cyclophanes

Cyclophanes are a class of hydrocarbons in which two or more aromatic rings are connected by aliphatic or partially saturated linkers to form a single macrocyclic framework. The linking units force the rings into close spatial proximity, often leading to substantial through-space interactions between the aromatic rings. A well-known subset is the paracyclophanes, in which two benzene rings are held parallel and face-to-face by short bridges, such as the two ethylene bridges in [2.2]paracyclophane.

Classification of cyclophanes is based on the topology and the lengths of the bridging units. The bridges may connect at ortho, meta, or para positions on the rings, giving ortho-, meta-, or para-cyclophanes. When the two rings are linked by two or more identical short chains, the notation [n.n] paracyclophane is often used, where n indicates the number of atoms in each bridge. Unsymmetrical variants [n.m] also exist, with bridges of different lengths.

Structural and chemical features include rigid, constrained geometries and, in many cases, restricted rotation that can render certain cyclophanes chiral despite lacking stereogenic centers. The close arene–arene contacts influence reactivity, spectroscopy, and host–guest behavior, making cyclophanes useful as models for arene–arene interactions and as rigid scaffolds in supramolecular design.

Synthesis typically involves macrocyclization strategies to assemble the aromatic rings with the bridging links, followed by steps to close the macrocycle and, when desired, to introduce stereochemical control. Applications span fundamental studies of aromatic interactions, materials science, and catalysis, where the unique geometry of cyclophanes provides a distinct structural platform.