Carbon-Oxygen Bond Cleavage Reactions by Electron Transfer. 1. Electrochemical Studies on the Formation and Subsequent Reaction Pathways of Cyanoanisole Radical Anions

Article abstract of DOI:10.1021/ja00322a013

The radical anions of three isomers of cyanoanisole have been electrochemically generated and subsequently shown to react by at least three different reaction pathways in dry N,N-dimethylformamide (DMF).The o-cyanoanisole radical anion (E_pc = -2.3 V vs.SCE) dimerizes (k₂ = 3.2*10² M^-1 s^-1) to form an intermediate dimeric dianion.The dianion, which can be oxidized (E_pa = -1.1 V vs.SCE), undergoes a slow intramolecular disproportionation reaction to form o-cyanophenoxide ion, methide ion, and unreduced substrate (k₃ = 1.9*10^-2 s^-1).Subsequent protonation results in the formation of methane and p-cyanophenol in an overall two-electron process.The m-cyanoanisole radical anion (E_pc = -2.3 V vs.SCE) is very stable (t_1/2 > 10³ s) under anhydrous DMF conditions.Overall, slow carbon-carbon bond cleavage with loss of cyanide occurs competitively with β-carbon-oxygen bond cleavage to produce anisole and m-cyanophenol, respectively.The final products of the reduction of p-cyanoanisole are p-cyanophenol and methane; however, the radical anion of p-cyanoanisole (E_pc = -2.5 V vs.SCE) undergoes a relatively rapid unimolecular fragmentation reaction (k₁ = 7 s^-1).The initial products of the fragmentation are p-cyanophenoxide ion and the methyl radical, which is reduced further to methide ion.Hydrogen atom abstraction reactions by the methyl radical can also occur in the bulk solution to produce methane.