Reactivity of Anodically Generated 4‑Methoxystilbene Cation Radicals: The Influence of Ortho-Substituted Hydroxymethyl, Aminomethyl, and Carboxylic Acid Groups

The
effect of ortho′-substituted side chains bearing nucleophilic
groups such as CH2OH, CH2NHR, and CO2H on the reactivity of anodically generated 4-methoxy- and 3,4-dimethoxystilbene
cation radicals was investigated, and results were compared with those
of substrates where the nucleophilic groups such as OH and NHR are
directly attached to the aromatic ring. It was found that when ortho′-substituted
groups such as CH2OH or CH2NHR are present in
the other ring, only direct intramolecular cation–nucleophile
reactions occur to give bisbenzopyrans or bisisoquinolines. Crossover
products (previously obtained when the ortho′ substituents
were OH and NH2) such as the fused benzoxepanes/fused benzoazepanes
were not formed. When the ortho′ substituent is COOH, direct
intramolecular cation–nucleophile reaction occurs to give the
corresponding bis-δ-lactones in high yield. The presence of
an additional 3-methoxy substituent resulted in the formation of other
fused polycyclic products due to competing aromatic substitution reactions.
Reaction pathways leading to the different products and reasons for
the difference in behavior shown by the present stilbenes are presented.
The results have provided additional insight into the reactivity and
behavior of anodically generated stilbene cation radicals.

本研究考察了带有CH₂OH、CH₂NHR、CO₂H等亲核基团的邻位取代侧链，对阳极生成的4-甲氧基与3,4-二甲氧基芪阳离子自由基(cation radicals)反应活性的影响，并将实验结果与亲核基团（如OH、NHR）直接连于芳香环的底物所得结果进行了对比。研究发现，当另一芳环上带有CH₂OH或CH₂NHR等邻位取代基团时，仅发生分子内阳离子-亲核试剂(intramolecular cation–nucleophile)反应，生成双苯并吡喃(bisbenzopyrans)与双异喹啉(bisisoquinolines)。此前当邻位取代基为OH与NH₂时可得到的交叉产物（如稠合苯并氧杂卓(fused benzoxepanes)/稠合苯并氮杂卓(fused benzoazepanes)）在此实验中并未生成。当邻位取代基为COOH时，则发生分子内阳离子-亲核试剂反应，以高收率得到对应的双δ-内酯(bis-δ-lactones)。若芳环上额外带有3-甲氧基取代基，则因竞争性芳香取代反应(aromatic substitution reactions)，会生成其他稠合多环产物。本文阐明了生成不同产物的反应路径，以及本次实验所用芪类底物表现出反应行为差异的原因。本研究结果为深入理解阳极生成的芪阳离子自由基的反应活性与行为特征提供了新的认知。