(Invited) One-Step Aerobic Oxygenation of Hydrocarbons By Electron Transfer

Abstract

Photoinduced oxygenation of neat cyclohexane in the presence of O2 occurred under visible light irradiation of 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) which acts as a super photooxidant [1-3]. The products detected by GC and NMR analyses were cyclohexanol, cyclohexanone and cyclohexane hydroperoxide with 70, 40, 210% yields, respectively. When cyclohexane was replaced by n-butane, n-pentane and 3-methylpentane, the oxygenation reactions also took place to form the corresponding oxygenated products under the otherwise same photochemical reaction conditions. The reaction mechanism was clarified by femtosecond and nanosecond laser flash photolysis measurements. Femtosecond laser irradiation of DDQ in cyclohexane results in formation of the singlet excited state of DDQ (1DDQ*; * denotes the excited state) detected as a transient absorption band at 480 nm. The band of 1DDQ* decayed with increasing a new absorption band at 590 nm and 630 nm, which are assigned to 3DDQ* and DDQ semiquinone radical anion (DDQ•–), respectively. Thus, the photocatalytic reactions are initiated by electron-transfer oxidation of cyclohexane (CyH) with 1DDQ* to form the cyclohexane radical cation (CyH•+), which reacts with DDQ•– to produce the cyclohexyl radical (Cy•) and DDQ semiquinone radical (DDQH•). The triplet excited state (3DDQ*) generated by the intersystem crossing of 3DDQ* is also reacts with CyH to produce radical ion pair. Cy• reacts with O2to produce the final oxygenated products via radical chain process. [1] Ohkubo, K.; Fujimoto, A.; Fukuzumi, S. J. Am. Chem. Soc. 2013, 135, 5368. [2] Ohkubo, K.; Hirose, K.; Fukuzumi, S. Chem.–Eur. J. 2015, 21, 2855. [3] Fukuzumi, S.; Ohkubo, K. Asian J. Org. Chem. 2015, 4, 836 (Focus Review).