Double-Brønsted acidic deep eutectic solvents and their applications as solvents and catalysts in chemical transformations

• Novel Double Brønsted-acidic Deep Eutectic Solvents (DESs) were realized. • DESs were realized with a predictive approach based on theoretical melting curves. • Their DESs identity was confirmed via theoretical/experimental melting points comparison. • The novel DESs were proficiently used for aldol condensation reaction. • The novel DESs were used for indole synthesis from alkynes without any other catalyst. The number of novel Deep Eutectic Solvents (DESs) liquids reported in literature is increasing in recent years thanks to the high number of H-bond capable molecules that possess the features to give stable liquids when mixed in binary systems. In this work a predictive approach based on the theoretical melting curves crossing points was used to generate a novel class of DESs: double Brønsted-acidic DESs. These liquids were identified as DESs thanks to the comparison of the experimental melting points with the theoretical curves and with ionic conductivity measures. Considering that the catalytic properties of the DESs are correlated with the properties of the forming molecules, the novel DESs were tested as acidic reaction media in model reactions. In the Claisen-Schmidt transformation, the double-acidic DESs gave excellent results. The indole synthesis from alkynes involved for the first time to our knowledge the use of simple Brønsted-acid catalysis without any metal or any other catalyst.