| Résumé | The reaction of tri-tert-butyl phosphite with tert-butoxy radicals yields the phosphate and a tert-butyl radical via an intermediate tetra-tert-butoxyphosphoranyl radical. Using EPR spectroscopy and deuterium labeling, it has been shown that, at temperatures from 78 to -100°, this reaction occurs without any "memory" as to which tert-butoxy attacked the phosphite. However, the β scission of the phosphoranyl to phosphate and tert-butoxy exhibits a significant secondary β-deuterium isotope effect. After correction for the isotope effect involved in the disproportionation of tert-butyls (kd(CH3)3C/ kd(CD3)3C = 1.4 ± 02 from 80 to -20°, it being assumed that the combination rate constants kc(CH3)3C and kc(CD)3C are equal), the isotope effect per deuterium for decay of the phosphoranyl can be represented by log (kβH/kβD) = -0.029 + 0.063/θ where θ = 2.3RT kcal/mol. These isotope effects are discussed in relation to isotope effects in analogous reactions, and the advantages in terms of sensitivity and available temperature range of our procedure are pointed out. |
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