Calix[4]arenes, molecular platforms for supramolecular structures

A new strategy for the synthesis of receptor molecules comprises the combination of medium-sized molecules to which functional groups for intermolecular interactions can be attached. Most efforts concern the functionalization of calix[4]arenes, but other building blocks like cyclodextrins and octols are used as well. Selective mono-, 1,2-di, 1,3-di-, and tetraalkylation of calix[4]arenes can be achieved by variation of the alkylation conditions. The calix[4]arenes can be (selectively) functionalized at the “upper rim” by a variety of methods including ipso-nitration. Combination of the calix[4]arenes with crown ethers to calixcrowns gave receptors with a high K+/Na+-selectivity and with terphenyls calixspherands were obtained that form kinetically stable complexes with Na+, K+ and Rb+. Bridging of calix[4]arene with a salophene moiety gave a receptor for urea that can effectively transport urea through a supported liquid membrane. Calix[4]arenes can be combined covalently to double and triple calixarenes and with selectively functionalized octols. When substituted with hydrogen bond donors and acceptors like 2-pyridone, calixarenes undergo self-association to larger aggregrates.