Chlorinated Cubane-1,4-dicarboxylic Acids

Herein, we report radical chlorination
of cubane-1,4-dicarboxylic
acid leading preferentially to one monochlorinated cubane dicarboxylate
(ca. 70%) that is accompanied by four dichlorinated derivatives (ca.
20% in total). The exact positions of the chlorine atoms have been
confirmed by X-ray diffraction of the corresponding single crystals.
The acidity constants of all dicarboxylic acids in water were determined
by capillary electrophoresis (3.17 ± 0.04 and 4.09 ± 0.05
for monochlorinated and ca. 2.71 ± 0.05 and 3.75 ± 0.05
for dichlorinated cubanes). All chlorinated derivatives as well as
the parent diacid showed high thermal stability (decomposition above
250 °C) as documented by differential scanning calorimetry. The
probable reaction pathways leading to individual isomers were proposed,
and the energies of individual transition states and intermediates
were obtained using density functional theory calculations (B3LYP-D3BJ/6-311+G(d,p)).
The relative strain energies for all newly prepared derivatives as
well as for hypothetical hexahalogenated (fluorinated, chlorinated,
brominated, and iodinated) derivatives of cubane-1,4-dicarboxylic
acids were predicted using wavefunction theory methods. The hexafluorinated
derivative was identified as the most strained compound (57.5 kcal/mol),
and the relative strain decreased as the size of halogen atoms increased
(23.7 for hexachloro, 16.7 for hexabromo, and 4.0 kcal/mol for the
hexaiodo derivative).