Unstable intermediates. Part 169. Electron capture processes in organic phosphates: an electron spin resonance study

Abstract

A range of mono-, di-, and tri-alkyl phosphates have been exposed to ⁶⁰Co γ-rays at 77 K and at ambient temperature, together with their solutions in CD₃OD at 77 K. Monoalkyl phosphates (as the sodium, potassium, magnesium, or calcium salts) gave alkyl radicals on electron capture at 77 K, but a low yield of ·PO₃^2– was detected after irradiation at ambient temperature. The major electron loss centre was formed by loss of α-hydrogen (R₂ĊOPO₃^2–). In general, the ³¹P hyperfine coupling for these radicals was in the range 0–10 G, but for the radical MeĊHOPO₃^2– in the calcium salt a coupling of 45 G indicated considerable hyperconjugative interaction. Dialkyl phosphates gave the phosphoryl radicals ROPO₂^– on irradiation at 77 K, together with alkyl radicals. Some sodium and potassium salts also gave relatively low yields of the parent anions, (RO)₂PO₂^2–(phosphoranyl radicals). These were not obtained from CD₃OD solutions, but the phosphoryl radicals and alkyl radicals were detected. Only phosphoryl radicals were obtained on irradiation at ambient temperature. The major electron-loss centres were again R₂ĊOPO₂(OR)^–. Trialkyl phosphates at 77 K gave predominant yields of the parent phosphoranyl anions, (RO)₃PO^–, although phosphoryl radicals were also detected. On annealing, there was no evidence for the conversion of phosphoranyl radicals into phosphoryl radicals. Solutions in CD₃OD gave the same species, but the phosphoryl radicals were obtained in relatively greater yields. Again, the major electron-loss centres seemed to be R₂ĊOPO(OR)₂ radicals. A doublet species having A∥ 237 and A⊥ 179 G was also obtained from MeOPO₃Na₂, MeOPO₃Mg, and other methyl esters after irradiating at 77 K and slight annealing. In some cases, an extra 8 G splitting was observed on these features. Possible structures for this species are discussed.