Preparation, Structure, and Properties of Symmetrically 1,3-Difunctionalized Penta- and Hexafluorobicyclo[1.1.1]pentanes Michael D. Levin, † Steven J. Hamrock, ‡ Piotr Kaszynski, § Alexander B. Shtarev, † Galina A. Levina, † Bruce C. Noll, † Martin E. Ashley, † Richard Newmark, ⊥ George G. I. Moore, ⊥ and Josef Michl* ,† Contribution from the Department of Chemistry and Biochemistry, UniVersity of Colorado, Boulder, Colorado 80309-0215, and 3M Company, Saint Paul, Minnesota 55144 ReceiVed April 3, 1997 X Abstract: 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 pK 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. Introduction There are several reasons for which the presently unknown analogues of [n]staffanes (1) fluorinated on the bridge carbons would be useful to synthesize and investigate. First, their bridgehead hydrogens might be sufficiently acidic for direct functionalization, facilitating attempts to use these rodlike molecules as modules in molecular-size construction sets. 1 Second, it would be interesting to compare the properties of fluorinated oligomeric or polymeric [n]staffanes with those of the parent [n]staffanes. Among others, we would like to evaluate the effects of fluorination on electronic interaction through these relatively rigid spacers, e.g., on the long-range propagation of spin density from a terminal bridgehead position. 2 Third, the fluorine atoms present in the neighboring bridges of the bicyclo[1.1.1]pentane cage are forced to be much closer than the sum of their van der Waals radii, and it would be interesting to see how the crowding is accommodated, what effect it has on cage strain, and whether any unusual reactivity or spectral properties might result from the nonbonded interactions. We have started by investigating the first member of the series (1, n ) 1) and now report that (i) dimethyl pentafluorobicyclo- [1.1.1]pentane-1,3-dicarboxylate (2) and dimethyl hexafluoro- † University of Colorado. ‡ Present address: 3M Company. § Present address: Department of Chemistry, Vanderbilt University, Nashville, TN 37235. ⊥ 3M Company. X Abstract published in AdVance ACS Abstracts, December 15, 1997. (1) Kaszynski, P.; Friedli, A. C.; Michl, J. J. Am. Chem. Soc. 1992, 114, 601. (2) McKinley, A. J.; Ibrahim, P. N.; Balaji, V.; Michl, J. J. Am. Chem. Soc. 1992, 114, 10631. 1 Chart 1. Bicyclo[1.1.1]pentanes 12750 J. Am. Chem. Soc. 1997, 119, 12750-12761 S0002-7863(97)01051-2 CCC: $14.00 © 1997 American Chemical Society