Bridge-Chlorinated Bicyclo[1.1.1]pentane-1,3-dicarboxylic Acids

Radical chlorination of bicyclo[1.1.1]­pentane-1,3-dicarboxylic
acid is highly selective, and up to four chlorine atoms can be introduced
relatively easily without damage to the strained bicyclic cage. Combined
with hydrodechlorination with TMS3SiH, direct chlorination
provides access to five of the 15 possible chlorinated diacids. Their
configuration has been established by X-ray diffraction. Their pKa values have been measured by capillary electrophoresis
and calculated at the B3LYP-D3BJ/6-311+G­(d,p)-level. The results are
in good agreement and reflect the expected trend, from 2.78 ±
0.08 and 4.14 ± 0.10 in the parent to 1.07 ± 0.03 and 2.31
± 0.03 in the tetrachlorinated diacid. Strain energy relative
to the parent diacid was calculated for all 15 chlorinated diacids
and shows a dramatic increase with successive chlorination, due to
nonbonded Cl–Cl repulsions.