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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 19 F NMR at 80–120 °C, and the activation parameters for the rearrangement of 1 to 2 are: Δ G ‡ (100 ° C )=28.7 kcal/mol, Δ H ‡ =26.7 kcal/mol, and Δ S ‡ =−5.5 eu (log A =12.7, E a =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: Δ G ‡ ( E )≅28.5 kcal/mol and Δ G ‡ ( Z )≅31.1 kcal/mol. A favorable, polar biradical transition state for rearrangement is proposed to account the exceptional reactivity of these alkenyl pentafluorocyclopropanes. |
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