Generation and reactivity of the 4-aminophenyl cation by photolysis of 4-chloroaniline

4-Chloroaniline and its NN-dimethyl derivative are photostable in cyclohexa ne but undergo efficient photoheterolysis in polar media via the triplet st ate and give the corresponding triplet phenyl cations. CASSCF and UB3LYP ca lculations show that the 4-aminophenyl triplet cation has a planar geometry and is stabilized by > 10 kcal mol(-1) with respect to the slightly bent s inglet. The triplet has a mixed carbene-diradical character at the divalent carbon. This species either adds to the starting substrate forming 5-chlor o-2,4 ' -diaminodiphenyls (via an intermediate cyclohexadienyl cation) or i s reduced to the aniline (via the aniline radical cation) in a ratio depend ing on the hydrogen-donating properties of the solvent. Transients attribut able to the triplet aminophenyl cation as well as to the ensuing intermedia tes are detected. Chemical evidence, for the generation of the phenyl catio n is given by trapping via electrophilic. substitution with benzene, mesity lene, and hexamethylbenzene (in the last case the main product is a 6-aryl- 3-methylene-1,4-cyclohexadiene). Relative rates of electrophilic attack to benzene and to some alkenes and five-membered heterocycles are measured and span over a factor of 15 or 30 for the two cations. The triplet cation for med under these conditions is trapped by iodide more efficiently than by th e best pi nucleophiles. However, in contrast to the singlet cation, it does not form ethers with alcohols, by which it is rather reduced.