Triazolated calix[4]arenes from 2-azidoethylated precursors: is there a difference in the way the triazoles are attached to narrow rims?

Abstract

A series of cone calix[4]arenes modified with 2-azidoethyl groups at one, two (distal or proximal), and four positions of their narrow rims was prepared. These compounds were studied as the azide components of copper(I)-catalyzed azide–alkyne cycloaddition (CuAAC) with arylacetylenes and aryl propargyl ethers. The study showed that relatively mild or much stronger conditions need to be applied for the complete conversion of the azides into the triazoles, and this depended on the number and mutual arrangement of 2-azidoethyl groups at the narrow rims of calix[4]arenes as well as on the alkyne type selected. Under the optimized conditions a series of calixarenes bearing 2-(4-R-1-triazolyl)ethyl or 2-(4-ROCH₂-1-triazolyl)ethyl (R = phenyl, 2-naphthyl) groups at the narrow rims was prepared, and some bis- and tetrakis(triazoles) from the series were studied for their complexation with transition metal cations. The same complexation study was performed for the related calixarenes bearing (1-R-4-triazolyl)methyl (R = 2-naphthylmethyl) substituents. The triazolated calixarenes of both types were efficient at binding the cations, but the complexation preferences and the cation-binding modes were found to depend drastically on the side by which the triazole units were attached to the calixarene core.