Radical cations from aniline

Let us now compare the pA, values (in AN) of the cation-radicals derived from aniline, N-methylaniline, and A-phenylaniline 5.5, 4.2, and 1.8, respectively (Jonsson et al. 1996). An N-methyl substitnent produces only a marginal effect on the pA, valne of the aniline cation-radical. At the same time, the effect of the A-phenyl snbstitnent in aniline is considerable. The phenyl group (electron-withdrawing) effect on the aniline cation-radical acidylation is self-obvious. 

Polvfaniline) and oob/(pyrrole) Typical, simple, "in-a-beaker" syntheses of poly(pyrrole) and poly(aniline) may now be illustratively described. Both involve use of a chemical oxidant to generate a radical cation from the monomer, which initiates the polymerization. In the case of poly(pyrrole), 2.5 M of FeCla, the oxidant, is taken in MeOH. Pyrrole (liquid) is then added slowly with stirring and cooling, in a ca. 2 5 pyrrole/FeCla molar ratio. The polymer so obtained is filtered, washed and dried. In the case of poly(aniline), aniline (liquid) is taken either neat or on a substrate, and mixed with oxidant solution, which is typically 0.1 M ammonium peroxydisulfate in 2 M HCl. The solution is cooled in an ice bath to 0 C, while stirring for several hours. The polymer obtained is washed, usually with MeOH, and dried. Variations of even these simple procedures involve use of other solvent media, e.g. acetonitrile/water mixtures, and other oxidants, e.g. CUBF4. 

The mechanism proposed for the production of radicals from the N,N-dimethylaniline/BPO couple179,1 involves reaction of the aniline with BPO by a Sn-2 mechanism to produce an intermediate (44). This thermally decomposes to benzoyloxy radicals and an amine radical cation (46) both of which might, in principle, initiate polymerization (Scheme 3.29). Pryor and Hendrikson181 were able to distinguish this mechanism from a process involving single electron transfer through a study of the kinetic isotope effect. 

In order to understand these results it is necessary to consider the nature of the intermediates formed upon photolysis of arylamines. The absorption spectra of transients produced upon photolysis of aniline and various alkyl ring-substituted arylamines was obtained by Land and Porter (18) in different solvents using a flash photolysis apparatus. On this basis they identified both an anilinyl radical (PhNH-) and an anilinyl radical cation (PhNHj). The radical cation is present in polar media (H2O) but absent in cyclohexane. From these results, a homolytic cleavage... 

Aromatic molecules can be polymerized catalytically on clean metal surfaces, or electrochemically to produce oriented polymer films. Initial adsorption of aromatic molecules occurs by electron donation from the aromatic molecule to the surface. This electron donation creates radical cations that can polymerize. Molecular orientation in the films depends on the stable bonding configuration of the radical cation. Thiophene, pyridines, and pyrrole all polymerize with the ring substantially perpendicular to the surface, whereas aniline polymerizes with the phenyl rings parallel to the surface. The catalytically... 

One-electron oxidation of aniline derivatives gives a radical-cation in which the unpaired electron is distributed over both the nitrogen atom and the aromatic system. The further reactions of these intermediates more resemble those of aromatic compounds than of aliphatic amines. Some of the radical-cations are very stable in solution Wurster s blue, prepared by oxidation of tetramethyl-1,4-pheny ene-diamine [152], and Wurster s red from N,N-dimethyI-l,4-phenylenediamine [153] have been known since 1879. They were recognised as radical-cations by Mi-chaelis [154]. 

Electron transfer can be accomplished by quenching of a micelle trapped chromophore by ions capable of ion pairing with the micelle surface. For example, excited N-methylphenothiazine in sodium dodecylsulfate (SDS) micelles can exchange electrons with Cu(II). The photogenerated Cu(I) is rapidly displaced by Cu(II) from the aqueous phase so that intramicellar recombination is averted, Fig. 5 (266). Similarly, the quantum yield for formation of the pyrene radical cation via electron transfer to Cu(II) increases with micellar complexation from 0.25 at 0.05 M SDS to 0.60 at 0.8 M SDS (267). The electron transfer quenching of triplet thionine by aniline is also accelerated in reverse micelles by this mechanism (268). 

The essential step is believed to be the escape of the electron from the coulomb-valence force potential well. A subsequent proton transfer from the radical cation to the solvent should depend on its acidity. For example, the transient spectra in Table II show that proton transfer takes place for phenol and anisole, in which cases the radicals were identified as neutral phenoxyl and phenoxymethyl, respectively. On the other hand, irradiating aniline gave both the neutral and protonated... 

Co(0-NH2)TPP] was polymerized onto glassy-carbon electrodes from an electrolytic solution containing the monomer and tetraethylammonium perchlorate as the electrolyte by cycling the electrode potential in the oxidative way (26). The polymerization of this monomer appears to proceed through a radical cation of the porphyrin with a mechanism similar to that of the oxidative electropolymerization of aniline (27). 

Photoinduced electron transfer between amines and aromatic hydrocarbons occurs to generate radical cations of amines and radical anions of aromatic hydrocarbons. Pac and Sakurai reported the photoaddition of N,N-dimethylaniline to anthracene via photoinduced electron transfer [60]. In benzene, the 4n + 4n) photocyclodimer of anthracene is produced as a sole isolable product, although an emission due to the exciplex formed from anthracene and JV,N-dimethylaniline is observed. In acetonitrile, the addition of dimethylaniline to anthracene occurs via their radical ions to give 9,10- dihydro-9-(4 -dimethylaminophenyl)anthracene as the major product. However, the photoamination on anthracene takes place even in benzene when iV-methylani-line is used as an electron donor. Sugimoto and his coworkers reported the intramolecular photoaddition of anilines to aromatic hydrocarbons to give cyclic amino compounds (Scheme 16) [61-63]. 

Quantitative separation of the n contact and pseudocontact contributions to the lanthanide induced shifts (LIS) in aniline and m-and p-toluidines has been reported. (395, 396) The contact shift patterns are estimated from the rr-spin density distribution of the appropriate cation radical or from the Ni(acac)2 induced shifts. The separation of the shifts was checked by comparing the relative contact shift contribution with the <5 ) value of Golding and Halton (389) and the remaining pseudocontact contribution with the calculated values of Bleaney s theory. (380)... 

It was discovered several decades ago that spontaneous thermal electron transfer from encapsulated aromatic organics to the zeolite framework is possible. Although the exact location of the electron on the framework was not determined, dehydration of the zeolite, the zeolite topology and the nature of the co-cations in the zeolite were found to be important in generating the radical species. NH4-Y zeolite rather than Na-Y was necessary for formation of radical cations of 1,1-diphenylethylene, triphenylamine, quinoline, perylene, aniline and p-phenylenediamine [126]. Recent studies have shown that stable, radical cations of a,(U-diphenylpolyenes can be formed thermally on activated Na ZSM-5 [127]... 

Amine radical-cations have been generated by the treatment of para-substituted anilines with ceric ion (Stone and Waters, 1962 Fox and Waters, 1964). When the para position is free, the initial radical-cation can react further for example, the oxidation of triphenylamine with lead tetra-acetate in the presence of boron trifluoride (Allara ei al., 1965) or with iodine (Stamires and Turkevich, 1963) gives the radical cation Ph3N+ , and, when excess of triphenylamine is used, the former oxidant leads to the radical-cation of A(, .A/, W, A -tetraphenylbenzidine. The only radical observed by the oxidation of dimethylanihne either electrochemically (Mzoguchi and Adams, 1962) or with lead tetraacetate and boron trifluoride (Allara et al., 1965) is the radical-cation of iV, .A, .A7, iV -tetramethylbenzidine. The relatively stable (hindered) anilino radical (40) has been generated from the corresponding aniline by flash photolysis audits e.s.r. spectrum has been measured in n-hexane (Land and Porter, 1961). The electronic spectrum of this radical is very similar to that of the unsubstituted anilino radical, detected during flash photolysis of aniline, but this radical is so short-lived that it has not yet been detected by e.s.r. 

It is clear from the recorded CID data that aniline radical cations are produced in the protonation of aniline, followed by loss of a hydrogen atom. The distonic species are differentiated by a significant signal at m/z 76 for benzyne ions. 

FIGURE 26. Selected geometrical parameters of the aniline radical cation (2 B ). Bond lengths in angstroms and bond angles in degrees were obtained from UB3LYP/6-311++G(d,p) optimization... 

FIGURE 28 (PLATE 4). Summary of electronic distribution in aniline radical cation (a) Bond distances (A), NBO charges [bracket, in au] and Wiberg indices (parentheses, in au). (b) Topology of the electron density determined from AIM p(r) = electron density, L = Laplacian of the density defined as L(r) = — V2p(r) and e = ellipticity of the bond critical point, (c) Isosurfaces of the electron localization function, ELF = 0.87 the values s are the populations of the valence basins... 

TABLE 11. Fundamental vibrational frequencies (cm 1) and associated potential energy distributions of the aniline radical cation in its lowest-lying doublet state. Calculated values were obtained from scaled UB3LYP/6-311++G(3df,2p) harmonic frequencies... 

FIGURE 30. Normal displacements of vibrational modes of aniline radical cation (251). The assignment of the normal vibrations and associated frequencies are presented in Table 12. The numbers given within the ring correspond to the normal modes numbered from Q1 to Q36 in Table 11... 

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