Biomimetic Oxidative Dimerization of Anodically Generated Stilbene Radical Cations: Effect of Aromatic Substitution on Product Distribution and Reaction Pathways

A systematic study of the electrochemical
oxidation of 1,2-diarylalkenes
was carried out with the focus on detailed product studies and variation
of product type as a function of aromatic substitution. A reinvestigation
of the electrochemical oxidation of 4,4′-dimethoxystilbene
under various conditions was first carried out, and all products formed
were fully characterized and quantitated. This was followed by a systematic
investigation of the effect of aromatic substitution on the nature
and distribution of the products. The aromatic substituents were found
to fall into three main categories, viz., substrates in which the
nature and position of the aromatic substituents gave rise to essentially
the same products as 4,4′-dimethoxystilbene, for example, tetraaryltetrahydrofurans,
dehydrotetralins, and aldehydes (p-MeO or p-NMe2 on one ring and X on the other ring, where
X = o-MeO or p-alkyl, or m- or p-EWG; e.g., 4-methoxy-4′-trifluoromethylstilbene);
those that gave rise to a mixture of indanyl (or tetralinyl) acetamides
and dehydrotetralins (or pallidols) (both or one ring substituted
by alkyl groups, e.g., 4,4′-dimethylstilbene); and those where
strategic placement of donor groups, such as OMe and OH, led to the
formation of ampelopsin F and pallidol-type carbon skeletons (e.g.,
4,3′,4′-trimethoxystilbene). Reaction pathways to rationalize
the formation of the different products are presented.