Spontaneous Hydrogen Atom Transfer on Ionization: Characterization of Enol Radical Cations in Cryogenic Matrices

Abstract

Organic radical cations are important intermediates in a wide variety of chemical and biological processes. Because of the unpaired spin and charge, radical cations are very reactive and undergo transformations typical for both free radicals and/or cations. Unimolecular processes in radical cations such as valence isomerization, rearrangement, and fragmentation are often significantly different than those of their neutral precursors. The deficit of one electron leading to a reduced strength of key bonds manifests itself in unusual structures and reactivities of radical cations.1 A special feature of radical cation chemistry is the possibility of hydrogen atom transfer which may occur spontaneously upon ionization if a cyclic transition state with five members can be formed. Mass spectrometric experiments showed that this occurs in many heteroatomcontaining compounds leading to so-called distonic ions whose structure often does not correspond to that of a persistent neutral species, but which are nevertheless usually more stable than their conventional isomers.2 Distonic ions have also been investigated in condensed phase where they may occur as products of radiation or radical chemistry (cf. Section VI of ref 2c). However, apart from a few exceptional cases, the only distonic ions which form by spontanous intramolecular hydrogen atom transfer in solid media are those arising from aliphatic esters3 where it is thought to occur by quantum mechanical tunneling.3c This Account is concerned with another class of compounds that show a similar behavior, namely enones (or related compounds) which are disposed for 1,5hydrogen transfers via 6-membered ring transition states to yield dienols as shown here: