Electrochemical Study of the Nonaqueous Oxidation of Dipyrrolic Compounds

Article abstract of DOI:10.1021/jo00269a037

With use of cyclic voltammetry, electrochemical oxidation potentials vs Ag/AgCl in acetonitrile of 26 pyrromethanes, 10 pyrromethenes, and 2 pyrroketones are measured.Compounds of these types are valuable intermediates in the organic synthesis of porphyrins.Analysis of the measured potentials shows that substituent partial potentials previously established for monopyrroles (Tabba and Smith, J.Org.Chem. 1984, 49, 1870) can usually be used to accurately predict the experimental data, except under circumstances where pyrromethanes possess a terminal α-methyl group.AnECE mechanism is proposed for the electrochemical oxidation of pyrromethanes 1, which are shown to afford pyrromethene salts 2 by way of a two-electron oxidation at a potential approximating the corresponding monopyrrole calculated value.An additional wave at higher potential probably corresponds to oxidation of the pyrromethene 2, and this potential is pH sensitive.A pyrromethane (42) bearing a gem-dimethyl function at the interpyrrole carbon and acetonepyrrole (43) fail to afford pyrromethene upon oxidation, and their insulated dipyrrole or polypyrrole systems behave as simple monopyrroles.