ZnCl₂-Based Deep Eutectic Solvent as Solvent-Catalyst in the Michael Addition Reaction of Pyrrole to Maleimide

The use of deep eutectic solvents (DESs) as catalysts presents indisputable advantages, for example, their simplicity of preparation, high biodegradability, and recyclability, as well as zero toxicity and their effectiveness as environmentally friendly reaction media. However, aspects related to their reactivity and catalytic activity are still unclear. In this work, we explore the versatility of ChCl/ZnCl₂ DES in the formation of C-C bonds through the Michael-type addition of pyrrole to maleimide, where ChCl/ZnCl₂ DES leads to catalysis and chelation of the substrates, thus describing a recommended method for the construction of C-C bonds with high atomic economy. We describe experimental and theoretical aspects that explain the ability of ChCl/ZnCl₂ DES in the presence of water to act as a catalyst in the formation of C-C bonds between pyrrole and maleimide. The potential energy surface showed that the ChCl and the zinc-zincate species 2ZnCl₂·3H₂O, formed by the interaction between zinc chloride and water, decrease the relative free Gibbs energy values for all the species involved in the reaction mechanism (TSs, intermediates, product), favoring the kinetics and thermodynamics of the Michael addition.