Regioselective Electrochemical Oxidation of One of the Identical Benzene Rings of Carbazole to 1,4-Quinone on the MWCNT Surface and Its Electrocatalytic Activity

Carbazole-1,4-quinone is a key structural unit of naturally occurring carbazole-quinone alkaloids, for instance, 3-methyl carbazole-1,4-quinone (Murrayaquinone), that are widely used in pharmaceutics and medicine. These compounds are generally synthesized from tetrahydro ketone derivatives by multistep synthetic routes. Until now, there is no direct method reported for the partial oxidation of carbazole to carbazole-1,4-quinone by chemical, biochemical, or electrochemical reaction routes. Literature survey on the oxidation of carbazole and its derivatives suggests that polymeric or oligomeric forms are the sole products, but not, monomeric carbazole-1,4-quinone. Herein, we report a simple electrochemical oxidation of surface-confined carbazole to a highly redox active carbazole-mono-1,4-quinone, where only one of the identical benzene rings of carbazole gets selectively oxidized, on a cathodically activated multiwalled carbon nanotube (MWCNT) surface anchored to a glassy carbon electrode (GCE/MWCNT*@Car-Qn; * = cathodically activated, Car-Qn = carbazole-1,4-quinone) in pH 7 phosphate buffer solution, unlike the time consuming conventional multistep synthetic routes. The GCE/MWCNT*@Car-Qn showed a well-defined surface-confined peak at E-1/2 = 215 +/- 5 mV versus Ag/AgCl with a Nertisan pH dependence in character. This new hybrid system showed efficient electrocatalytic oxidation and sensing of hydrazine in a neutral pH solution. No such electrochemical features were noticed for carbazole on the GCE surface. Collective physico-chemical (scanning electron microscopy, transmission electron microscopy, Fourier transform infrared, HRMS, and NMR) and electrochemical characterizations of the MVVCNT*@Car-Qn formation process revealed that in situ formed H2O2 at cathodic potentials and iron impurity in MWCNT, forming electro-Fenton species, are responsible for the selective electrochemical oxidation of carbazole to carbazole-1,4-quinone on the MWCNT surface.