Polarographic Properties and Electrochemical Reduction of 1,2-Dimethyl-3-indolyl Heteroaryl Ketones

The reduction of a series of 1,2-dimethyl-3-indolyl heteroaryl ketones, where the heteroaryl components are 2-benzoxazolyl, 2-behzpthiuzolyl. 1-methyl-2-benz-imidazolyl, 4- and 2-pyridyl, in aprotic and prone solution are described by means of DC. and AC. polarography, cyclic voltammetry and coulometry. In acetonitrile a two-step reduction mechanism with a reversible first and an irreversible second one-electron transfer has been observed, confirmed by AC.-polarographic and cyclovoltammetric measurements. The reduction potentials, between ?1.377 and ?1.787 V for the first step, and between ?1.937 and ?2.250 V (vs. Ag/AgCl) for the second, could not be correlated with the LUMO energies from HMO calculations which was explained as being due to a distorted configuration of the π systems. In protic media (ethanol/water) reduction occurs in an irreversible one-step mechanism. Measurements of the pH dependence of the half-wave potential in the pH* range 0–12 and the evaluated transfer coefficients show that the reduction step represents the transfer of two electrons and two protons yielding secondary alcohols and no dimerization products under any condition. These findings are supported by coulometric and macroscale electrolysis of the ketones; the products isolated were characterized by IR. and ¹H-NMR. data.