A doubly destabilized antiaromatic cyclopentadienyl cation: Solvolysis of a 5-trifluoroacetoxy-5-heptafluoropropyl 1,3-cyclopentadienela

The 5-trifluoroacetoxy-5-heptafluoropropylcyclopentadiene 15 rearranges to the isomeric trifluoroacetate 16 with a rate constant 5 x 10(5) less than t hat for solvolysis of the corresponding 5-CH3 derivative 5. Labeling of 15 with O-18 shows the rearrangement occurs by a [1,5]-sigmatropic rearrangeme nt. Solvolysis of 16 occurs at a rate 4 times slower than its formation fro m 15 and leads to the extensively rearranged fulvene 18, implicating format ion of the doubly destabilized cation 20. Carbocation formation from 15 is retarded by a factor of 10(20) relative to the model 11, showing cumulative destabilizing effects due to formation of the antiaromatic cyclopentadieny l carbocation and electron withdrawal by the fluoroalkyl group.