Synthesis of Novel Calix[4]arenes Having Benzothiazolylacetamidoalkoxy Pendants and Their Potential Application as Ag+-selective Electrodes

Abstract

Calix[4]arenes, a class of macrocyclic compounds, are superb supramolecular building block for fabricating ion and molecular receptors. Appropriately functionalized calix[4]arenes have been used as carriers in ion selective membranes. Predominantly, calixarenes containing ester, ether, carboxylic acid and carbamate have been reported for using as ion-selective electrode (ISE) for alkali metal ions. Only few calixarene derivatives reported by Reinhoudt and Kim have been employed as ISE for heavy metal ions such as Cu, Hg and Cd. Kimura and coworkers reported calix[4]arene derivatives incorporating substituents such as allyl, benzyl and propargyl groups being used as soft neutral carrier for silver ion sensors. Mercaptobenzothiazoleappended calix[4]arene was used as Ag-ISE which showed the selectivity towards Ag with good discrimination against Hg. We are interested in the development of new Ag receptors for use as ionophores in Ag-selective electrodes. Herein, we report syntheses of compounds 9 and 10 bearing two benzothiazolylacetamidobutoxy groups at the narrow rim of p-tert-butylcalix[4]arene and compounds 11 and 12 consisting of benzothiazolylacetamidoethoxy groups on the narrow rim of calix[4]arene. The preliminary studies using the synthesized compounds for Ag-selective electrodes towards metal ions were carried out to see the effect of substituent groups on sensitivity and selectivity of the ionophore. Benzothiazolylacetamidoalkoxy calix[4]arenes, 9-12, have been prepared in two steps as shown in Scheme 1. The first step involved the conversion of amine groups in calix[4]arene derivatives 1-4 into corresponding cyanoacetamido groups. Using modified Salol procedure, the conversion of compound 1 to compound 5 (50%) was achieved by heating 1 with ethyl cyanoacetate. However, compounds 2-4 could not react with ethyl cyanoacetate to produce compounds 6-8. Therefore, succinimide cyanoacetate was employed instead. The cyanoacetate ester containing the succinimide group was prepared from reaction of cyanoacetic acid with Nhydroxysuccinimide in the presence of dicyclohexylcarbodiimide in dry ethyl acetate. The amines 2-4 were reacted with succinimide cyanoacetate in dry CH2Cl2 to give cyanoacetamido compounds 6-8 in 19%, 34% and 40% yields, respectively. The cyanoacetamido compounds 5-8 were then heated with 2-aminothiophenol to give the final products 912 in 59%, 64%, 64% and 20% yields, respectively. All of the compounds showed characteristic peaks of ptert-butylcalix[4]arene (or calix[4]arene) building blocks and were in cone conformations which were substantiated by two doublets of the Ar-CH2-Ar protons (J = 13-14 Hz) around 3-4 ppm in H NMR spectra. The benzothiazolylacetamido groups showed two doublets and two triplets of the benzene proton of the benzothiazolyl group, one singlet for the proton in the alpha position of the carbonyl group and