Application of a novel Deep Eutectic Solvent as a capable and new catalyst for the synthesis of tetrahydropyridines and 1,3-thiazolidin-4-ones

A novel Deep Eutectic Solvent (ETPP-Br/THF-TCA-DES) was prepared by a mixture of ethyl triphenylphosphonium bromide (MTPP-Br) and tetrahydrofuran-2,3,4,5-tetra-carboxylic acid (THF-TCA, mole ratio 7:3) in a cheap, simple, and non-toxic method, characterized by FT-IR, TGA/DTA, densitometer, eutectic point, and 1 H NMR techniques and used as a capable and new catalyst for the synthesis of two sets of compounds: i) the four new [a(1-4)] and the ten [a(5-15)] known alkyl 1,2,6-trisubstituted-4-[(hetero)arylamino]-1,2,5,6-tetrahydropyridine-3-carboxyl-ates, and ii) the two new [b(1-2)] and the eight [b(3-10)] known 1,3-thiazolidin-4-ones in solvent-free conditions, short reaction time, high yields, and easy recycling and separation of the DES catalyst. There is a nice consistency between the proposed structure of the DES compound, the integration values of the 1 H NMR peaks and the ratio of ETPP-Br to THF-TCA obtained from the eutectic point phase diagram. Also, the decrease in splitting patterns of the peaks in DES, compared to the two starting materials can be a good evidence of hydrogen bond formation between the two components.