Exploration of Synthesis and Coordination Chemistry of Thiazole-Containing Calix[3]Pyrroles

Abstract

Calix[n]pyrroles have been used as powerful and specific receptors and as extractants for anions and ion pairs over the past 20 years. In the recent past, calix[4]pyrroles have been used as systems that can move ions and ion pairs over lipophilic membranes [1]. Unfortunately, the coordination chemistry of calix[n]pyrroles hardly explored. The ring-contracted analog of calix[n]pyrrole, calix[3]pyrrole, was synthesized and recently published [2]. The structure of calix[3]pyrrole and its analogues is strained, and they show an appealing strain-induced ring expansion response. On the other hand, the synthesis of calix[3]pyrrole requires multistep reactions including cyclization of hexaketone precursor and Paar–Knorr synthesis with the total yield of 4%. Extended use of calix[3]pyrrole analogues necessitates further refinement of the synthesis process.Here, we demonstrate the synthesis of new calix[3]pyrroles analogue, calix[1]pyrrole[2]thiazole (3). Direct macrocyclization between bis(α-bromoketone) and 2,2-dimethylpropane (bis-thioamide) produced the calix[1]furan[2]thiazole (1), with 60% yield. The furan unit of 1 was converted into pyrrole to give calix[1]pyrrole[2]thiazole 3 (Figure 1).Single-crystal X-ray analysis revealed a cone-shaped conformation of calix[1]pyrrole[2]thiazole 3, with all three heterocycles pointing in the same direction through strong intramolecular hydrogen bonding. However, unlike calix[3]pyrrole, calix[1]pyrrole[2]thiazole was completely stable under acidic conditions, which may be due to the basic nature of the thiazolimine units.Metal coordination of calix[1]pyrrole[2]thiazole was attempted and its Zn(II), Ag(I) and Pd(II) complexes were obtained by appropriate metalation procedures. The central metal ion of the Zn(II) complex is coordinated by a conical 3 and an axial ethyl group to behave as a monoanionic tridentate ligand. In contrast, it acts as a bidentate neutral ligand for Pd(II) or Ag(I) complexation. The synthesis of meso-aza calix[1]pyrrole[2]thiazole is in progress. Figure 1