Arylmethyl radicals

The situation with polyarylmethanes is very similar. Due to the stabilization of free valence in arylmethyl radicals, the bond dissociation energy (BDE) of the bond C—02 for example, in triphenylmethyl radical is sufficiently lower than in alkylperoxyl radicals. This radical is decomposed under oxidation conditions (room temperature), and the reaction of Ph3C with dioxygen is reversible ... 

Hendry and Russell (15) have shown how polyarylmethanes and polyarylethylenes may retard autoxidations of other hydrocarbons at 60° to 90 °C. because the carbon radicals involved react relatively slowly and incompletely with oxygen, the free arylmethyl radicals contribute to a fast, crossed, termination reaction. A similar effect has been reported for benzyl radicals above 300°C. (11). These effects can usually be overcome by sufficient oxygen pressure hence, our restriction at the beginning of this section. 

The enormous effect of perchlorophenyl groups on the stability of arylmethyl radicals has been well documented by a series of reports by Ballester. Thus, perchlorotriphenylmethyl has been shown to have a half-life on the order of 100 years in solution at room temperature in contact with air. It has therefore been termed an inert free radical. Even perchlorodiphenyl(chloro)methyl has been shown to be stable.The perchlorophenyl group may be expected to exert a similar stabilizing effect on triplet DPCs. 

Perchlorophenyl groups, which have an enormous effect on the stability of aryl-methyl radicals, have only a shght effect on the stability of DPCs. This fact suggests how unstable triplet DPCs are and how much more difficult they are to stabilize than arylmethyl radicals. 

Although the work of Gomberg and those who followed provided good evidence for the existence of arylmethyl radicals, there was still skepticism regarding simple aliphatic radicals such as methyl and ethyl. As noted above, the modern history of these species began with the suggestions of Crum Brown (equation 2). ... 

Carbon-bromine cleavage from upper triplet states has also been observed by two-color photolysis of 55 and for the ketones 83-85 [69]. In the latter cases cleavage is followed by a neophyl-type rearrangement and decarboxylation yielding the arylmethyl radical (Scheme 4). 

In general, for benzyl and other arylmethyl radicals there is a large spectral red shift between the observed absorption maxima of the ground-state radicals and the emission maxima for the excited states. The origin of this shift lies in the low oscillator strength for the D0 —> Z), transition which renders it nearly unobservable for most of the radicals studied. Thus, the observed absorption is due to the Dq — Dn transition. Excitation into this band partially leads to relaxation to the >, state followed by emission. The effect of substituents on the emission has been studied extensively and has shown that substitution usually leads to a spectral red-shift. 

Unlike the arylmethyl radicals, diphenyl ketyl radicals have substantial extinction coefficients for the D0 —> Dx absorption and emission that shows the expected mirror image relationship to the absorption band. Ketyl radical lifetimes are generally in the 10-ns range and show only minor temperature dependence. Deuterium substitution of the hydroxy group has a pronounced effect on lifetime,... 

TABLE 7 Spectral and Lifetime Data for Fluorescence from Excited Arylmethyl Radicals in Low-Temperature (77 K) Matrices... 

In these compounds, the intermediate arylmethyl radical is long-lived. However, further excitation of the radicals in benzene results in production of the bromine atom-benzene charge transfer complex ( max 550 nm), an indication of photo-induced cleavage of the remaining C-Br bond. This is further supported by observation of the corresponding vinyl products. 

Table 13 Rate Constants for the Reactions of Excited Arylmethyi and Substituted Arylmethyl Radicals wth Added Quenchers in Room-Temperature Solution...

The convenient entry to arylmethyl radicals via homolytic photodissociation of the corresponding halomethylaryl precursors has been used to study the ground and excited state reactivities of -substituted benzyl radicals with 02172,173 and the excited state properties and reactivities of a series of arylmethyl radicals174,175. 

Phen and the radical anion of the alkene. Secondary electron transfer from allylsilane to Phen produces the radical cation of allylsilane and neutral Phen. The radical cation of allylsilane is cleaved by assistance of acetonitrile to generate an allyl radical. The allyl radical adds to the radical anion of the alkene to give the allylated anion which is converted into the product upon protonation. Alkyl and arylmethyl radicals can be generated in a similar manner from tetraalkyl tin compounds and arylmethylsilanes, respectively [124]. These radicals add regioselectively to the -position to the cyano groups in the radical anions of alkenes. 

The feasibility of coupling defects was studied with Monte Carlo conformational searches (MM3 force field, with parametrization for arylmethyl radicals) for 50 in the gas phase. The representative low-energy conformation (Fig. 30) had... 

Alonsono and co-workers [146] have used substituted 1-naphthyl and phe-nylphosphonium chlorides as precursors for the generation of the corresponding arylmethyl radicals and cations in both nanosecond LFP and product studies. For instance, the salt 101 has a quantum yield for cation formation of 0.71 in methanol and the sole product observed was the corresponding methyl ether. No transient radical was observed in this solvent. In contrast, in 5% acetonitrile in dioxane, the radical was observed but now the cation was absent. No fluorescence was observed in either solvent suggesting that Si is very reactive. Redox potentials indicate that the conversion of the radical/radical ion pair to the cation/triphenyl-phosphine pair would be exothermic by some 25 kcal/mol. Therefore, both heterolytic cleavage from Si or homolytic cleavage followed by electron transfer were suggested as possible pathways for cation formation. 

Radicals and radical ions provide fruitful subjects of research. Room temperature fluorescence from the arylmethyl radicals Ph3C, Ph2CH- and PhCH2 and theoretical studies of rotational barriers in the benzyl cation, radical and anion as well as the singlet and triplet states of diphenylcarbene are typical examples of such contemporary studies. A very detailed paper considers the problems of the state assignment and reactivity of excited states of p-substituted benzyl radicals.Ketyl radicals containing the enthrone moiety and the 4-(methyl sulphonate) benzophenone ketyl radical anion are related studies in this field. 

Values are log (relative rates) of abstraction. An arylmethyl radical intermediate is formed. 

Yrs are two-centre two-electron repulsion energies. For aromatic ions, the onsite electron pair densities, i.e., the diagonal elements of the spinless second order density matrix are lower than those of any classical structure. For radicals, however, the pair densities are increased relative to those in the corresponding classical structures. Thus the resonance energy of the ion exceeds that of the radical by 21 Ec I [33]. FIMO and PMO do not differentiate between the formation of an arylmethyl radical and its carbocation. Empirically, radicals are better described by these methods this has been related to the constant Coulson charge order Q = 1 for arylmethyl radicals as opposed to the variable rt-charge order 1 on arylmethyl cations [16,39]. The actual PPP values of Ec have been correlated to an excellent accuracy to their PMO-o) counterparts [16]. 

This group comprises several characteristic series of neutral radicals odd polyenes, odd a,co-diphenylpolyenes, arylmethyl radicals, and odd... 

FIGURE 4.5. NBMO coefficients in some arylmethyl radicals. 

In these reactions, the key step is loss of a proton from the initially formed radical cation to form an arylmethyl radical. The driving force is the production of a highly stabilized radical. In other cases, the radical cation can combine with an anion to form a neutral radical which then undergoes further oxidation. Thus the first step in the oxidation of anthracene involves a process of this kind. 

Table II presents the originally obtained percent of reaction at the methyl group and corrected relative rates of hydrogen abstraction. Also shown are the calculated relative energy differences between the arylmethyl radical and the initial arene. It can be seen that a large range of experimental reactivities, nearly three powers of ten, is encountered.

The correlation of the rates of formation of arylmethyl free-radicals by molecular oibital theory will be discussed. Different levels of sophistication among pi-electron methods lead to conflicting conclusions concerning the degree of possible electron localization and the importance of non-bonded interactions with the principal radical site. The arylmethyl radical systems have been generated both by hydrogen abstraction... 

Decarboxylation of the acyloxy radical then competes with electron transfer (k f) for formation of ion pairs. The rates of electron transfer for both substituted 1-naphthylmethyl 2 and benzyl substrates 6 follow Marcus theory in both the normal and inverted region when correlated with the oxidation potential of the arylmethyl radical. The meta-methoxy compounds give high yields of ion-derived products because the oxidation potentials of their arylmethyl radicals place them near the maximum on the Marcus plot therefore, kg is competitive with fcco2- This work has been reviewed in the previous volume of this Handbook and in other places." ... 

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