In-Cage Formation of Carbanions in Photoinduced Electron-Transfer Reaction of Carboxylate Ions

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• 作者：Hajime Yokoi ; Tomoharu Nakano ; Wataru Fujita ; Katsuya Ishiguro ; and Yasuhiko Sawaki
• 刊名：Journal of the American Chemical Society
• 出版年：1998
• 出版时间：December 9, 1998
• 年：1998
• 卷：120
• 期：48
• 页码：12453 - 12458
• 全文大小：128K
• 年卷期：v.120,no.48(December 9, 1998)
• ISSN：1520-5126

文摘

To develop a novel method for the photochemical generation of carbanions, the photoinduced electron-transfer reaction of carboxylate salts was investigated. The photolysis of tetra-n-butylammonium or metalion/crown ether salts of diphenylacetate (Ph₂CHCO₂^-) sensitized with 1-cyanonaphthalene or 1,4-dicyanonaphthalene in THF or benzene yielded Ph₂CH₂ as the protonated product of the corresponding carbanion(Ph₂CH^-). This reaction was not affected by the presence of oxygen, and nanosecond transient absorptionspectroscopy allowed the observation of Ph₂CH^- immediately after the laser pulse. The suggested reactionmechanism involves (1) photoexcitation of the sensitizer, (2) the one-electron oxidation of the carboxylateion, (3) the decarboxylation of the resulting carboxyl radical generating Ph₂CH, and (4) the back electron-transfer from the sensitizer radical anion to the radical to yield the carbanion, which occurs smoothly withincage of the geminate radical ion pair. When the sensitizer employed was 9,10-dicyanoanthracene, which hasa less negative reduction potential, Ph₂CH was observed by laser flash photolysis. The free energy change(-G) on the electron transfer is the crucial factor determining the radical/anion selectivity.