In situ and real-time monitoring of mechanochemical milling reactions using synchrotron X-ray diffraction

We describe the only currently available protocol for in situ, real-time monitoring of mechanochemical reactions and intermediates by X-ray powder diffraction. Although mechanochemical reactions (inducing transformations by mechanical forces such as grinding and milling) are normally performed in commercially available milling assemblies, such equipment does not permit direct reaction monitoring. We now describe the design and in-house modification of milling equipment that allows the reaction jars of the operating mill to be placed in the path of a high-energy (∼90 keV) synchrotron X-ray beam while the reaction is taking place. Resulting data are analyzed using conventional software, such as TOPAS. Reaction intermediates and products are identified using the Cambridge Structural Database or Inorganic Crystal Structure Database. Reactions are analyzed by fitting the time-resolved diffractograms using structureless Pawley refinement for crystalline phases that are not fully structurally characterized (such as porous frameworks with disordered guests), or the Rietveld method for solids with fully determined crystal structures (metal oxides, coordination polymers).

Solvent-free enantioselective Friedländer condensation with wet 1,1'-binaphthalene-2,2'-diamine-derived prolinamides as organocatalysts

Wet unsupported and supported 1,1'-binaphthalene-2,2'-diamine (BINAM) derived prolinamides are efficient organocatalysts under solvent-free conditions at room temperature to perform the synthesis of chiral tacrine analogues in good yields (up to 93%) and excellent enantioselectivies (up to 96%). The Friedländer reaction involved in this process takes place with several cyclohexanone derivatives and 2-aminoaromatic aldehydes, and it is compatible with the presence of either electron-withdrawing or electron-donating groups at the aromatic ring of the 2-aminoaryl aldehyde derivatives used as electrophiles. The reaction can be extended to cyclopentanone derivatives, affording a regioisomeric but separable mixture of products. The use of the wet silica gel supported organocatalyst, under solvent-free conditions, for this process led to the expected product (up to 87% enantiomeric excess), with its reuse being possible at least up to five times.

Mechanochemistry of fullerenes and related materials

The low or lack of solubility of fullerenes, carbon nanotubes and graphene/graphite in organic solvents and water severely hampers the study of their chemical functionalizations and practical applications. Covalent and noncovalent functionalizations of fullerenes and related materials via mechanochemistry seem appealing to tackle these problems. In this review article, we provide a comprehensive coverage on the mechanochemical reactions of fullerenes, carbon nanotubes and graphite, including dimerizations and trimerizations, nucleophilic additions, 1,3-dipolar cycloadditions, Diels-Alder reactions, [2 + 1] cycloadditions of carbenes and nitrenes, radical additions, oxidations, etc. It is intriguing to find that some reactions of fullerenes can only proceed under solvent-free conditions or undergo different reaction pathways from those of the liquid-phase counterparts to generate completely different products. We also present the application of the mechanical milling technique to complex formation, nanocomposite formation and enhanced hydrogen storage of carbon-related materials.

Solvent-free chemoselective oxidation of thioethers and thiophenes by mechanical milling

Organosulphur compounds can be easily and selectively oxidized to sulfones using a small excess of Oxone(®) (1.6 eq.) under solventless mechanical milling conditions. This green procedure has been efficiently applied to a series of model compounds and to the desulphurization of medium/high sulphur content paraffins (up to 3000 mg kg(-1)).

A Simple and Green Protocol for 2H-Indazolo[2,1-b]phthalazine-triones Using Grinding Method

A robust, facile, and solvent-free route for the three-component synthesis of 2H-indazolo[2,1-b]phthalazine-triones in the presence of a catalytic amount ofp-toluene sulfonic acid utilizing grinding has been developed. Short reaction time, simple operation, and high yields are the advantages of this protocol.

A simple and straightforward synthesis of phenyl isothiocyanates, symmetrical and unsymmetrical thioureas under ball milling

Anilines were efficiently transformed into isothiocyanates (5.0 equiv. CS₂) or thioureas (1.0 equiv. CS₂) under ball milling and promoted by KOH.

Mechanochemistry assisted asymmetric organocatalysis: A sustainable approach

Ball-milling and pestle and mortar grinding have emerged as powerful methods for the development of environmentally benign chemical transformations. Recently, the use of these mechanochemical techniques in asymmetric organocatalysis has increased. This review highlights the progress in asymmetric organocatalytic reactions assisted by mechanochemical techniques.

Real-time and in situ monitoring of mechanochemical milling reactions

Chemical and structural transformations have long been carried out by milling. Such mechanochemical steps are now ubiquitous in a number of industries (such as the pharmaceutical, chemical and metallurgical industries), and are emerging as excellent environmentally friendly alternatives to solution-based syntheses. However, mechanochemical transformations are typically difficult to monitor in real time, which leaves a large gap in the mechanistic understanding required for their development. We now report the real-time study of mechanochemical transformations in a ball mill by means of in situ diffraction of high-energy synchrotron X-rays. Focusing on the mechanosynthesis of metal-organic frameworks, we have directly monitored reaction profiles, the formation of intermediates, and interconversions of framework topologies. Our results reveal that mechanochemistry is highly dynamic, with reaction rates comparable to or greater than those in solution. The technique also enabled us to probe directly how catalytic additives recently introduced in the mechanosynthesis of metal-organic frameworks, such as organic liquids or ionic species, change the reactivity pathways and kinetics.

Mechanosynthesis of amides in the total absence of organic solvent from reaction to product recovery

The synthesis of various amides has been realised avoiding the use of any organic solvent from activation of carboxylic acids with CDI to isolation of the amides. Mechanochemistry was the key point of the process allowing rapid formation of the amide bond and efficient water-based purification of the final products.

Ag nanoparticles from the mechanochemical decomposition of Ag oxalate

The present work focuses on the chemical reactivity of Ag oxalate powders under mechanical processing conditions. The powders were submitted to mechanical loads in the presence of an aqueous solution containing a polymeric surfactant. A gradual decrease of the total mass of powders was observed, ascribable to the occurrence of a decomposition process. X-ray diffraction and UV-vis spectrophotometric analyses indicated that the Ag oxalate decomposes into metallic Ag and gaseous carbon dioxide. Transmission electron microscopy showed that metallic Ag exists in the form of particles with average size of about 5 nm. The formation of nanometer-sized Ag particles can be related to the plastic deformation and attrition processes taking place at the points of contacts between neighboring particles during the mechanical loading at collision.

Supramolecular concepts and new techniques in mechanochemistry: cocrystals, cages, rotaxanes, open metal-organic frameworks

Mechanochemical reactions effected by milling or grinding are an attractive means to conduct chemical reactions dependent on molecular recognition and to systematically explore different modes of molecular self-assembly. The natural relationship between milling mechanochemistry and supramolecular chemistry arises primarily from the ability to avoid bulk solvent, which simultaneously avoids limitations of solution-based chemistry, such as solubility, solvent complexation, or solvolysis, and makes the resulting process highly environmentally friendly. This tutorial review highlights the use of mechanochemistry for the synthesis of supramolecular targets in the solid state, such as molecular hydrogen- or halogen-bonded complexes, molecular and supramolecular cages, open frameworks and interlocked architectures. It is also demonstrated that the molecular self-assembly phenomena that are well-established in solution chemistry, such as reversible binding through covalent or non-covalent bonds, thermodynamic equilibration and structure templating, are also accessible in milling mechanochemistry through recently developed highly efficient methodologies such as liquid-assisted grinding (LAG) or ion- and liquid-assisted grinding (ILAG). Also highlighted are the new opportunities arising from the marriage of concepts of supramolecular and mechanochemical synthesis, including organocatalysis, deracemisation and discovery of new molecular recognition motifs.

Solvent-free, microwave-assisted N-arylation of indolines by using low palladium catalyst loadings

Indoline-based compounds are abundant in nature, and the indoline skeleton is an often-encountered scaffold in a range of biologically active alkaloids, pharmaceutically active compounds, and functional molecules (e.g., sensitizers for solar cells). The wide range of uses warrants further interest in the structural modification of this class of compounds. A series of substituted N-aryl indolines is prepared by a solvent-free, palladium-catalyzed procedure. The procedure requires only low loadings of catalyst, uses microwave irradiation, and starts from commercially available substrates. The method proceeds in good yields and in short reaction times with aryl bromides, chlorides, and iodides, also on 2-substituted indolines. The combination of solvent-free methods with microwave heating will further increase in importance in the search for more environmentally acceptable synthesis methods.

Alternative energy input: mechanochemical, microwave and ultrasound-assisted organic synthesis

Microwave, ultrasound, sunlight and mechanochemical mixing can be used to augment conventional laboratory techniques. By applying these alternative means of activation, a number of chemical transformations have been achieved thereby improving many existing protocols with superior results when compared to reactions performed under traditional conditions. The purpose of this critical review is to highlight the advances in this general area by presenting such newer applications in organic synthesis (175 references).

Ball-milling modification of single-walled carbon nanotubes: purification, cutting, and functionalization

Single-walled carbon nanotubes (SWNTs) can be successfully cut with relatively homogeneous sizes using a planetary mill. The optimized conditions produce highly dispersible SWNTs that can be efficiently functionalized in a variety of synthetic ways. As clearly shown by Raman spectroscopy, the milling/cutting procedure compares very favorably with the most common way of purifying SWNTs, namely, treatment with strong oxidizing acids. Moreover a similar milling process can be used to functionalize and cut pristine SWNTs by one-step nitrene chemistry.

Switchable selectivity during oxidation of anilines in a ball mill

A solvent-free method for the direct oxidation of anilines to the corresponding azo and azoxy homocoupling products by using a planetary ball mill was developed. Various oxidants and grinding auxiliaries were tested and a variety of substituted anilines were investigated. It was possible to form chemoselectively either azo, azoxy, or the nitro compounds from reaction of aromatic anilines. The selectivity of the solvent-free reaction is switchable by applying a combination of oxidant and grinding auxiliary. Furthermore, a comparison with other methods of energy input (microwave, classical heating, and ultrasound) highlighted the advantages of the ball mill approach and its high energy efficiency.