Polyhalogenated Bicyclo[1.1.1]pentane-1,3-dicarboxylic Acids

Herein we report the highly selective radical chlorination of 2,2-difluorobicyclo[1.1.1]­pentane-1,3-dicarboxylic acid. Together with radical hydrodechlorination by TMS3SiH, four new bicyclo[1.1.1]­pentane cages carrying two fluorine and one to three chlorine atoms in bridge positions have been obtained. The exact positions of all halogen atoms have been confirmed by X-ray diffraction. The acidity constants (pKa) for all new derivatives have been determined by capillary electrophoresis, and these experimental values show excellent agreement with pKas predicted by DFT methods. Extensive DFT calculations have been used to rationalize the selective formation of four out of nine possible F2Cl1–4 isomers of bridge-halogenated bicyclo[1.1.1]­pentanes and to obtain relative strain energies for all possible isomers.