Anodic Amide Oxidations in the Presence of Electron-Rich Phenyl Rings: Evidence for an Intramolecular Electron-Transfer Mechanism

The anodic oxidations of amides in the presence of mono-, di-, and trialkoxyphenyl rings were examined. Although literature reduction potentials suggest that these oxidations would lead to either selective aromatic ring oxidation or mixtures, the chemoselectivity of the reactions was found to be dependent on the substitution pattern of the phenyl ring. For example, the anodic oxidations of ((3-methoxyphenyl)acetyl)pyrrolidine, ((2-methoxyphenyl)acetyl)pyrrolidine, ((3-methoxy-4-(pivaloyloxy)phenyl)acetyl)pyrroiidine, and ((3,5-dimethoxy-4-(pivaloyloxy)phenyl)acetyl)pyrrolidine all led to selective methoxylation of the pyrrolidine ring. The anodic oxidations of ((4-methoxyphenyl)acetyl)pyrrolidine and ((3,4-dimethoxyphenyl)acetyl)pyrrolidine led to selective methoxylation of the benzylic carbon. Mechanistic studies indicate that both amide and aryl oxidation processes compete under the reaction conditions, but that intramolecular electron transfer leads to the selective formation of products. Evidence for this mechanism was obtained by examining the cyclic voltammogram of ((3-methoxyphenyl)-acetyl)pyrrolidine, competition studies, and the preparative electrolysis of ((4-methoxyphenyl)dimethylacetyl)pyrrolidine. The methoxylated amides were cyclized to form tricyclic amides using titanium tetrachloride.