PREPARATION, STRUCTURE, AND PROPERTIES OF SYMMETRICALLY 1,3-DIFUNCTIONALIZED PENTAFLUOROBICYCLO[1.1.1]PENTANES AND HEXAFLUOROBICYCLO[1.1.1]PENTANES

Exhaustive direct fluorination of dimethyl bicyclo[1.1.1]pentane-1,3-d icarboxylate leads to dimethyl pentafluorobicyclo[1.1.1]pentane-1,3-di carboxylate (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 react ion followed by treatment with SmI2. Na/NH3 reduction of the disodium salt 10 causes cage C-C bond cleavage. Single-crystal X-ray diffractio n analysis of 3 revealed very short nonbonded F-F separations of 2.41 Angstrom and an interbridgehead distance of 1.979 Angstrom, long compa red with 1.875 Angstrom in 1,3-diacetylbicyclo[1.1.1]pentane [19; cf. 1.954 Angstrom calculated (MP2/6-31G) for 2,2,4,4,5,5-hexafluorobicyc lo[1.1.1]pentane (13)]. Calculation suggests a strain energy of 101 kc al/mol (MP2/6-31G) for the hexafluorinated cage, compared with 68 kca l/mol for the parent bicyclo[1.1.1]pentane (20). The remarkably low pK (a) values of 4 [0.73 and 1.34; cf. 3.22 and 4.26 for the parent diaci d 24] originate in a direct field effect of fluorine atoms, combined w ith an increased s character of the exocyclic hybrid orbital on the br idgehead 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 simul ations, and GIAO-HF/6-31G calculations of chemical shifts, revealed l arge and stereospecific long-range H-1-C-13, H-1-F-19, C-13-F-19, and F-19-F-19 spin-spin coupling constants.