Macrocyclic receptor molecules with a pendant carboxylic acid group for the complexation of urea

The synthesis of 5,5′‐di‐ tert ‐butyl‐3‐(carboxymethyl)biphenyl crown ethers and several lariat ethers with pendant carboxylic acid groups, together with the determinations of the p K a values of these crown ether carboxylic acids, are described. In addition, the complexation of urea by these crown ethers has been investigated. Both the larger‐ring biphenyl crown ethers (≥ 29 ring atoms) and lariat ethers (≥ 27 ring atoms) solubilized urea in chloroform. The data for urea complexation by the lariat ethers show that, for assistance in urea complexation, it is important for the carboxylic acid group to be located close to the macrocyclic ring. When the carboxylic acid group of the lariat ethers is located in an appropriate position, urea is complexed by 21–27% of these macrocyclic ligands, as is also the case with the biphenyl crown‐ether carboxylic acids. These results demonstrate that a pendant carboxylic acid group can assist in the complexation of a neutral molecule.