The Influence of Multiple Fluorination on the Ring Opening of [2.2.2]Propellane

Effect of electronegative substituents on ring opening of [2.2.2]propellane was examined by quantum chemical calculations at CASSCF and CASPT2 level of theory. Comparison of the results with those obtained with MR-AQCC calculations indicated that calculations beyond CASSCF level were needed for reliable description of the process. Calculations show a strong effect of fluorination on the central bond elongation energy profile, with the effect being most pronounced for the perfluoro derivative. Moreover, for rearrangement of the most of multiply fluorinated derivatives, including perfluoro[2.2.2]propellane, CASPT2 calculations predict a single minimum potential energy well, which hampers rearrangement to significantly more stable fluorinated 1,4-dimethylenecyclohexane analogues. This observation is in harmony with high stability of synthetically available stable heavily fluorinated derivatives which, in contrast to the parent [2.2.2]propellane, do not rearrange to the thermodynamically most stable fluorinated 1,4-dimethylene-cyclohexane isomer.