Calixphyrins. Hybrid macrocycles at the structural crossroads between porphyrins and calixpyrroles*

Calixphyrins are a class of hybrid molecules that lie at the structural crossroads between porphyrins and calixpyrroles. Porphyrins, long known for their versatile metal cation coordination chemistry, are macrocycles that contain only sp2-hybridized bridging meso carbon atoms within their framework. Calix[n]pyrroles, on the other hand, are porphyrin analogs that contain pyrroles bridged exclusively by sp3 meso carbon centers, and in recent years have been shown to display remarkable anion-binding properties. Calix[n]phyrins bear analogy to both the porphyrins and calixpyrroles and are macrocyclic analogs that contain a mixture of sp2and sp3-hybridized meso carbon bridges. This leads to partial interruptions in the conjugation pathway of the molecule, introduces novel structural features, and leads to interesting anion and cation recognition properties. It also allows for modular syntheses. In the present paper, the chemistry of calix[n]phyrins, still at an early stage of exploration, is reviewed.