Influence of the leaving group and of the annelation in the electroreduction of 2-methyl-substituted quinones

Abstract

In the present work, several 2-methylquinones, of the series benzo- (BQ), naphtho- (NQ) and anthraquinone (AQ), substituted at the C-2 side chain with groups of different nucleofugacity were prepared and submitted to electrochemical studies (cyclic voltammetry and electrolysis), in aprotic medium (DMF + 0.1 mol l −1 TBAP), using mercury electrodes, to observe the role of the 2-methyl substituents and of the annelation in their electroreductions. The analysis of the voltammograms showed that, in each series, mainly in the naphthoquinone case, which was more extensively investigated, the presence and the nature of the leaving group influence the cathodic reduction greatly. The comparison of the electrochemical behaviour allows the classification of the quinones in three main groups. The simplest one approaches the electrochemical behaviour of the unsubstituted quinone and in the other two, the substituents work as leaving groups (LG) and are lost, after uptake of one or two electrons, generating reactive intermediates, which form electroreductive species, giving rise, then, to complex voltammograms. More specifically, the reduction mechanism involves a one electron transfer to form a radical anion, which fragments, expelling a good leaving group such as the halide (-I, -Br, -Cl), -ONO 2 and -OTs, generating radicals that can follow different and competitive pathways, including generation of quinomethides and their addition products. For leaving groups of weak nucleofugacity, like –OAc or –OCHO, the cleavage occurs only after two-electron uptake. The influence of the substituents on the first wave redox potential ( E pIc), represented by positive shifts is in the order Br > OTs ∼ I > Cl > ONO 2 > OCHO ∼ OAc > OH > H and is related to the nucleofugal ability or the fragility of the C-LG. The extent of annelation of the quinone moiety also influences the feature of the cyclic voltammogram, through stabilization of the electrogenerated species. The feasibility of reduction, comparing the same substituents, is BQ ∼ NQ > AQ, and, the order of stability of the electrogenerated species is opposite: AQ > NQ > BQ.