Thermal Transformations of Cubane Derivatives

The thermal degradation of the cubane moiety in 1,4-cubanedicarboxylic acid, its dimethyl ester, and 1,4-dibromocubane in a condensed state were studied by calorimetry. The kinetic parameters and the composition of products, as determined by ¹H and ¹³C NMR spectroscopy, are consistent with a biradical reaction mechanism. The presence of carboxyl groups capable of resonance conjugation stabilizes a biradical and increases the rate of isomerization of the acid and ester by a factor of 20, as compared with that of cubane. Bromine as a substituent has almost no effect on the rate of cubane thermolysis. The primary biradical is rapidly isomerized to corresponding cyclooctatetraene derivatives. A detailed reaction scheme is presented, and the relative rates of individual steps of the process are considered.