Abstract
Singlet O2 is a key reactive oxygen species responsible for photodynamic therapy and is generally recognized to be chemically reactive towards C=C double bonds. Herein, we report the hydroperoxidation/lactonization of α-ethereal C–H bonds by singlet O2 (1Δg) under exceptionally mild conditions, i.e., room temperature and ambient pressure, with modest to high yields (38~90%) and excellent site selectivity. Singlet O2 has been known for > 90 years, but was never reported to be able to react with weakly activated C–H bonds in saturated hydrocarbons. Theoretical calculations indicate that singlet O2 directly inserts into the α-ethereal C–H bond in one step with conservation of steric configuration in products. The current discovery of chemical reaction of singlet oxygen with weakly activated solvent C–H bonds, in addition to physical relaxation pathway, provides an important clue to a 35-year-old unresolved mystery regarding huge variations of solvent dependent lifetime of singlet O2.
Highlights
Singlet O2 is a key reactive oxygen species responsible for photodynamic therapy and is generally recognized to be chemically reactive towards C=C double bonds
We demonstrate that singlet oxygen can chemically react with C–H bonds of various ethereal solvent molecules, leading to the formation of hydroperoxide and lactone products (Tables 1 and 2)
The site selectivity of singlet O2 is most probably governed by the C–H bond strength of the α-ethereal C–H bond and the activation energy, rather than by its electrophilicity
Summary
Singlet O2 is a key reactive oxygen species responsible for photodynamic therapy and is generally recognized to be chemically reactive towards C=C double bonds. Singlet O2 has been known for > 90 years, but was never reported to be able to react with weakly activated C–H bonds in saturated hydrocarbons. The current discovery of chemical reaction of singlet oxygen with weakly activated solvent C–H bonds, in addition to physical relaxation pathway, provides an important clue to a 35year-old unresolved mystery regarding huge variations of solvent dependent lifetime of singlet O2. When strained hydrocarbons (such as cubanes) were used to quench the excited state of singlet oxygen, the quenching rate is one order faster than that predicted by the solvent C–H bond vibration-induced perturbation model[9]. There is a large variation and inconsistency between the experimentally measured singlet oxygen lifetimes and theoretically calculated values derived from solvent C–H bond vibrationinduced physical perturbation model[7,8,9]. To-date, the origin of the huge lifetime variations mystery[6,7,8,9, 12]
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