The power of mechanochemistry in the chemical transformation of carbohydrates

Abstract

In recent decades, green chemistry and the rise of new, cheaper devices have focused on chemical transformations initiated by physical stress. Humans have, for millennia, harnessed mechanochemical phenomena in processes like grinding and forging without a complete understanding of their chemical aspects. The field of mechanochemistry, devoted to elucidating these processes, has emerged much more recently. Mechanochemical transformations occur as a result of mixed physical effects. Although not all physical phenomena are suitable for chemical transformation, many chemical transformations caused by physical stress are rarely unavoidable. Although cavitation (the tiny implosion of microbubbles) has been a harmful phenomenon for a long time, it has recently been found suitable in chemical transformations. Cavitation can be induced not only by fluid flow but also by ultrasound or laser light. In this review, we provide a brief overview of the historical development and everyday role of the field of mechanochemistry. We present the theoretical background of the change of chemical bonds under physical impact, which sheds light on the limitations of simulations of mechanochemical transformations from the point of view of theoretical chemistry, reaction kinetics and reaction mechanisms. Complex instrumentation is not necessarily required for mechanochemical transformations because the limitations of liquid and solid phase reactions and the most commonly used devices demonstrate that simpler alternatives exist. Many mechanochemical transformations occur in a hidden way due to physical forces. Mechanochemical syntheses are generally well-scalable or convertible into continuous production, and these properties of the mechanochemical transformations can significantly reduce development and production costs. Carbohydrate chemical transformations require sophisticated methods that are often complex, but the application of mechanochemistry can simplify or make them greener. Cavitation technologies accelerate reactions, but the radicals generated by cavitation can also cause undesirable side reactions. However, the radical formation via cavitation can also be favourable, eliminating the need for conventional radical-forming compounds, as demonstrated in the case of the phenol-in reaction. Reaction mechanism studies of solid phase reactions are more complex because online reaction monitoring and sampling are challenging. Solvent-free synthesis, often implemented using grinding or extrusion, increases the green chemical nature of the process and can reduce purification costs. In solvent-free reactions, classical side reactions in solution are absent or limited, which can increase the efficiency of syntheses requiring high purity. The mechanochemical preparation of simple organic molecules is not uncommon, but the study of such oligo- and polysaccharides is still in the experimental phase. The physical impact is most often limited to the efficient production of nanoparticles. We have demonstrated the usability of mechanochemical methods in polyand oligosaccharide transformations, especially in cyclodextrin derivatisation. Preparing cyclodextrin derivatives is often challenging because of their limited solubility and difficulties achieving a desired regio- and stereoselectivity. Mechanochemical activation can exploit the different reactivity of the three types of hydroxyls, which can be advantageous in regio- and stereoselective syntheses. The preparation of various cyclodextrin polymers is a good example of how the cross-linking reaction is not only more efficient, but the isolation of the products is also more effective in solvent-free reactions. Extrusion has been known for two hundred years, but the organic chemical reactions occurring during the process are less studied. Because complex systems are in organic material extrusion, such as food production, synthetic reactions are less studied, and research focuses primarily on degradation processes. The mechanochemistry in carbohydrate conversion is gaining ground and will likely play an even more influential role shortly, exploiting the advantages of green chemistry.