Inverted spin trapping. Part V. 1,1,1,3,3,3-Hexafluoropropan-2-ol as a solvent for the discrimination between proper and inverted spin trapping

Abstract

1,1,1,3,3,3-Hexafluoropropan-2-ol (HFP) has been tested as a solvent for spin trapping experiments. It sustained proper spin trapping of alkyl and aryl radicals generated in secondary cleavage reactions with no obvious complications, and also other neutral, stable radicals could be generated and kept stable for long periods in this solvent.

The extreme persistency of radical cations in HFP was shown to be at least partly due to a strong attenuation of nucleophile reactivity. Hard nucleophiles reacted > 10⁷ times slower with tris(4-bromophenyl)aminium ion in HFP than in acetonitrile, and also soft nucleophiles experienced large rate decreases, for example by a factor of > 10³ for trinitromethanide ion. This means that the inverted spin trapping mechanism, possible under conditions in which a spin trap and a nucleophile is treated by an oxidizing agent, becomes severely impeded in HFP. This was demonstrated in a number of cases, for example, benzotriazolate ion, trinitromethamde ion and 3,5-lutidine. Even rather small proportions of HFP in dichloromethane (10–30%) had a completely inhibiting effect on the inverted spin trapping mechanism, and only triethyl phosphite and related esters underwent this reaction type in neat HFP, yielding trialkoxyphosphonio spin adducts from PBN. In addition, triphenylphosphine reacted in this way.

Anions of imides become protonated in HFP, an additional factor inhibiting inverted spin trapping from potential imidyl radical sources. Thus conditions could be established for the unambiguous trapping of imidyl radicals. No radical from a ring-opened imidyl could be detected.