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

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 H-1 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.