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 SmI2.
Na/NH3 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 1H−13C,
1H−19F, 13C−19F,
and 19F−19F spin−spin coupling
constants.
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