A Joint Action of Deep Eutectic Solvents and Ultrasound to Promote Diels–Alder Reaction in a Sustainable Way

Abstract

The need to reduce environmental impact of chemical processes has induced a surge of attention in the choice of solvent and methodologies to carry them out. In this context, we studied the Diels–Alder reaction using N-ethylmaleimide as dienophile and changing the nature of the diene in deep eutectic solvents (DES) under both conventional heating and ultrasonic activation. DES obtained by the combination of different hydrogen bond acceptors and donors allowed assessing the role played by solvent nature. DES proved suitable solvent media for the target reaction, allowing higher yields and faster reactions compared to conventional organic solvents. The trend of yields, both in silent and under ultrasound irradiation, was rationalized considering solvent viscosity and structural organization. In particular, thanks to a combined approach of resonance light scattering (RLS) and 1H NMR investigation, a relationship between performance of the reaction and thickness and strength of the hydrogen bond network of the solvent was identified. Moreover, DES could be recycled up to eight times without any loss in yield. Using ultrasonic activation in combination with DES proved beneficial providing good yields in drastically reduced reaction times. From the standpoint of Green Chemistry, the combined use of ultrasound and DES allowed reducing energy consumption by more than 99% compared with silent reactions and proved beneficial for the scalability of the process.

Diels–Alder Cycloaddition Reactions in Sustainable Media

“…They observed that the experimental rate constants were dependent on the percentage of urea in the carbohydrate melt; however, the best correlation was found to be with the solvent viscosity, which is in agreement with previous reports by the authors [67]. The Diels-Alder reaction of 9-anthracenemethanol (54) and N-ethylmaleimide (55a) in different solvent media under conventional heating and ultrasonic activation has been reported [68]. The influence of the solvent medium in the yield of the reaction was assessed by comparing DESs, obtained by the combination of different HBAs (e.g., choline chloride, methyltriphenylphosphonium chloride, (±)-menthol) and HBDs (e.g., glycerol (Gly), ethylene glycol (EG), urea), with water and organic solvents.…”

“…The Diels-Alder reaction of 9-anthracenemethanol (54) and N-ethylmaleimide (55a) in different solvent media under conventional heating and ultrasonic activation has been reported [68]. The influence of the solvent medium in the yield of the reaction was assessed by comparing DESs, obtained by the combination of different HBAs (e.g., choline chloride, methyltriphenylphosphonium chloride, (±)-menthol) and HBDs (e.g., glycerol (Gly), ethylene glycol (EG), urea), with water and organic solvents.…”

Deep Eutectic Solvents and Multicomponent Reactions: Two Convergent Items to Green Chemistry Strategies

“…Several DESs have been tested for the Diels–Alder reaction, using N ‐ethylmaleimide as dienophile and changing the nature of the diene under both conventional heating and ultrasonic activation. Using ultrasonic activation in combination with DES proved good yields in drastically reduced reaction times [143] (Scheme 20 ).…”

How do arenediazonium salts behave in deep eutectic solvents? A combined experimental and computational approach

“…[26] Despite the increasing use of DESs in organic synthesis has already brought to their employment within the framework of important processes (e.g. aldol condensation, [28] Diels-Alder cycloaddition, [29] electrocyclization reactions, [30] nucleophilic acyl substitution [31] among others), they have never been used as solvents in the reactions of diazonium salts, excluding two very recent examples. The first describes one-pot diazotization/coupling reactions in choline chloride/tartaric acid DES; [32] the second describes the electro-grafting of arenediazonium salts on carbon surfaces in the presence of ethaline.…”