Mechanism of Rearrangement of β-(Acyloxy)alkyl Radicals

Article abstract of DOI:10.1021/ja00318a039

Rate constants for the free radical rearrangement, CF3C(O)CMe2CH2 (1d) -> CF3C(O)OCH2CMe2 (2d), have been measured in CF2ClCFCl2 by kinetic EPR spectroscopy.This reaction is very significantly faster (k₁^{75 deg C} = 7.0E4 s^-1) than the related rearrangement, CH3C(O)OCMe2CH2 (1a) -> CH3C(O)OCH2CMe2 (2a) (k₁^{75 deg C} = 4.5E2 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 rearrangement 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 k₁ values for 1a and 1d that these rearrangements involve a charge-separated transition state.The 1a -> 2a rearrangement is very much faster in water (k₁^{75 deg C} = 2.1E4 s^-1) than in hydrocarbon solvents, which provides additional support for a charge-separated transition state.