1-Vinyl-1,2,2,3,3-pentafluorocyclopropane: an extraordinarily rapid vinylcyclopropane rearrangement

The synthesis of 1-vinyl-1,2,2,3,3-pentafluorocyclopropane (1) and the kinetics of its thermal, unimolecular rearrangement to 1,4,4,5,5-pentafluorocyclopentene (2) are reported. Kinetic data were obtained by gas-phase 19F NMR at 80–120 °C, and the activation parameters for the rearrangement of 1 to 2 are: ΔG‡(100 °C)=28.7 kcal/mol, ΔH‡=26.7 kcal/mol, and ΔS‡=−5.5 eu (log A=12.7, Ea=28.4 kcal/mol). Vinyl(pentafluorocyclopropane) (1) with a half-life of only 38 min at 110 °C, is one of the most thermally labile vinylcyclopropanes known. The isomeric E- and Z-1-propenyl-1,2,2,3,3-pentafluorocyclopropanes (3) similarly have relatively low activation parameters: ΔG‡(E)≅28.5 kcal/mol and ΔG‡(Z)≅31.1 kcal/mol. A favorable, polar biradical transition state for rearrangement is proposed to account the exceptional reactivity of these alkenyl pentafluorocyclopropanes.