Looking for a Paradigm for the Reactivity of Phenonium Ions

The addition of a photogenerated phenyl cation to an alkene offers an entry, under mild conditions, to the phenethyl cation/phenonium ion system in organic solvents. Taking advantage of this, a product study has been carried out in parallel with a computational characterization of the intermediates. Thus, 4-methoxy- and 4-dimethyaminophenyl cations have been photogenerated from the corresponding chlorobenzenes in the presence of mono-to tetrasubstituted olefins in polar or protic media. The chemistry that occurs has been correlated with the degree of anchimeric assistance offered by the phenyl group, as predicted by calculations. Two limiting situations arise, the first one when starting from mono- or 1,2-disubstituted alkenes. In these cases, calculations evidence a large stabilization of the intermediate (phenonium character), particularly with electron-donating substituents on the aromatic ring, and the main products in acetonitrile are phenethyl chlorides. Variation of the ion-stabilizing solvent such as trifluoroethanol (TFE) leads to alkyl and hydride migration before chloride addition. In contrast, with less electron-rich aromatics and highly substituted alkenes the stabilization of the intermediate is modest and the typical reaction is deprotonation to yield an allylbenzene or, in TFE, the formation of a phenethyl ether. In between these two extremes, there are intermediate cases, more easily directed by a change in the conditions. These generalizations and the mildness of the method help to assess the synthetic potential of this reaction.