Making photochemically generated phenyl cations visible by addition to aromatics: Production of phenylcyclohexadienyl cations and their reactions with bases/nucleophiles

The benzenediazonium cation and its 4-fluoro, 4-chloro, 4-bromo, 4-methyl, and 4-methoxy derivatives were photolyzed in 1,1,1,3,3,3-hexafluoroisopropyl alcohol (HFIP) with 20-ns pulses of 308-nm light from a XeCl excimer laser. This leads to a pronounced and permanent depletion of the parent compounds (quantum yields 0.89-0.96), but no signal from a transient is seen. However, on addition of aromatics such as, e.g., mesitylene, strong signals due to species with lambda(max) at 250-260 and 350-390 nm are detected. Ln the absence of bases/nucleophiles other than the aromatics, these species, which do not react with oxygen, have a lifetime in the microsecond to millisecond range. On the basis of their absorption spectra and their reactivity with typical bases/nucleophiles such as halides, alcohols, and ethers, the transients are identified as cyclohexadienyl cations formed from the photoproduced "invisible" phenyl cations by addition to the ring of the added aromatics. On the basis of competition data for reaction with HRP and aromatic, the substituents Me, MeO, and Cl lead to a weak increase in selectivity of the phenyl cation, whereas F and Br do not influence the selectivity.