Anilino radicals

The 7T system of PN is closely related to that of the benzyl and anilino radicals. Thus, it is no surprise that the triplet absorption spectrum of PN is very similar to the experimental spectra of the benzyl (CgHsCH ) and anilino (CfiHsNH ) radicals. Benzyl radical has a medium intensity feature at 316 nm and a very weak band at 452 nm. The anilino radical has a medium intensity band at 308 nm and a weak transition at 400 nm. ... 

Penkert, F.N., Weyhermuller, T., Bill, E., Hildebrandt, P., Lecomte, S., and Wieghardt, K. Anilino radical complexes of cobalt(III) and manganese(IV) and comparison with their phenoxyl analogues,... 

So we were induced to try this approach, too, and we started synthesis work in the field of s-triazines. The result of our primary working hypothesis was disappointing derivatives bearing anilino radicals showed no herbicidal effects. Surprisingly, however, the herbicidal activity reappeared in the structure 2-chloro-4,6-bis-diethylamino-s-triazine, compound G-25804 shown previously. The awareness that we were confronted with a completely new herbicidal matrix with apparently superior usefulness led us to intensive work around the s-triazine ring system. 

It should be pointed out that the radicals produced by reactions (68)-(70) can be also produced directly from the parent compounds by reaction with O radicals. The phenoxide ion is oxidized by O to the phenoxyl radical (Neta and Schuler, 1975), toluenes undergo hydrogen abstraction by O- to produce benzyl radicals (Neta et al., 1972 Neta and Schuler, 1973), and aniline is converted to the anilino radical either by hydrogen abstraction or by electron transfer. 

The mechanism of 3-haloaniline phototransformation was investigated in more detail in a nanosecond flash photolysis and product analysis study in methanolic solution [52], Photosolvolysis, as witnessed by the formation of anisidine, was again the major pathway, in particular for 3-fluoroaniline. A photoreductive pathway yielding aniline was also observed, being most important for 3-bromoaniline transient absorption indicated the formation of anilino radicals by singlet state dehalogenation as an intermediate on this pathway. Homolytic C - Br cleavage was additionally evidenced. The possibility to observe the triplet-triplet absorption spectra of 3-fluoro- and 3-chloroaniline pointed to an unreactive triplet state [52]. 

The mechanism of 4-chloroaniline photochemistry was independently studied by Guizzardi et al. in organic solvents they reached very similar conclusions [57]. These authors pointed out that the aminophenyl cation has a triplet-diradical character which fully explains its reactivity in organic solvents [57]. However, in aqueous solutions the cation reacted with hydroxyl ions with a rate constant of 3.1 x 1010 M s, which can only be interpreted as a deprotonation reaction [55]. The carbene 4-iminocyclohexa-2,5-dienylidene thus must exist in aqueous solutions, even though its properties have not yet been characterized. This is partly due to an expected low extinction coefficient, similar to the neutral anilino radical [55]. Following these arguments, the primary pathways of 4-chloroaniline photolysis in polar solvents may be pictured as shown in Scheme 7. 

The first two derivatives in the series, monuron and metobromuron, are related to the 4-haloanilines. Their primary photochemistry has been studied by Boulkamh and Richard by means of nanosecond absorption spectroscopy [80]. The transients detected from both compounds in aqueous solution could be assigned to the N-substituted 4-iminocarbene, imino-p-benzoquinone-O-oxide and anilino radical from a complete analogy of their spectral and reactive behavior with that of the species obtained from 4-chloroaniline [55,57]. The quantum yields of carbene formation were determined to be = 0.051 for monuron and

halogen-substituted phenylurea derivatives underwent the same heterolytic dehalogenation process as the 4-haloanilines, which could be understood with reference to the protonability of the amine nitrogen, as in the case of 4-chloro-N,N-dimethylanilinc [55]. 

OH, O and Nj radicals were reported to react with chloro-and hydroxy-derivatives of aniline at diffusion-controlled rates with k> dm mol s and the rates are nearly identical for all isomers of chloroaniline. The OH radical reaction involves both addition and direct H abstraction and the extent of the two reactions is determined by the position of the substituent. The initially formed OH adduct then undergoes dehydration to give the anilino radical. The attack of the OH radical at the carbon bonded to Cl is a minor pathway (not more than 15%) as was confirmed by the detection of Cl ions. 

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. 

Oxidation of the syringic acid is considered to result in the formation of an unstable semiquinone, which undergoes decarboxylation with concurrent uptake of O2 to form the 2,6-dimethoxy-l,4-benzoquinone (41). The third peak from 475 to 521 ppm, with maxima at 483, 504, and 515 ppm, corresponds to azobenzene nitrogens. These most likely arise from nitrogen to nitrogen coupling reactions between the anilino radicals and free aromatic amino groups in the fulvic acid molecules ... 

Figure 23 displays selected optimized geometrical parameters of the anilino radical in both lowest-lying doublet (2A") and quartet (4A") states. In both electronic states, the C—N bond distances are almost identical. The doublet state exhibits a quinoidal structure, whereas the quartet state geometry is characterized by long C2-C3 and C5-C6 distances (1.51 A). This basically corresponds to a triradical electronic structure in which two electrons are centered on two allyl-type moieties. 

The phenylnitrenium cation (PhNH+), formally generated upon ionization of the anilino radical, exhibits a singlet ground state ( A ), followed by a triplet state (3A"), with a singlet-triplet energy gap of AESt(P1iNH+) = 0.84 0.1 eV (81 kJmol-1)210,211. 

Figure 25 displays various results probing the charge distribution of the anilino radical. Although this molecule has only one N—H bond, the properties of the atoms and bonds in both sides are almost identical (for example, the basin population for J(CA,ps,C2,nh ) is 2.51 e, while it is 2.50 e for V(Cl so,C6orAo)). The order of the Cl— C2 and C3—C4 bond is reduced due to the resonance structures similar to those of Scheme 1, indicated by the 2.50 and 2.64 e populations of V(C2,C2) and V(C3,C4), respectively, compared... 

TABLE 10. Vibrational frequencies (cm-1), infrared intensities, normal modes and potential energy distribution of the anilino radical (C6H5N"H). Values obtained from scaled UB3LYP/6-311 + + G(3df,2p) calculations... 

FIGURE 24. Normal displacements of vibrational modes of the anilino radical (2A"). The assignment of the normal vibrations and associated frequencies are presented in Table 10. The numbers given within the ring correspond to the modes Q1-Q33 described in Table 10... 

In general, the electronic partition in the anilino radical is not much different from that of the isoelectronic benzyl and phenoxy radicals12. Figure 25e demonstrates that the ortho- and para-carbon atoms have positive spin populations (excess of a-spin electrons), whereas the ipso- and meta-carbons have negative spin populations (excess of /1-spin... 

FIGURE 25 (PLATE 2). Summary of electronic distribution in the anilino radical (2 A") (a) Bond distances (A), NBO charges [bracket, in au] and Wiberg indices (parentheses, in au). (b) Topology of the electron density determined from atom-in-molecule calculations p(r) = electron density, L = Laplacian of the density defined as L(r) = — V2p(r) and s = ellipticity of the bond critical point, (c) Laplacian map of the density, (d) Isosurfaces of the electron localization function, ELF = 0.87 the values are the populations of the valence basins, (e) Spin density in the molecular plane... 

Although the hot aniline molecules undergo the aryl-amino C—N bond cleavage (Scheme 2), this reaction is inefficient for anilines in the excited states. In contrast, the anilino-alkyl C—N bond homolysis has been observed for several aniline derivatives. For example, methyl radical (Me") and anilino radical (PhN Me) have been detected as the products of the photoirradiation of tetramethylphenylenediamine (25) and N,N-dimethylaniline (lb), respectively, in 3-methylpentane at 77 K124,125. 

At this point it is worth considering the spectrum of the benzyl radical [76]. The electronic absorption spectrum of the benzyl radical consists of a strong sharp band at 312nm and a weak forbidden band at 450nm. The absorption spectrum of the anilino radical is also known and it very much resembles that of benzyl [77], The anilino radical has a strong sharp absorption band at 308 nm. The major difference in the spectra of anilino and benzyl radical is that the forbidden band in benzyl is much more intense in the anilino radical and it is shifted to 400 nm. It has been found that the... 

The latter point is not trivial. Recall that Reiser et al [18] reported in 1965 that photolysis of phenyl azide in an organic glass produces a new UV-VIS absorption spectrum. The spectrum was attributed to triplet phenyl nitrene because of Wasserman s [63] EPR work and the similarity of the spectrum to that of benzyl and anilino radical. However after Chapman and Le Roux s work appeared one could legitimately ask whether Reiser s spectrum is that of triplet phenyl nitrene or of dehydroazepine, or a mixture of both species. 

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