2-Methoxynaphthalene, radical anion protonation

Addition to alkenes can be sensitized by both electron-donors and electron-acceptors, and it is most likely that the reactive species is the alkene radical anion or the alkene radical cation, respectively. 1,1-Diphenylethylene can be converted to the Markownikov addition product with methanol (2.48) using the electron-donating sensitizer t-methoxynaphthalene no added proton acid is needed. Using... 

Arnold reported that the photoaddition of methanol and trifluoroethanol to 1,1-diphenylethene occurs in the photoreaction using methoxynaphthalenes as an electron donating sensitizer [35, 113]. The proposed mechanism for this reaction is shown in Scheme 34. The radical anion of the alkene is first produced by photoinduced electron transfer from the electron donating sensitizer to the alkene and it is protonated in a Markownikoff fashion to form the 1,1-diphenylethyl radical. The resulting radical is then oxidized by the radical cation of the electron donating sensitizer to generate the cation of the alkene. Finally, a nucleophilic attack of alcohol on this cation affords the alkoxylated product. 

Protonation becomes a rapid reaction in protic solvents and in the presence of acids, as demonstrated for, e.g., -butyl acrylate in aqueous solution [207], methyl acrylate in EtOH [208], cinnamates in the presence of phenol in DMF [209], and benzaldehyde in ethanolic buffer solution [210]. Rate constants for protonation of aromatic radical anions (anthracene [211], naphthalene, 2-methoxynaphthalene, 2,3-dimethoxynaphthalene) by a number of proton donors including phenols, acetic acid, and benzoic acids in aprotic DMF were found to vary from 5.0 X 10 M- s-> (for anthracene, in the presence of p-chlorophenol) to 6.2 x lO s (for anthracene, in the presence of pentachlorophenol) [212]. For dimedone, PhOH, or PhC02H the rate of protonation depends on the hydrogen-bond basicity of the solvent and increases in the order DMSO < DMF MeCN [213],... 

Few examples have been reported involving the complementary photo-EOCAS reaction, which corresponds to path g in Scheme 14.2, but with reversed charges [15], Typical reactions involved a methoxyarene (e.g. 1,4-dimethoxybenzene or 1-methoxynaphthalene) as donors and an electron-poor olefin (e.g. acrylonitrile) as acceptors [16]. As an example, the photochemicaUy produced radical anion of the acrylonitrile was protonated by MeOH as solvent, and then coupled with the aryl... 

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