Mechanochemistry Assisted Asymmetric Organocatalysis: A Sustainable Approach

Introduction
Green chemistry involves innovation in chemical research and engineering that encourages the design of processes to minimize the use and production of hazardous materials and also reduce the use of energy. These requirements are fulfilled by preventing or minimizing the use of volatile and toxic solvents and reagents, minimizing chemical wastage, development of atom-economical processes and recyclable supported catalyst that are less toxic, biodegradable and can be used at low loading.

To address many of these issues mechanochemical methods such as ball-milling and grinding with pestle and mortar have emerged as powerful techniques. The mechanical energy generated by grinding two solids or one solid and one liquid substance results in the formation of new surfaces and cracks by breaking the order of the crystalline structure, and this results in the formation of products.

Grinding and ball-milling are widely applied to pulverize minerals into fine particles, in the preparation and modification of inorganic solids. Recently, their use in synthetic organic chemistry has increased considerably, due to the need for development of sustainable methodologies, and has been widely used in solvent-free non-asymmetric transformations.

On the other hand, demands for the development of stereoselective synthesis of organic molecules have noticeably amplified in recent times. In this regard, catalytic asymmetric synthesis involving the use of chiral organocatalysts has emerged as a powerful tool from the infancy to the maturity of asymmetric organocatalysis. The use of organocatalysts for catalysing asymmetric reactions may allow several advantages, such as lower toxicity compared to metal analogues, robustness, no requirement of an inert atmosphere, provision of high stereoselectivity, and the ability to be used for the synthesis of opposite enantiomers by using enantiomeric catalysts. Organocatalyts also provide an insight into biological catalytic processes, as a number of these catalysts work by the phenomenon of enzyme mimicry. These advantages of chiral organocatalysts also meet many of the requirements of green chemistry.

Recently developed, organocatalytic asymmetric transformations assisted by mechanochemical techniques proved to be an excellent alternative to atom-economical stereoselective transformations under solvent-free reaction conditions. This review gives an overview of the solvent-free asymmetric organocatalytic transformation assisted by mechanochemical techniques, viz. ball-milling and grinding with pestle and mortar.

This article is published online in the Beilstein Journal of Organic Chemistry and is free to access.