1- Naphthoxy radical

FIGURE 13.1 Spin densities at different positions of phenoxy and 1-naphthoxy radicals. Values at the meta and meto-hke positions are not shown because they are very low. Reproduced with permission from Ref 43. Copyright (2001) Japanese Photochemistry Association. 

Using the D values of the dialkylanilines at 298K and Equation 13.13, the time required for 1-phenylethyl and 1-naphthoxy radicals to move to the locations amenable to combine at the 2- and 4-positions of 1-naphthoxy, ca. 3.1 and 5.0 A, respectively, correspond to ca. 2.4-24 and 6.3-63 ns in unstretched PEG and ca. 24—240 and 63-630 ns in unstretched PE46, depending on the specific diffusion coefficient employed. The times required for formation of the keto intermediates of 2-AN and 4-AN from lb by the radical clock method described above are ca. 3 and 14 ns, respectively, in unstretched or sttetched PEG and ca. 2-3 and 26-33 ns, respectively, in unstretched LDPE. In stretched LDPE, where translocation increases... 

It is important to note that a 2-phenylpropanoyl radical (from which the radical clock based rate constants were derived) should be more reactive than a 1 -phenylethyl radical toward an electron-rich 1-naphthoxy radical since the odd electron density of the former radical is more localized. " Therefore, absolute comparisons between k, k2B, and rate constants from irradiations of (/f)-3b and k A and k A from irradiations of lb, even in the same medium, are not very useful. 

It is proposed that 32 reacts from its nn excited state by the nitro-to-nitrite (33) inversion followed by nitrite homolysis, when the naphthoxy radical must diffuse away from the cages to obtain the dimerization intermediate 35. However, the source of oxidizing agents is not identified. In comparison, o-nitro-ferf-butylbenzenes 37 are excited to undergo intramolecular H-atom transfer and cyclization to give indol-IV-oxides 40 (equation 34)38. The discrepancy may arise from the nature of the excited state, e.g. that of 37 may react from its njr state. 

Quinol Derivatives. Feichtmayr and Scheibe (61,62) noted that photodissociation occurs for a large number of substituted quinols in carbon tetrachloride solution both at room temperature and at low temperature in glasses. The reversible formation of naphthoxy radicals from tetrachloro-l(4H)naphthalenone is represented as follows ... 

At the moment of their birth (i.e., when the excited singlet states of the aryl esters undergo lysis), the geminal radical pairs mustbe in positions that make their shape similar to that of their precursor ester the radical center of the acyl part is very near the oxygen atom of the aryloxy part, and all subsequent diffusion of the two species starts from this orientation. As a result, addition of an acyl radical to its aryloxy partner is favored spatially at the nearer adj acent ortho) position(s) than at the more distant meta and para positions. However, the ability of the acyl radical to add to each of the positions of an aryloxy radical is expected to depend on the energies of the adducts" ° (which are keto intermediates that enolize thermally with time to the eventual products Equation 13.5 shows an example of the keto intermediate for acyl addition to the ortho position of phenoxy). The spin densities at the 2-, 4-, and oxy-positions of 1-naphthoxy (and of phenoxy) from ESR measurements and HF/6-31G level calculations (in parentheses) are shown in Fig. 13.1. Those for the 2-naphthoxy radical by ESR measurements " and MNDO-UHF calculations are collected in Table 13.1. These considerations explain why the yields of products from addition at the meta positions are very low. 

TABLE 13.1 Spin Densities (Atomic Units) of Carbon Atoms and Oxygen of the 2-naphthoxy Radical from MNDO-UHF Calculations and ESR Data... 

The fraction of the acyl/naphthoxy radical pairs that escape from their initial cages (/tesc) should be known. That fraction can be approximated from the yields of (Bz)2, the dimerization product of the decarbonylated radical. In practice, the fractions of NOL and (Bz)2 arising from the /tesc and /tesc pathways cannot be determined easily. As long as the relative yield of (Bz)2 is very small, the values for /t2A and calculated from Equations 13.10 and 13.11 are overestimated by amounts that are usually less than the experimental error in determining the relative yields of the 2-AN, 4-AN, 2-BN, 4-BN, BzON, and... 

TABLE 13.5 Rate Constants and Activation Energies for Combination of 2-phe-nylpropanoyl/l-naphthoxy Radical Pairs and Relative Rate Constants (at 295 K) for Combination of 1-phenylethyl/l-naphthoxy Radical Pairs from Irradiation of lb in Unstretched (u) and Stretched (s) Polyethylene Films... 

The /t2b/ 4b ratios from la and benzyl 1-naphthyl ether (3a) are compared in Table 13.6." Although the ratios from 3a may be affected shghtly by the very small amounts of out-of-cage reactions indicated by the presence of very low relative yields of (Bz)2 from irradiations in LDPE, the pattern is clear. Addition of benzyl radicals to the closer 2-position of 1-naphthyl is favored when the benzyl/l-naphthoxy radical pair is produced directly and, therefore, the benzyl radical center is closer to the 2-position than to the 4-position of 1-naphthoxy. 

Because the energies of the enantiomeric (prochiral) 1-phenylethyI/l-naphthoxy radical parrs from (R)-3h and their rate constants leading to (/f)-3b and (5)-3b are the same,S = 2Bj v/(l inv Fret). wheref i vand/ retaretheprobabilitiesthataradical pair will form the (S)- and (F)-enantiomers of 3b, respectively. The expressions for Finv, Fret, and S based on Scheme 13.5 are very complex, and even it does not describe all of the processes involved in the tumbling of the 1-phenylethyl radicals because F, really should not be described by one rate constant. To do so requires the introduction of Ft 3, and Ft 4B, defined as the specific tumbling rate constants inside a cage for... 

Aroxy. A group of the type —OR", where R" is an aromatic hydrocarbon radical or a substituted aromatic radical a benzenoid group linked through oxygen, as phenoxy (—OCeHs), naphthoxy (—OC10H7) an aromatic ester group. 

Volume differences between the guest molecules containing naphthoxy and phenoxy and the hole free volumes of the polyethylenes do not appear to be the reason for these results. An indication of what may be responsible is found in the 2a/ 4a ratios from 2a regardless of the polyethylene type or its unstretched/stretched state, the ratios remain near unity. If the rotational motion of the coin-shaped phenoxy moiety of a radical pair is faster than (or comparable to) to the rate combination of phenylacetyl to phenoxy, the overall effect on the relative rates would be the same as if there were translational motions between the two radicals. 

As the viscosity of the -aIkanes increases, radical movements are attenuated and so are the rates of the three possible in-cage recombination processes, A 2b, tb, and k. However, there is no a priori reason to believe that the three rate constants are equally dependent on viscosity. As mentioned above, if translational diffusion of aradical from its initial solvent cage is the most sensitive motion, the reciprocal of the fraction of in cage combination of a radical pair can be expressed as a linear function of (1/t))". Although this expression does not always hold in solvents of low viscosity, like hexane, it was observed that (2 x (5)-3b + 2-BN + 4-BN)in cage. the relative yield of in-cage 1-phenylethyl/l-naphthoxy combination products from irradiations of (I )-3b, does increase qualitatively with increasing viscosity. ... 

Because the radical pairs in this case are singlets, no spin change is needed for their combination and their rate constants for combination after the 1-phenylethyl radical center reaches the proximity of C(2) or C(4) of 1-naphthoxy, kc.2B and k, should be >10 regardless of the microviscosity. Thus, they should be much faster than the viscosity-dependent rates, k. 2B and k. 4B, at which the radicals of the parr move... 

Results from irradiations in polyethylene films. Given these analyses, the data from irradiations of (/ )-3b in polyethylene films will now be discussed (Table 13.7). As mentioned above, the ability of the radical pairs from (/ )-3b to diffuse translationally within a cage is related qualitatively to the 2-BN/4-BN ratios whereas the ability of the 1-phenylethyl radical to tumble along the translational course that brings it to combine at either the 2- or 4-position of its 1 -naphthoxy partner is related to or %ee4B. Although a 1-phenylethyl radical center is attracted... 

Few examples of direct comparisons of rates of reaction of different radicals with a common species are in the literature. In one, the (nondelocalized) tert-butyl radical was found to react more rapidly than pivaloyl radical with an electron-deficient partner, acrylonitrile, in 2-propanol. This is not a good analogy to the comparison between 1-phenylethyl and 2-phenylpropanoyl being made here because we suspect that 1-naphthoxy is more electron-rich than acrylonitrile, polyethylene is much less polar than 2-propanol, and the odd-electron in a 1-phenylethyl radical is delocalized, (a) Jent, F. Paul, H. Roduner, E. Heming, M. Fischer, H. Int. J. Chem. Kinet. 1986, 18, 1113. 

Naphthoxy and naphthoxy-type radicals no detailed A/-data Fa 4, La 7, Ri 1... 

---