‘Inverted spin trapping’. Reactions between the radical cation of α-phenyl-N-tert-butylnitrone and ionic and neutral nucleophiles

Abstract

Tris(4-bromophenyl)aminium ion, TBPA˙⁺, is both an efficient electron transfer oxidant and an electrophile. Its reactivity toward nucleophiles reflects this dichotomy, in that bond formation occurs with nucleophiles that are not readily oxidized, such as carboxylate, cyanide and chloride ion, whereas less oxidation-resistant nucleophiles, such as bromide, iodide and trinitromethanide ion react with electron transfer.

This property has been utilized to study spin-trapping reactions with α-phenyl-N-tert-butylnitrone (PBN) under oxidative conditions. The reaction between TBPA˙⁺ and a solution of PBN and nucleophile gave the corresponding spin adducts from both categories of nucleophiles, provided the nucleophilic/solvolytic reactivity of the spin adduct was not too high. Spin adducts from the oxidation-resistant nucleophiles [F^–, CN^–, CNO^–, pyridine(s), succinimidate(s), triethyl phosphite, RCO₂^–] must then be formed in the reaction between the nucleophile and the radical cation of PBN, generated in an initial electron-transfer step (‘inverted spin trapping’). Only in the case of the more easily oxidizable nucleophiles [SCN^–, (NO₂)₃C^–, N₃^–] does proper spin trapping occur, i.e., the radical is formed by the TBPA˙⁺/nucleophile reaction and is then trapped by PBN.