Preparation, Structure, and Properties of Symmetrically 1,3-Difunctionalized Penta- and Hexafluorobicyclo[1.1.1]pentanes

Exhaustive direct fluorination of dimethyl bicyclo[1.1.1]pentane-1,3-dicarboxylate leads to dimethyl pentafluorobicyclo[1.1.1]pentane-1,3-dicarboxylate ( 2 ) and hexafluorobicyclo[1.1.1]pentane-1,3-dicarboxylate ( 3 ). The latter was hydrolyzed to the diacid ( 4) and converted to the 1,3-dibromo and 1,3-diiodo analogues ( 5 and 6 ) by the Hunsdieker reaction followed by treatment with SmI 2 . Na/NH 3 reduction of the disodium salt 10 causes cage C−C bond cleavage. Single-crystal X-ray diffraction analysis of 3 revealed very short nonbonded F−F separations of 2.41 Å and an interbridgehead distance of 1.979 Å, long compared with 1.875 Å in 1,3-diacetylbicyclo[1.1.1]pentane [ 19; cf. 1.954 Å calculated (MP2/6-31G*) for 2,2,4,4,5,5-hexafluorobicyclo[1.1.1]pentane ( 13 )]. Calculation suggests a strain energy of 101 kcal/mol (MP2/6-31G*) for the hexafluorinated cage, compared with 68 kcal/mol for the parent bicyclo[1.1.1]pentane ( 20 ). The remarkably low p K a values of 4 [0.73 and 1.34; cf. 3.22 and 4.26 for the parent diacid 24 ] originate in a direct field effect of fluorine atoms, combined with an increased s character of the exocyclic hybrid orbital on the bridgehead carbon in 4 (calculated 34% in 13 ) relative to 24 (calculated 30% in 20 ). Analysis of the strongly coupled nuclear spin systems of 2 and 3, based on a combination of two-dimensional NMR, spectral simulations, and GIAO-HF/6-31G* calculations of chemical shifts, revealed large and stereospecific long-range 1 H− 13 C, 1 H− 19 F, 13 C− 19 F, and 19 F− 19 F spin−spin coupling constants.