Radical anilinyl

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... 

Frontier electron density distribution for the 3-methylcarbamyl-4-methyl anilinyl radical... 

In contrast, the much lower enthalpy computed for 3 lb, compared to 3lc, means that the N-H BDE of the anilinyl radical 8b is much lower than the C-H BDE of the 3-pyridylmethyl radical 8c. The results in Table 5 show that this is indeed the case, not only for R=Ph and R =3-pyridyl, but also for R=R =Ph and R=R —H.77 The data in Table 5 indicate that, not just for lb and lc but in general, triplet nitrenes are ca. 20 kcal/mol more thermodynamically stable than comparably substituted triplet carbenes. 

The results in this paper support an N-C bond cleavage mechanism (Schemes I and II) for the photolysis of both TDI and MDI based polyurethanes. The substituted anilinyl radicals observed no doubt are formed by diffusion from a solvent cage after the primary N-C bond cleavage. Although not specifically shown in this paper, the reported photo-Fries products (6) are probably formed by attack of the carboxyl radical on the phenyl ring before radical diffusion occurs. The solvent separated anilinyl radicals rapidly abstract hydrogens from the solvent to give the reported aromatic amine product (6). 

The addition/elimination mechanism (equation 11) for formation of phenoxyl radicals by OH reaction with phenols is now documented for a vast number of substituted phenols , catechols , resorcinol , hydroquinones and hydroxylated heterocyclics . The addition/elimination mechanism is also operative in OH reactions with anilines to yield the nitrogen-centered anilinyl radicals (equation i2) 408-in... 

By comparison with phenoxyl radicals, the isoelectronic anUinyl radicals protonate much more readily pATa values in the range 4-7 have been reported for the equilibria between various anilinyl radicals and their corresponding aniline radical cations 45i... 

In order to interpret the results for MDI-PUE and TDI-PU, the laser flash photolysis measurements of several model systems were performed. The transient spectra of the p-toluidinyl radical, recorded upon laser flash photolysis (Aex=248 nm) of p-toluidine (1.4 X 10 M in THF), has a distinct maximum at approximately 310 nm and a broad, diffuse absorbance above 400 nm (Figure 3). The results for p-toluidine are in agreement with previously reported spectra for anilinyl type radicals (12.13). Comparing the transient spectra for p-toluidine with TDI-PU and MDI-PUE, it is quite obvious that the p-toluidinyl radical in THF (Figure 3) is essentially identical to the 300-330 nm and > 400 nm portions of the transient spectra of MDI-PUE and TDI-PU in Figures 1 and 2. 

The phenoxyl radical can be compared with the isoeiectronic anilinyl and benzyl radicals ... 

The gas-phase proton affinity (AH) and the gas basicity (AG) are available for the anilinyl-, phenyl-, benzyl- and phenoxyl radicals. These values are displayed in table 5. 

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