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.

本研究针对1,2-二芳基烯烃（1,2-diarylalkenes）的电化学氧化开展系统性探究，重点聚焦产物的详细表征以及产物类型随芳环取代基变化的规律。首先在多种反应条件下对4,4'-二甲氧基二苯乙烯（4,4′-dimethoxystilbene）的电化学氧化过程进行了重新考察，并对所有生成产物完成了全面表征与定量分析。随后系统探究了芳环取代基对产物类型与产物分布的影响。研究发现芳基取代基可分为三大类别：第一类为芳环取代基的性质与位置所生成的产物与4,4'-二甲氧基二苯乙烯基本一致，例如四芳基四氢呋喃（tetraaryltetrahydrofurans）、脱氢四氢萘（dehydrotetralins）与醛类；此类底物的单芳环上分别连有p-MeO（对甲氧基）或p-NMe2（对二甲氨基），另一芳环连有取代基X，其中X可为邻甲氧基、烷基、间位或对位吸电子基团（electron-withdrawing group, EWG），例如4-甲氧基-4'-三氟甲基二苯乙烯。第二类为芳环（单环或双环）被烷基取代的底物，可生成茚基乙酰胺（indanyl acetamides，或四氢萘基乙酰胺）与脱氢四氢萘（或pallidol）的混合物，例如4,4'-二甲基二苯乙烯。第三类为供电子基团（如甲氧基、羟基）处于特定位置的底物，可生成安五素F（ampelopsin F）与pallidol型碳骨架，例如4,3',4'-三甲氧基二苯乙烯。本文还提出了可合理解释各类产物生成路径的反应途径。