Generation and Reactivity of the 4-Aminophenyl Cation by Photolysis of 4-Chloroaniline

Abstract

4-Chloroaniline and its N,N-dimethyl derivative are photostable in cyclohexane but undergo efficient photoheterolysis in polar media via the triplet state and give the corresponding triplet phenyl cations. CASSCF and UB3LYP calculations 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 singlet. The triplet has a mixed carbene-diradical character at the divalent carbon. This species either adds to the starting substrate forming 5-chloro-2,4'-diaminodiphenyls (via an intermediate cyclohexadienyl cation) or is reduced to the aniline (via the aniline radical cation) in a ratio depending on the hydrogen-donating properties of the solvent. Transients attributable to the triplet aminophenyl cation as well as to the ensuing intermediates are detected. Chemical evidence for the generation of 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-membered heterocycles are measured and span over a factor of 15 or 30 for the two cations. The triplet cation formed under these conditions is trapped by iodide more efficiently than by the best pi nucleophiles. However, in contrast to the singlet cation, it does not form ethers with alcohols, by which it is rather reduced.

About possibility of the triplet mechanism of the Meerwein reaction

“…These cations can either be deprotonated to form styrene VII (arylation) or add nucleophile to form (2-chloroethyl)benzene VIII (chloroarylation) (Scheme 2). Similar results have been obtained by Mella et al [56][57][58] during photoinduced ionic Meerwein arylation of olefins.…”

Theoretical study of the triplet state aryl cations recombination: A possible route to unusually stable doubly charged biphenyl cations

“…). Albini and coworkers have shown that the B3LYP/6–31G( d ) method is satisfactory for describing the nature of the triplet ( 3 B 2 ) state of the p ‐H 2 NC 6 H 4 + cation, but due to the peculiar nature of its singlet ( 1 A′) state, which can be described by two electronic configurations (π 6 σ 0 and π 4 σ 2 ), a better strategy for a computational investigation on the singlet state reactivity would be using a multiconfigurational approach (complete active space self‐consistent field (CASSCF)(8,8)/6–31G(d) method). The latter approach reveals that the 1 A′ state is poorly described by any single configuration; two large coefficients in the lowest configuration interaction eigenvector are found: 0.81 and −0.47, respectively …”

Metal‐Free Cross‐Coupling Reactions of Aryl Sulfonates and Phosphates through Photoheterolysis of Aryl–Oxygen Bonds

“…The electrophilic character of these salts limits the choice of nucleophiles that can be used in the photoarylation reactions, since there will be competing thermal reactions. However, aryl cations could also be photogenerated starting from electron‐donating substituted aryl halides, such as chloro‐ or fluoroanilines,11 chlorophenols, and chloroanisoles12 (Scheme ). Delocalization of the positive charge on the electron‐donating substituents in the resulting aryl cation as well as the use of a polar medium resulted in efficient photoheterolysis.…”