Calixarene - Structure

Structure

Calixarenes are characterised by a three-dimensional basket, cup or bucket shape. In calixarenes the internal volume is around 10 cubic nanometers. Calixarenes are characterised by a wide upper rim and a narrow lower rim and a central annulus. With phenol as a starting material the 4 hydroxyl groups are intrannular on the lower rim. In a resorcinarene 8 hydroxyl groups are placed extraannular on the upper ring. Calixarenes exist in different chemical conformations because rotation around the methylene bridge is not difficult. In calixarene 4 up-down conformations exist: cone (point group C2v,C4v), partial cone Cs, 1,2 alternate C2h and 1,3 alternate D2d. The 4 hydroxyl groups interact by hydrogen bonding and stabilize the cone conformation. This conformation is in dymamic equilibrium with the other conformations. Conformations can be locked in place with proper substituents replacing the hydroxyl groups which increase the rotational barrier. Alternatively placing a bulky substituent on the upper rim also locks a conformation. The calixarene based on p-tert-butyl phenol is also a cone. Calixarenes are structurally related to the pillararenes.

Calixarene with para-tert-butyl substituents 3D representation of a cone conformation

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