Photosensitized Oxidation of Alkyl Phenyl Sulfoxides. C−S Bond Cleavage in Alkyl Phenyl Sulfoxide Radical Cations

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• 作者：Enrico Baciocchi ; Tiziana Del Giacco ; Osvaldo Lanzalunga ; Paolo Mencarelli ; Barbara Procacci
• 刊名：Journal of Organic Chemistry
• 出版年：2008
• 出版时间：August 1, 2008
• 年：2008
• 卷：73
• 期：15
• 页码：5675-5682
• 全文大小：298K
• 年卷期：v.73,no.15(August 1, 2008)
• ISSN：1520-6904

文摘

The 3-cyano-N-methylquinolinium perchlorate (3-CN-NMQ⁺ ClO₄⁻)-photosensitized oxidation of phenyl alkyl sulfoxides (PhSOCR₁R₂R₃, 1, R₁ = R₂ = H, R₃ = Ph; 2, R₁ = H, R₂ = Me, R₃ = Ph; 3, R₁ = R₂ = Ph, R₃ = H; 4, R₁ = R₂ = Me, R₃ = Ph; 5, R₁ = R₂ = R₃ = Me) has been investigated by steady-state irradiation and nanosecond laser flash photolysis (LFP) under nitrogen in MeCN. Steady-state photolysis showed the formation of products deriving from the heterolytic C−S bond cleavage in the sulfoxide radical cations (alcohols, R₁R₂R₃COH, and acetamides, R₁R₂R₃CNHCOCH₃) accompanied by sulfur-containing products (phenyl benzenethiosulfinate, diphenyl disulfide, and phenyl benzenethiosulfonate). By laser irradiation, the formation of 3-CN-NMQ^• (λ_max = 390 nm) and sulfoxide radical cations 1^•+, 2^•+, and 5^•+ (λ_max = 550 nm) was observed within the laser pulse. The radical cations decayed by first-order kinetics with a process attributable to the heterolytic C−S bond cleavage leading to the sulfinyl radical and an alkyl carbocation. The radical cations 3^•+ and 4^•+ fragment too rapidly, decaying within the laser pulse. The absorption band of the cation Ph₂CH⁺ (λ_max = 440 nm) was observed with 3 while the absorption bands of 3-CN-NMQ^• and PhSO^• (λ_max = 460 nm) were observed just after the laser pulse in the LFP experiment with 4. No competitive β-C−H bond cleavage has been observed in the radical cations from 1−3. The C−S bond cleavage rates were measured for 1^•+, 2^•+, and 5^•+. For 3^•+ and 4^•+, only a lower limit (ca. >3 × 10⁷ s⁻¹) could be given. Quantum yields (Φ) and fragmentation first-order rate constants (k) appear to depend on the structure of the alkyl group and on the bond dissociation free energy (BDFE) of the C−S bond of the radical cations determined by a thermochemical cycle using the C−S BDEs for the neutral sulfoxides 1−5 obtained by DFT calculations. Namely, Φ and k increase as the C−S BDFE becomes more negative, that is in the order 1 < 5 < 2 < 3, 4, which is also the stability order of the alkyl carbocations formed in the cleavage. An estimate of the difference in the C−S bond cleavage rate between sulfoxide and sulfide radical cations was possible by comparing the fragmentation rate of 5^•+ (1.4 × 10⁶ s⁻¹) with the upper limit (10⁴ s⁻¹) given for tert-butyl phenyl sulfide radical cation (Baciocchi, E.; Del Giacco, T.; Gerini, M. F.; Lanzalunga, O. Org. Lett. 2006, 8, 641−644). It turns out that sulfoxide radical cations undergo C−S bond breaking at a rate at least 2 orders of magnitude faster than that of corresponding sulfide radical cations.