Free-Radical Aromatic Nitromethylation Promoted by Cerium(IV)

Article abstract of DOI:10.1021/jo00336a008

A search for metal salt oxidants, other than manganese(III) acetate, capable of effecting aromatic nitromethylation with nitromethane was undertaken.Of the many screened, cerium(IV) salts showed the most promise.Cerium(IV) ammonium nitrate gave high, though initially erratic, yields (essentially 100percent yield of isomeric α-nitroxylenes from toluene and α-nitrotoluene from benzene).However, nitration products and aromatic aldehydes were generated as well.Product formation was followed as a function of time in these reactions, and nitromethylation products were shown to be converted to the corresponding isomeric tolualdehydes or benzaldehyde upon prolonged heating.Cerium(III) nitrate was shown to be primarily responsible for promoting this side reaction.Cerium(IV) acetate, generated in solution by ozonolysis of either cerium(III) nitrate or a mixture of cerium(III) acetate and cerium(III) nitrate in acetic acid, also was shown to promote high yields of nitromethylation products.Furthermore, this reaction was free of side products.Isomer studies and relative rates for nitromethylation were determined for both cerium(IV) salts in comparison with manganese(III) acetate.A Hammett treatment in each case gave similar ρ values (vs. ?⁺): -2.3 from manganese(III) acetate, -2.0 from cerium(IV) ammonium nitrate, and -1.9 from cerium(IV) acetate, indicating a common intermediate in each case.The nitromethyl radical, believed to the substituting entity, exhibits marked electrophilic properties.