| Abstract
| - 4-Chloroaniline and its N,N-dimethyl derivative are photostable in cyclohexane but undergo efficientphotoheterolysis in polar media via the triplet state and give the corresponding triplet phenylcations. CASSCF and UB3LYP calculations show that the 4-aminophenyl triplet cation has a planargeometry and is stabilized by >10 kcal mol-1 with respect to the slightly bent singlet. The triplethas a mixed carbene−diradical character at the divalent carbon. This species either adds to thestarting substrate forming 5-chloro-2,4‘-diaminodiphenyls (via an intermediate cyclohexadienylcation) or is reduced to the aniline (via the aniline radical cation) in a ratio depending on thehydrogen-donating properties of the solvent. Transients attributable to the triplet aminophenylcation as well as to the ensuing intermediates are detected. Chemical evidence for the generationof the phenyl cation is given by trapping via electrophilic substitution with benzene, mesitylene,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-memberedheterocycles are measured and span over a factor of 15 or 30 for the two cations. The triplet cationformed under these conditions is trapped by iodide more efficiently than by the best π nucleophiles.However, in contrast to the singlet cation, it does not form ethers with alcohols, by which it israther reduced.
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