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.

Chemoenzymatic oligomerisation of l-amino acids was successfully developed using mechanochemical techniques.

The ability of mechanochemistry to alter established chemical selectivity is demonstrated. A copper(I)-catalyzed mechanochemical aldehyde/alkyne/amine coupling using calcium carbide as the acetylene source provides selective access to 1,4-diamino-2-butynes, which contrasts classical approaches that provide propargylamine-type products. Solventless milling conditions were found to be essential to unmask A coupling products with new compositions.

Mechanochemical chemoenzymatic peptide and amide bond formation catalysed by papain was studied by ball milling.

Recent progress in the field of mechanochemistry has expanded the discovery of mechanically induced chemical transformations to several areas of science. However, a general fundamental understanding of how mechanochemical reactions by ball milling occur has remained unreached. For this, we have now implemented in situ monitoring of a mechanochemically induced molecular rearrangement by synchrotron X‐ray powder diffraction, Raman spectroscopy, and real‐time temperature sensing. The results of this study demonstrate that molecular rearrangements can be accomplished in the solid state by ball milling and how in situ monitoring techniques enable the visualization of changes occurring at the exact instant of a molecular migration. The mechanochemical benzil–benzilic acid rearrangement is the focal point of the study.

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.

This work reports the experimentally studied mechanochemical formation of rhodacycles by ball milling pyridine-and quinoline-derived substrates and [Cp*RhCl 2 ] 2 in the presence of NaOAc. Ex-situ analysis of the mechanochemical reactions using powder X-ray diffraction (PXRD), solidstate UV-vis spectroscopy and ATR-FTIR spectroscopy revealed the formation of unexpected cocrystals between the substrates and the rhodium dimer prior to the CÀ H activation step. This sequence of events differs from the generally accepted steps in solution in which cleavage of [Cp*RhCl 2 ] 2 is initiated by acetate ions. Additionally, the mechanochemical approach enabled the synthesis of the six-membered rhodacycle [Cp*Rh(2-benzilpyridine)Cl], a metal complex repeatedly reported as inaccessible in solution. Altogether, the results of this investigation clarify some of the fundamental aspects of mechanochemical cyclometallations.

While the understanding of the supramolecular chemistry of steroidal hormones is largely based on receptor binding studies in vitro and in vivo, their solid-state molecular recognition properties remain unexplored. Here, we use mechanochemical cocrystallization and single crystal X-ray structure analysis to gain insight into the solid-state complexation of sex hormones with arenes, by systematic investigation of the ability of two important estrogens ß-estradiol (bes) and estrone (est) to form cocrystals with 1,2-dimethylnaphthalene, phenanthrene, anthracene, 9,10-anthraquinone, phenanthridine, benzo[h]quinoline, and perfluoronaphthalene. Cocrystallization of bes reveals the formation of a novel hydrogen-bonded lattice host, exhibiting rectangular channels occupied by arene guests. In striking contrast to bes, its 17-keto-analogue est did not yield cocrystals with any of the explored arenes except perfluoronaphthalene, revealing association via arene-perfluorarene π•••π stacking. The results reveal previously unknown solid-state complexation behavior of important estrogen hormones, demonstrating how minor changes in the steroid structure, in particular switching from a 17-hydroxyl to a 17-keto group, can result in extraordinary changes to their solid-state self-assembly. In that respect, solid-state chemistry of steroids appears to mirror their important signaling role in biological systems, as very small modifications to the steroid structure lead to large changes in cocrystallization propensity.