Preparation, structure and stereodynamics of phosphorus-bridged calixarenes

The pyrolysis of several dialkylphosphate ester derivatives of calix[4]arenes yielded the same phosphorus bridged compound 7. Under the pyrolytic conditions the phosphate groups may be cleaved or intermolecularly transferred. X-ray crystallography of the bridged calixarenes 7 and 8 shows that they exist in a chiral 'flattened cone' (fc) conformation. The bridged calixarenes undergo in solution a dynamic process with a barrier of about 10.1 kcal mol^-1 for 7 and 8 and 13.1 kcal mol^-1 for 10, respectively. The dynamic processes result in enantiomerization of the systems. Pyrolysis of partially phosphorylated calix[6]arenes resulted in the formation of two products (11 and 12), each consisting of two subunits of three proximal rings bridged by a phosphate group. The rotational barriers for 11 and 12 are 14.4 and 8.8 kcal mol^-1, indicating that the bridged calix[6]arene system 12 is appreciably more flexible than 11.