Bettoni Marta, Del Giacco Tiziana, Stradiotto Marina, Elisei Fausto
†Dipartimento di Ingegneria Civile ed Ambientale, ‡Dipartimento di Chimica, Biologia e Biotecnologie, and §Centro di Eccellenza Materiali Innovativi Nanostrutturati (CEMIN), Università di Perugia, 06123 Perugia, Italy.
J Org Chem. 2015 Aug 21;80(16):8001-8. doi: 10.1021/acs.joc.5b01052. Epub 2015 Aug 13.
The photo-oxidation of 4-methoxybenzyl methyl sulfide (1a), benzyl methyl sulfide (1b), and 4-cyanobenzyl methyl sulfide (1c) has been investigated in the presence of N-methoxy phenanthridinium hexafluorophosphate (MeOP(+)PF6(-)) under nitrogen in CH3CN. The steady-state photolysis experiments showed for the investigated sulfides exclusively the formation of the corresponding benzaldehyde as the oxidation product, reasonably due to a deprotonation of the sulfide radical cations. Photo-oxidation of 1a-1c occurs through an electron transfer process. Indeed, laser flash photolysis measurements showed an efficient formation of sulfide radical cations, detected in their dimeric form [(4-X-C6H4CH2SCH3)2(+•)] at ≈520 nm. At longer delay times, the absorption of the dimer radical cation was replaced by an absorption band assigned to the (α-thio)benzyl cation (thionium ion, λmax = 420-400 nm), formed by oxidation of the benzyl radical and not by that of the (α-thiomethyl)benzyl radical, as expected if a Cα-H bond cleavage is operative. This finding highlights a particular stability of this kind of cation never reported before, even though its involvement in one-electron oxidation mechanisms of various sulfides has already been invoked. Density functional theory calculations allowed identification of a significant charge and spin delocalization involving both the phenyl ring and the sulfur atom of the radical cations.
在氮气氛围下，于乙腈中，研究了4 - 甲氧基苄基甲基硫醚（1a）、苄基甲基硫醚（1b）和4 - 氰基苄基甲基硫醚（1c）在N - 甲氧基菲啶鎓六氟磷酸盐（MeOP(+)PF6(-)）存在下的光氧化反应。稳态光解实验表明，对于所研究的硫醚，仅生成相应的苯甲醛作为氧化产物，这合理地归因于硫醚自由基阳离子的去质子化。1a - 1c的光氧化通过电子转移过程发生。实际上，激光闪光光解测量表明有效地形成了硫醚自由基阳离子，在约520 nm处检测到其二聚体形式[(4 - X - C6H4CH2SCH3)2(+•)]。在更长的延迟时间下，二聚体自由基阳离子的吸收被一个归属于（α - 硫代）苄基阳离子（硫鎓离子，λmax = 420 - 400 nm）的吸收带所取代，该阳离子是由苄基自由基氧化形成的，而不是由（α - 硫代甲基）苄基自由基氧化形成的，正如预期的那样，如果Cα - H键断裂起作用的话。这一发现突出了这种阳离子前所未有的特殊稳定性，尽管其参与各种硫醚的单电子氧化机制已被提及。密度泛函理论计算能够确定自由基阳离子的苯环和硫原子之间存在显著的电荷和自旋离域。
