Radical anions from anisole

A number of papers report investigations of the pyrolytic cleavage of aromatic hydrocarbons. The oxidation and pyrolysis of anisole at 1000 K have revealed first-order decay in oxygen exclusively via homolysis of the O—CH3 bond to afford phenol, cresols, methylcyclopentadiene, and CO as the major products.256 A study of PAH radical anion salts revealed that CH4 and H2 are evolved from carbene formation and anionic polymerization of the radical species, respectively.257 Pyrolysis of allylpropar-gyltosylamine was studied at temperatures of 460-500 °C and pressures of 10-16 Torr. The product mixture was dominated by hydrocarbon fragments but also contained SO2 from a proposed thermolysis of an intermediate aldimine by radical processes.258... 

Ohashi et al. [136,139] have proposed a scheme describing the formation of ortho adducts and substitution products from anisole and the three dimethoxyben-zenes with acrylonitrile, methacrylonitrile, and crotonitrile (Scheme 39). Here, the ortho cycloadduct is supposed to be formed directly from an encounter complex or exciplex, whereas the substitution product arises via formation of an ion pair from the complex, followed by protonation of the radical anion and radical... 

When the irradiation was performed in MeOD, incorporation of one deuterium atom into the a-methyl groups could be demonstrated. The reaction proceeds via photoinduced electron transfer from the excited anisole to acrylonitrile, possible in an exciplex [139], The radical anion of acrylonitrile abstracts a proton from the solvent and the resulting radical combines with the radical cation of anisole forming a a-complcx that finally loses a proton. 

For substituted benzenes, the electronic nature of the substituent is responsible for the stability of the radical anion intermediate initially formed. Thus, in the case of anisole (3), the electron transfer is slower due to the presence of the electron-donating methoxy group, and from the two possible anion-radicals V and VI, the first one is less stable due to electron repulsion of vicinal carbanion and the electron-rich oxygen substituent. After taking a proton, radical VO is generated, which after sueeessive capture of an electron and a proton yields the final product 2,5-dihydroderivative 4 (Scheme 13.3). 

Let ns direct onr attention to the difference between the anion-radicals 804 and COj". While the latter is a one-electron rednctant (see section 1.7.4), the former is a one-electron oxidant. One-electron transfer from a snbstrate to the snlfate radical mostly follows diffusion rates. For instance, rate constants of one-electron oxidation of benzene and anisole with SO4 are equal to 3 X 10 and 5 X 10 L mol s respectively (Goldstein and McNelis 1984). 

However, the presence in the anolyte of an anion, which oxidises more readily than the organic substrate and which appears in the substituted product, does not preclude ionisation of the substrate as the operative mechanism. Thus Parker and Burget showed that cyanation of anisole does not proceed at potentials where only cyanide is electroactive ionisation of the anisole is a prerequisite for substitution. " A further example is the chlorination of cyclohexene where, by operating at high electrode potentials at which the olefin is electroactive, chloride ion can be intercepted en route to the anode surface by cyclohexene radical cations diffusing away from it. ... 

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