Role of Sulfide Radical Cations in Electron Transfer Promoted Molecular Oxygenations at Sulfur

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• 作者：Edward L. Clennan ; Chen Liao
• 刊名：Journal of the American Chemical Society
• 出版年：2008
• 出版时间：March 26, 2008
• 年：2008
• 卷：130
• 期：12
• 页码：4057 - 4068
• 全文大小：521K
• 年卷期：v.130,no.12(March 26, 2008)
• ISSN：1520-5126

文摘

The methylene blue, N-methylquinolinium tetrafluoroborate, and pyrylium-cation-sensitizedphotooxygenations of 5H, 7H-dibenzo[b,g] [1,5]dithiocin, 1, and 1,5-dithiacyclooctane, 2, have beeninvestigated. The methylene blue sensitized reactions exhibit all of the characteristics of a singlet oxygenreaction including isotope effects for the formation of a hydroperoxysulfonium ylide and the ability of 1 and2 to quench the time-resolved emission of singlet oxygen at 1270 nm. The product compositions in theN-methylquinolinium tetrafluoroborate and pyrylium-cation-sensitized reactions are dramatically differentand are both different from that anticipated for the participation of singlet oxygen. This argues for differentreaction mechanisms for all three sensitizers. However, both the quinolinium and pyrylium-cation-sensitizedreactions display all of the characteristics of electron-transfer-initiated photooxygenations. Both sensitizerswere quenched at nearly diffusion-limited rates by 1 and 2. Laser flash photolysis of mixtures of eithersensitizer and 1 or 2 resulted in direct observation of the reduced sensitizer and the sulfide radical cation.In addition, electron-transfer reactions involving both sensitizers were shown to be exergonic. These resultsare consistent with the previously proposed outer sphere electron-transfer mechanism for N-methylquinolinium tetrafluoroborate and were used to argue for a new inner sphere mechanism for the pyrylium cationreactions.