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 TMS<sub>3</sub>SiH, direct chlorination provides access to five of the 15 possible chlorinated diacids. Their configuration has been established by X-ray diffraction. Their p<i>K</i><sub>a</sub> 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.