1- Phenylethyl radicals

Of wider significance was the generation of 2-phenylethyl radical by oxyhemoglobin-mediated oxidation of phenelzine (2-phenylethylhydrazine), which was shown to be more efficient in promoting alkali-labile sites than in producing direct DNA strand scission (Fig. 6) [25]. 

Fig. 6 Oxidation of phenelzine to carbon centered 2-phenylethyl radical...

The chemistry of radical sites adjacent to phosphatoxy centers elicited interest because of the involvement of such species in DNA degradation processes. These species can give rise to rearrangement, elimination, and substitution products, and for some time concerted eliminations and migrations as well as heterolysis to a radical cation and a phosphate anion were considered to be involved (Scheme 2). Recently, experimental studies of the l,2-dibenzyl-2-(diphenylphosphatoxy)-2-phenylethyl radical and complementary theoretical studies of l,l-dimethyl-2-(dimethylphosphatoxy)ethyl radical have been interpreted as indicating that a radical cation/anion pathway with initial formation of 49 is favored. ... 

Allylic radical are relatively stable, and the pentadienyl radical is particularly stable. In such molecules, E,E)- E,7)-, and (Z,Z)-stereoisomers can form. It has been calculated that (Z,Z)-pentadienyl radical is 5.6 kcal mol (23.4 kJ mol ) less stable than (F.Fj-peiitadienyl radical.2-Phenylethyl radicals have been shown to exhibit bridging of the phenyl group.It is noted that vinyl radical have ( )- and (Z)-forms and the inversion barrier from one to the other increases as the electronegativity of substituents increase. Enolate radicals are also known. " ... 

Yang 1981). Support for the theory of alkyl radical transfer was provided by Ortiz de Monellano et al. (1983) who used the electronic paramagnetic resonance spin-trapping technique to detect the 2-phenylethyl radical formed during microsomal biotransformation of phenelzine. Production of the a-(4-pyridyl 1 -oxide)-N-ferf-butylnitrone/2-phenyl-ethyl radical adduct was dependent on the presence of active microsomes, phenelzine, NADPH (or NADH), and spin trap a-(4-pyridyl l-oxide)-N-ferf-butylnitrone (Ortiz de Monellano et al. 1983). The addition of catalase and superoxide dismutase resulted in a 28.5 and 24 % decrease in radical production, respectively (Rumyantseva et al. 1991). The concentration of the a-(4-pyridyl l-oxide)-N-ferf-butylnitrone/2-phenylethyl radical adduct decreased significantly in the presence of metal chelators, i.e. EDTA, diethylenetriaminepentaacetic acid (DTPA), or deferoxamine mesylate. 

The self reaction of substituted phenylethyl radicals (1) has been widely investigated.92 96 The findings of these studies are summarized in Table 5.2. Unless R2 is very bulky (eg. r-butyl, see below ), combination is by far the dominant process with the value k /k typically in the range 0.05-0.16. Thus, a small amount of disproportionation is always observed. 

For radicals 1, ktA/ktc shows a marked dependence on the bulk of the substituent (R2). While phenylethyl radicals (2) and cumyl radicals (5) afford predominantly combination, there are indications of a substantial penultimate unit effect. The radicals 6, with an a-neopentyl substituent, give predominantly disproportionation, Termination in AMS polymerization might therefore also give substantial... 

Direct aromatization of the quinonoid intermediates is a photochemically allowed but thermally forbidden rearrangement (Scheme 5.6). When phenylethyl radicals are generated photochemically at 20 °C there is evidence95 of a-o coupling by way of the aromatized product 7. The products derived from these pathways can be trapped in thermal reactions by radical98 or acid1 catalyzed... 

Both S polymerization initiated by AlBMe176 180 (i.e. PS + 4) and MMA polymerization initiated by 1 J -azobis-l-phenylethane176 (i.e. PMMA + 1-phenylethyl radical) are reported lo give predominantly combination. Ito,7e has concluded that cross termination is not particularly favored over homotermination in S-MMA copolymerization. 

As a point of reference, relative rates for methyl methacrylate have also been included in Table 6. While addition to butadiene or isoprene is significantly faster as compared to methyl methacrylate for electrophilic or ambident radicals, little rate variation is found for the 1-phenylethyl radical. 

A high carbon monoxide pressure ( 5 atmos.) favours the formation of the butane. Possible mechanisms for its formation include homolytic cleavage of the benzyl-cobalt tetracarbonyl complex and recombination of the radicals to generate 2,3-diphenylbutane and dicobalt octacarbonyl, or a base-catalysed decomposition of the benzylcobalt tetracarbonyl complex (Scheme 8.4). The ethylbenzene and styrene could arise from the phenylethyl radical, or from the n-styrene hydridocobalt tricarbonyl complex. 

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... 

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... 

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. 

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. 

Tirrell and co-workers studied copolymerization of styrene (S) and acrylonitrile (A) using l,l -azobis(l-phenylethane)-a,q/-13C, which generates a monomeric styrene radical, 1-phenylethyl radical 87... 

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