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 ·PO32– was detected after irradiation at ambient temperature. The major electron loss centre was formed by loss of α-hydrogen (R2ĊOPO32–). In general, the 31P hyperfine coupling for these radicals was in the range 0–10 G, but for the radical MeĊHOPO32– 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)2PO22–(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.