Synthesis of Novel Anion Recognition Molecules as Quinazoline Precursors

Thiourea and structurally related urea derivatives are widely recognised for their ability to transport anions through hydrogen bonding interactions. The strength of these interactions correlates with the electronegativity of the ligand and the acidity of the NH hydrogens involved. Thiourea, being more acidic than urea, exhibits partial deprotonation in the presence of certain anions such as organic carboxylates, fluoride, and bromide, while remaining resistant to deprotonation by chloride. This behaviour suggests a degree of selectivity toward chloride ions. Additionally, while carbamide-containing molecules tend to aggregate—potentially reducing their ion-binding efficiency—thiourea derivatives show reduced aggregation, preserving their binding capabilities. In this study, we report the synthesis and characterisation of 21 novel thiourea derivatives obtained by reacting 2-aminobenzoylamino acid esters with various substituted phenyl isothiocyanates. Seven similar thiourea-containing molecules were made as a comparison—without the amino acids—by reacting aniline with the different phenyl isothiocyanates. The reaction kinetics were found to be influenced primarily by the electronic nature of the substituents on the phenyl ring. Electron-withdrawing groups (EWGs), such as para-nitro, 3,5-bis(trifluoromethyl), and fluorine, accelerated the reaction, while electron-donating groups (EDGs), such as para-methoxy, slowed it down. Interestingly, the nature of the amino acid precursors had no significant impact on reaction time; however, reactions with aniline proceeded the fastest. Solvent choice also played a role: reactions in N,N-dimethylformamide (DMF) proceeded faster than in acetone, although with reduced yields. Consequently, reaction conditions were optimised to balance time efficiency and product yield. To evaluate the chloride ion-binding properties of the synthesised compounds, 1H NMR titration experiments were conducted in deuterated dimethyl sulfoxide (DMSO-d6). The association constants (Ka) derived from these studies revealed a clear correlation with the electronic nature of the substituents. Compounds bearing EWGs exhibited enhanced chloride binding, while those with EDGs showed diminished binding affinity. Surprisingly, the presence of amino acid moieties led to a decrease in Ka values, despite the electron-withdrawing nature of the amide groups. This suggests that steric or conformational factors may play a role in modulating binding strength. Overall, the synthesised thiourea derivatives demonstrate mild, reversible chloride ion-binding behaviour, making them promising candidates for further development as selective anion receptors. The insights gained from this study contribute to a deeper understanding of structure–activity relationships in anion-binding systems and may inform the design of future supramolecular architectures with tailored ion recognition properties.