Electrochemically Mediated Oxidative Transformations of Substituted 4‑Methoxystilbenes: Effect of Ortho-Substituted Nucleophilic Groups

A systematic
study was undertaken to determine the influence of
ortho′-substituted nucleophilic groups (OH, NH2,
or NHR) on the reactivity of anodically generated 4-methoxy- and 3,4-dimethoxystilbene
cation radicals. The results showed that when ortho-substituted nucleophilic
groups such as OH and NHR are present in the other ring, both direct
and crossover intramolecular cation–nucleophile reactions occur
to give bisbenzofurans/bisindoles or fused bisbenzopyrans/bisquinolines,
respectively. Where an additional 3-methoxy substituent is present,
bridged oxocine/azocine products are formed in addition to the bisbenzopyrans/bisquinolines
and bisbenzofurans/bisindoles. Mechanistic rationalization of the
observed behavior is presented based on a generalized pathway involving
fast cation radical dimerization following electron transfer, followed
by direct and crossover trapping of the benzylic cations by the ortho-substituted
oxygen and nitrogen nucleophilic groups. In the instances where an
additional 3-methoxy group is present, the bridged oxocine/azocine
products are also formed as a result of competing aromatic substitution
(Friedel–Crafts reaction). The results have shed further light
and provided additional clarification on the reactivity of anodically
generated stilbene cation radicals.