Electrochemical oxidation of 2,2,6,6-tetramethylpiperidines in acetonitrile: mechanism of N-cyanomethylation

Abstract

Electrochemical oxidation of 2,2,6,6-tetramethylpiperidine (1a), its 4-oxo derivative (1b), and 2,2,6,6-tetramethylmorpholine (1c) in deoxygenated acetonitrile gave the corresponding N-cyanomethylated products (2). The process was investigated by cyclic voltammetry, controlled-potential electrolysis, and electrolysis in the cavity of an e.s.r. spectrometer. The sequence of cyanomethylation is proposed to be as follows: one-electron transfer from (1) to generate the radical cation, [graphic omitted]H⁺˙(3); deprotonation of (3) to give the aminyl radical, [graphic omitted]: (4); hydrogen abstraction by (4) from the solvent to afford the cyanomethylene radical; cross-coupling of (4) with the solvent-derived radical. The generation of the radical (4) was confirmed by the e.s.r. experiments. The decay of the 4-oxopiperidinyl radical (4b) obeyed first-order kinetics and exhibited a large primary deuterium isotope effect in [²H₃]acetonitrile, indicating that the hydrogen abstraction is rate-determining. The voltammetric behaviour of (1) in oxygen-saturated acetonitrile suggested that the aminyl radical (4) is oxidized at a potential more positive than is the starting amine (1).