Electrolysis of Substituted Heterocycles

In this Section we shall consider the electrolysis of substituted heterocyclic compounds, where the electrode reaction involves the substituent directly, or when it plays an essential role in determining the course of the reaction. The reductions leading to ring contractions have been discussed in Section IV, and some compounds treated in Section V might well have been included in Section VI. 

In the example given above, experimental conditions and the control of the electrical charge may orientate the reaction selectively to the mono-, di-, and tri substitution. The action of Lewis acid on substituted amine functions allows one to get back the iminium intermediates [38, 39]. Thus, the nucleophilic action in the course of the electrolysis could be seen as an iminium protection , and renders possible easy synthesis of the heterocycles as outlined below ... 

Although controlled substitution with molecular fluorine is difficult to attain, noteworthy is the recent success in perfluorinating both saturated and unsaturated heterocycles by electrolysis in anhydrous hydrogen fluoride. Cf. T. C. Simmons and F. W. Hoffmann, J. Am. Chem. Soc. 79, 3429 (1957), for the preparation of undecafluoropiperidine (from piperidine). The latter compound can be converted into pentafluoropyridine by passing it over an iron contact at 600° [R. E. Banks, A. E. Ginsberg, and R. N. Haszeldine, J. Chem. Soc. p. 1740 (1961)]. 

Quinone imine ketals have also been recognized to be quite useful for heterocycle synthesis. In a series of quinone mono- and bis-ketal chemistry, Swenton and coworkers carried out anodic oxidation of trifluoroacetamido-substituted p-methoxyphenols . For example, the readily available p-methoxyphenol derivative 88 underwent constant current electrolysis (60 mA) in 2% LiC104 in methanol, followed by hydrolysis with 5% aqueous KOH to afford quinone imine ketal 89 in 82% overall yield, through quinone monoketal 90 (Scheme 17). Furthermore, acid treatment of 89 with TsOH provided 5-methoxyindole (91). 

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