Photoinduced Generation of Superoxidants for the Oxidation of Substrates with High C-H Bond Dissociation Energies

Abstract

Oxidation of substrates, such as methane, with large C-H bond dissociation energies usually requires harsh reaction conditions, such as high temperature and pressure. The use of photoexcited states of oxidants enables the oxidation of substrates which would otherwise be unable to react thermally. This Review focuses on photoinduced generation of strong inorganic and organic oxidants, which enables the oxidation of substrates with strong C-H bonds. For example, photoexcitation of a Ce-IV-alkoxide complex results in the cleavage of Ce-O bond to produce an alkoxyl radical, which can abstract a hydrogen atom from methane to afford a methyl radical, leading to amination of methane with tert-butyloxycarbonyl (Boc)-protected monomethylhydrazine. Hydrogen atom abstraction from alkanes also occurs induced by the photoexcited state of tetrabutylammonium decatungstate, leading to alkylations and acylation of both aromatic and aliphatic N-tosylimines. Photoexcitation of Bi- and V-containing beta zeolites also results in hydrogen atom abstraction from methane to produce methanol at ambient temperature. The photoexcited state of a manganese(IV)-oxo complex binding with two Sc3+ ions has a surprisingly long lifetime (6.4 mu s), and is able to oxidize benzene to produce phenol. Chlorine radicals, that can be generated by photoinduced oxidation of chloride ion by the photoexcited states of oxidants or by photoexcitation of a chlorine dioxide radical, abstracts a hydrogen atom from alkanes including methane to produce alkyl radicals, which can be converted to the oxygenated products. Photoinduced oxidation of methane was also made possible by mixing photosystem II (PSII) and the membrane fraction of a particular form of methane monooxygenase. A PSII model reaction has been achieved by using p-benzoquinone derivatives as plastoquinone analogues with a non-heme Fe-II catalyst in the presence of H2O under photoirradiation, to yield dioxygen with the corresponding hydroquinone derivatives.