Partially bridge-fluorinated dimethyl bicyclo[1.1.1]pentane-1,3-dicarboxylates: Preparation and NMR spectra

Direct fluorination of dimethyl bicyclo[1.1.1]pentane-l,3-dicarboxylate, ob tained from [1.1.1]propellane prepared by an improved synthetic procedure, furnished esters of 14 of the 15 possible bridge-fluorinated bicyclo[1.1.1] pentane-l,3-dicarboxylic acids, isolated by preparative GC. Calculated geom etries reflect the substitution pattern in a regular fashion compatible wit h Bent's rules. Considerable additional strain is introduced into the bicyc lo[1.1.1]pentane cage by polyfluorination; it is calculated to be as high a s 33-35 kcal/mol for hexasubstitution. Three arrangements of the fluorine s ubstituents are especially strain-rich: geminal, proximate, and W-related. The H-1, C-13, and F-19 NMR spectra exhibit a striking variety of chemical shifts and long-range coupling constants, These are in good agreement with results calculated with neglect of the bridgehead substituents for all of t he chemical shifts by the GIAO-RHF/6-31Gh*//RHF/6-31G* and GIAO-RHF/6-31G*/ / MP2/6-31G* methods and for many of the coupling constants by the EOM-CCSD /6-311G*//MP2/6-311G* method. The proximate (4)J(FF) constants are particul arly Large (50-100 Hz) and show an inverse linear dependence on the calcula ted F-F distance in the range 2.43-2.58 Angstrom.