MECHANISTIC INSIGHTS, CATALYTIC PERFORMANCE, AND SUSTAINABILITY ASSESSMENT OF WATER VERSUS DEEP EUTECTIC SOLVENTS IN ORGANOCATALYSIS

Abstract

Organocatalysis has become one of the three principal pillars of modern asymmetric catalysis, alongside metal catalysis and biocatalysis. However, the environmental sustainability of its design relies on the reaction medium, as well as the design of the catalyst. Water and deep eutectic solvents (DESs) are two of the most promising green alternatives to traditional volatile organic solvents due to their low toxicity to the environment and special physicochemical characteristics. These media, with their common sustainability goals, have different mechanistic pathways and significant advantages and disadvantages in catalytic performance. In this review, the pros and cons of the use of water and DESs as reaction media in sustainable organocatalysis activities are critically examined and compared. It analyzes the rate-enhancing hydrophobic effect and interfacial catalysis of aqueous systems as well as the solvation by the hydrogen-bond-network, pre-organization of substrates and dual solvent–catalyst functionality of DESs. They are systematically tested on representative carbon–carbon and carbon–heteroatom bond-forming reactions such as aldol, Michael, Mannich and Diels–Alder reaction with a focus on reaction yield, stereoselectivity, catalyst recyclability, and operational efficiency. Both solvent systems are additionally assessed for their sustainability measure through green chemistry parameters, LCA (life-cycle assessment) and existing data on toxicity and biodegradability. The literature surveyed shows that neither water nor DES is superior in all respects, but rather the environment and catalytic properties are very much reaction type dependent, substrate compatible and process dependent. Finally, this review pinpoints the challenges that need to be addressed in future research: comprehensive life cycle data on DESs, hydrated DES–water hybrid systems and optimisation of scale up strategies. These challenges will enable to make informed choices for solvents and contribute to the development of sustainable organocatalytic processes.