent decades. However performing as methods, they often imply harsh conditions or the employment of toxic solvents and dangerous reagents. Mechanochemistry is undoubtedly one of the most promising technologies currently used for reducing any possible environmental impact, addressing the worldwide interest in counteracting environmental pollution. Following this line, we propose a new mechanochemical protocol for synthesizing various heterocyclic classes by exploiting thiourea dioxide (TDO)'s reducing proprieties and electrophilic nature. Simultaneously exploiting the low cost of a component of the textile industry such as TDO and all the advantages brought by a green technique such as mechanochemistry, we plot a route towards a more sustainable and eco-friendly methodology for preparing heterocyclic moieties.

The Mechanochemical Fries Rearrangement: Manipulating Isomer Ratios in the Synthesis of p-Hydroxyacetophenone at Different Scales

Herein, the first mechanochemical Fries rearrangement for the industrially important synthesis of para-hydroxyacetophenone, inside a ball mill and a twin-screw extruder, is presented. Our approach leads to a yield of 62 % in as little as 90 minutes while liquid-assisted grinding can shift the isomer ratio resulting in an excess of the desired para-product. The multigram scale-up by extrusion leads to 61 % yield in only three minutes while completely avoiding solvents. The extrusion temperature can even further be reduced by combining extrusion with a subsequent ageing step.

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.

Mechanochemical Access to a Short-Lived Cyclic Dimer Pd2 L2 : An Elusive Kinetic Species En Route to Molecular Triangle Pd3 L3 and Molecular Square Pd4 L4

Pd-based molecular square Pd₄ L₄ and triangle Pd₃ L₃ represent the molecular ancestors of metal-coordination polyhedra that have been an integral part of the field for the last 30 years. Conventional solution-based reactions between cis-protected Pd ions and 2,2'-bipyridine exclusively give Pd₄ L₄ and/or Pd₃ L₃ as the sole products. We herein show that, under solvent-free mechanochemical conditions, the self-assembly energy landscape can be thermodynamically manipulated to form an elusive cyclic dimer Pd₂ L₂ for the first time. In the absence of solvent, Pd₂ L₂ is indefinitely stable in the solid-state, but converts rapidly to its thermodynamic products Pd₃ L₃ and Pd₄ L₄ in solution, confirming Pd₂ L₂ as a short-lived kinetic species in the solution-based self-assembly process. Our results highlight how mechanochemistry grants access to a vastly different chemical space than available under conventional solution conditions. This provides a unique opportunity to isolate elusive species in self-assembly processes that are too reactive to both "see" and "capture".

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

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

Mechanochemical Solvent-Free Suzuki-Miyaura Cross-Coupling of Amides via Highly Chemoselective N-C Cleavage

Although cross-coupling reactions of amides by selective N-C cleavage are one of the most powerful and burgeoning areas in organic synthesis due to the ubiquity of amide bonds, the development of mechanochemical, solid-state methods remains a major challenge. Herein, we report the first mechanochemical strategy for highly chemoselective, solvent-free palladium-catalyzed cross-coupling of amides by N-C bond activation. The method is conducted in the absence of external heating, for short reaction time and shows excellent chemoselectivity for σ N-C bond activation. The reaction shows excellent functional group tolerance and can be applied to late-stage functionalization of complex APIs and sequential orthogonal cross-couplings exploiting double solventless solid-state methods. The results extend mechanochemical reaction environments to advance the chemical repertoire of N-C bond interconversions to solid-state environmentally friendly mechanochemical methods.

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.

Solvent-Free Mechanochemical Post-Polymerization Modification of Ionic Polymers

Despite their superior stability and facile handling, ionic polymers have limited solubility in most organic solvents, restricting the range of substrates and reaction conditions to which they can be applied. To overcome this solubility issue, the present study presents a solvent-free mechanochemical reaction. Specifically, a post-polymerization modification of ammonium-functionalized polyether was demonstrated using a solvent-free vibrational ball-milling technique. The formation of imine bonds between the ionic polymer and an aromatic aldehyde led to the complete conversion to imine within 1 h without any bond breakage on the polymer backbone. The viability of this approach for a wide range of aldehydes was also evaluated, highlighting the potential of the mechanochemical post-polymerization modification of polymers that are inaccessible by conventional solution approaches.

A Case Study on the Desired Selectivity in Solid-State Mechano- and Slow-Chemistry, Melt, and Solution Methodologies

Solution-based syntheses are omnipresent in chemistry but are often associated with obvious disadvantages, and the search for new mild and green synthetic methods continues to be a hot topic. Here, comparative studies in four different reaction media were conducted, that is, the solid-state mechano- and slow-chemistry synthesis, melted phase, and solution protocols, and the impact of the employed solvent-free solid-state versus liquid-phase synthetic approaches was highlighted on a pool of products. A moderately exothermic model reaction system was chosen based on bis(pentafluorophenyl)zinc, (C₆ F₅ )₂ Zn, and 2,2,6,6-tetramethylpiperidinyl oxide (TEMPO) as a stable nitroxyl radical, anticipating that these reagents may offer a unique landscape for addressing kinetic and thermodynamic aspects of wet and solvent-free solid-state processes. In a toluene solution two distinct paramagnetic Lewis acid-base adducts (C₆ F₅ )₂ Zn(η¹ -TEMPO) (1) and (C₆ F₅ )₂ Zn(η¹ -TEMPO)₂ (2) equilibrated, but only 2 was affordable by crystallization. In turn, crystallization from the melt was the only method yielding single crystals of 1. Moreover, the solid-state approaches were stoichiometry sensitive and allowed for the selective synthesis of both adducts by simple stoichiometric control over the substrates. Density functional theory (DFT) calculations were carried out to examine selected structural and thermodynamic features of the adducts 1 and 2. Compound 2 is a unique non-redox active metal complex supported by two nitroxide radicals, and the magnetic studies revealed weak-to-moderate intramolecular antiferromagnetic interactions between the two coordinated TEMPO molecules.

Direct Amidation of Esters by Ball Milling*

The direct mechanochemical amidation of esters by ball milling is described. The operationally simple procedure requires an ester, an amine, and substoichiometric KOtBu and was used to prepare a large and diverse library of 78 amide structures with modest to excellent efficiency. Heteroaromatic and heterocyclic components are specifically shown to be amenable to this mechanochemical protocol. This direct synthesis platform has been applied to the synthesis of active pharmaceutical ingredients (APIs) and agrochemicals as well as the gram-scale synthesis of an active pharmaceutical, all in the absence of a reaction solvent.

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.

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.

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.

Porous Molecular Capsules as Non-Polymeric Transducers of Mechanical Forces to Mechanophores

Mechanical grinding/milling can be regarded as historically the first technology for changing the properties of matter. Mechanically activated molecular units (mechanophores) can be present in various structures: polymers, macromolecules, or small molecules. However, only polymers have been reported to effectively transduce energy to mechanophores, which induces breakage of covalent bonds. In this paper, a second possibility is presented-molecular capsules as stress-sensitive units. Mechanochemical encapsulation of fullerenes in cystine-based covalent capsules indicates that complexation takes place in the solid state, despite the fact that the capsules do not possess large enough entrance portals. By using a set of solvent-free MALDI (sf-MALDI) and solid-state NMR (ss-NMR) experiments, it has been proven that encapsulation proceeds during milling and in this process hydrazones and disulfides get activated for breakage, exchange, and re-forming. The capsules are porous and therefore prone to collapse under solvent-free conditions and their conformational rigidity promotes the collapse by the breaking of covalent bonds.

N-Heterocyclic Carbene Acyl Anion Organocatalysis by Ball-Milling

The ability to conduct N-heterocyclic carbene-catalysed acyl anion chemistry under ball-milling conditions is reported for the first time. This process has been exemplified through applications to intermolecular-benzoin, intramolecular-benzoin, intermolecular-Stetter and intramolecular-Stetter reactions including asymmetric examples and demonstrates that this mode of mechanistically complex organocatalytic reaction can operate under solvent-minimised conditions.

Mechanosynthesis of Odd-Numbered Tetraaryl[n]cumulenes

A mechanochemical synthesis of one-dimensional carbon allotrope carbyne model compounds, namely tetraaryl[n]cumulenes (n=3, 5) was realized. Central for the mechanosynthesis of the cumulenic carbon nanostructures were the development of a mechanochemical Favorskii alkynylation-type reaction and the implementation of a solvent-free, acid-free reductive elimination with tin(II) chloride by ball milling.