Electroreductive dimerization of coumarin and coumarin analogues at carbon cathodes.
Electroreductive dimerization of coumarin and coumarin analogues at carbon cathodes.
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DOI：
10.1021/jo502272g
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发表时间：
2015-01
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影响因子：
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通讯作者：
Erick M. Pasciak;Jonathan T. Rittichier;Chun-Hsing Chen;Mohammad S. Mubarak;M. VanNieuwenhze;D. Peters
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文献类型：
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作者：
Erick M. Pasciak;Jonathan T. Rittichier;Chun-Hsing Chen;Mohammad S. Mubarak;M. VanNieuwenhze;D. Peters
用循环伏安法和控制电位电解法研究了香豆素（1）、6-甲基香豆素（2）、7-甲基香豆素（3）、7-甲氧基香豆素（4）和5，7-二甲氧基香豆素（5）在含0.10 M四氟硼酸四正丁基铵的二甲基甲酰胺中的碳阴极上的电化学还原。1-5的循环伏安图显示两个不可逆的阴极峰：（a）第一个峰来自香豆素的单电子还原，形成自由基-阴离子中间体，该中间体被介质质子化，得到中性自由基;（B）尽管该基团的大部分经历自偶联以产生氢化二聚物，剩余自由基的还原（最终产生二氢香豆素）引起第二个阴极峰。在对应于第一伏安峰的电位下，1-5的本体电解提供相应的氢化二聚物，为内消旋和dl非对映异构体的混合物。虽然内消旋形式占主导地位，但由于芳环上的取代基产生的空间效应，dl与内消旋的比率变化。1和4的等摩尔混合物的电还原产生，沿着预期的对称氢化二聚物，衍生自两种香豆素的不对称产物。提出了一个涉及自由基-阴离子和自由基中间体的机制方案来解释各种产物的形成。
Electrochemical reduction of coumarin (1), 6-methylcoumarin (2), 7-methylcoumarin (3), 7-methoxycoumarin (4), and 5,7-dimethoxycoumarin (5) at carbon cathodes in dimethylformamide containing 0.10 M tetra-n-butylammonium tetrafluoroborate has been investigated by means of cyclic voltammetry and controlled-potential (bulk) electrolysis. Cyclic voltammograms for reduction of 1-5 exhibit two irreversible cathodic peaks: (a) the first peak arises from one-electron reduction of the coumarin to form a radical-anion intermediate, which is protonated by the medium to give a neutral radical; (b) although most of this radical undergoes self-coupling to yield a hydrodimer, reduction of the remaining radical (ultimately to produce a dihydrocoumarin) causes the second cathodic peak. At a potential corresponding to the first voltammetric peak, bulk electrolysis of 1-5 affords the corresponding hydrodimer as a mixture of meso and dl diastereomers. Although the meso form dominates, the dl-to-meso ratio varies, due to steric effects arising from substituents on the aromatic ring. Electroreduction of an equimolar mixture of 1 and 4 gives, along with the anticipated symmetrical hydrodimers, an unsymmetrical product derived from the two coumarins. A mechanistic scheme involving both radical-anion and radical intermediates is proposed to account for the formation of the various products.