| Résumé | Rate constants for the free radical rearrangement, CF 3C(O)OCMe 2ĊH 2 (1d) → CF 3C(O)OCH 2ĊMe 2 (2d), have been measured in CF 2ClCFCl 2 by kinetic EPR spectroscopy. This reaction is very significantly faster (k 1 75°C = 7.0 × 10 4 s -1) than the related rearrangement, CH 3C(O)OCMe 2ĊH 2 (1a) → CH 3C(O)OCH 2ĊMe 2 (2a) (k 1 75°C = 4.5 × 10 2 s -1), in hydrocarbon solvents. The potential cyclic intermediate radical, 2-(trifluoromethyl)-4,4-dimethyl-1,3-dioxolan-2-yl (3d), does not undergo ring opening to 2d, at temperatures where the 1d → 2d rearangement is very fast. It is concluded that 3d is not an intermediate in the trifluoroacetoxyl migration. It is also concluded on the basis of the k 1 values for 1a and 1d that these rearrangements involved a charge-separated transition state. The 1a → 2a rearrangement is very much faster in water (k 1 75°C = 2.1 × 10 4 s -1) than in hydrocarbon solvents, which provides additional support for a charge-separated transition state. |
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