Kinetics of the 1,2-Migration of Carbon-Centered Groups in 2-Substituted 2,2-Dimethylethyl Radicals

Article abstract of DOI:10.1021/ja00335a037

The rearrangements RCMe2CH2^. -> RCH2C^.Me2 (k_r) (R = Ph, Me3CC<>C, Me3CC=O, and N<>C) have been studied over a range of temperatures by product analyses with use of the common competiting reaction RCMe2CH2^. + CCl4 -> RCMe2CH2Cl + CCl3^. (k_Cl).For R = H2C=CH the rearrangement was so fast that only the rearranged chloride, RCH2CMe2Cl, was produced.All these rearrangements occur via a 3-membered cyclic intermediate radical (or transition state).Various considerations led to the following Arrhenius equation for chlorine abstraction: log (k_Cl/M^-1 s^-1)=(8.14 +/- 0.42)-(5.52 +/- 0.63)/τ, where τ=2.3RT kcal/mol, and this equation is used to calculate Arrhenius parameters for migration of all but the H2C=CH group.Comparison of these parameters with those already available from kinetic EPR measurements leads to a choice of preferred Arrhenius parameters for all five rearrangements.The cyano group had an unexpectedly low mobility while the pivaloyl group underwent a surprisingly rapid 1,2-shift.Migratory aptitudes increase along the series R = N<>C < Me3CC<>C < Ph < Me3CC=O < H2C=CH, with k_r at 25 deg C = 0.9, 93, 762, 1.7*10⁵, and 1.0 * 10⁷ s^-1, respectively.The preferred pre-exponential factors all lie in the range 10^10.9 - 10^12.0, while the activation energies vary from 16.4 kcal/mol for R = N<>C to 5.7 kcal/mol for R = H2C=CH.These results are discussed in relation to the results of thermochemical kinetic calculations and to earlier work on the 1,2-migration of unsaturated groups in radicals.