Fluoro spin adducts and their modes of formation

Abstract

The reactions of two fluorinating reagents, XeF₂ and N-fluorodibenzenesulfonamide [(PhSO₂)₂N–F], with several spin traps have been investigated. In dichloromethane, the strong oxidant XeF₂ cleanly gives fluoro spin adducts with N-tert-butyl-α-phenylnitrone (PBN) or 5,5-dimethyl-1-pyrroline 1-oxide (DMPO) according to a mechanism mediated by the radical cation of the spin trap. In both cases, further fluorination takes place with replacement of the α hydrogen by fluorine.The much weaker oxidant (PhSO₂)₂N–F reacts with PBN or DMPO in dichloromethane giving both the fluoro adduct and an adduct formally derived from an N-centred radical, assigned the structure of PhSO₂N(F)–PBN˙ or (PhSO₂)₂N–DMPO˙, respectively. This type of reaction proceeds by a version of the Forrester–Hepburn mechanism, in which an acid HA, in this case HF, initially adds to the nitrone function to give a hydroxylamine derivative which is oxidized by (PhSO₂)N–F giving the fluoro spin adduct, a proton and the highly labile radical anion (PhSO₂)₂N–F˙^- . By decomposition of the latter to PhSO₂(F)N^- and PhSO₂˙, conditions are set up for propagation of the reaction by a new molecule of HA [now PhSO₂(F)NH] and thus formation of the PhSO₂(F)N spin adduct.