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 CeIV‐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 μs), 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 FeII catalyst in the presence of H2O under photoirradiation, to yield dioxygen with the corresponding hydroquinone derivatives.

How Photoactivation Triggers Protochlorophyllide Reduction: Computational Evidence of a Stepwise Hydride Transfer during Chlorophyll Biosynthesis

“…This suggests that photoactivation facilitates HYT by initiating eT from NADPH to PChlide, which has two effects: (i) the next step becomes exothermic and (ii) the BDE for homolytic cleavage of the breaking C–H bond decreases. Note that a photoexcited oxidant in a biomimetic reaction was also recently shown to work by decreasing the C–H BDE, 41 so this appears to be an effective strategy for light-activated H-transfer reactions and should be considered when designing artificial light-harvesting systems; to this end, simple BDE calculations can be very informative. The fact that the POR reaction involves the fastest known biological HYT is likely due to the photochemical nature of this reaction since photoactivation triggers eT and hence the decrease in H-donor BDE.…”

The Merger of Photocatalyzed Hydrogen Atom Transfer with Transition Metal Catalysis for C−H Functionalization of Alkanes and Cycloalkanes

“…Based on the mechanistic studies, a radical relay sequence involving photoinduced HAT catalysis, nickel-based asymmetric catalysis and sulfur dioxide insertion was proposed. Different from the previous studies, the nickel catalyst appears to provide Lewis acid activation of the α,β-unsaturated carbonyl substrate (18) and governs the stereoselective radical addition step (E (17), is a reaction component critical to achieving both the reactivity and selectivity. This study demonstrates the possibility of using abundant hydrocarbon feedstocks as the starting materials for high value-added asymmetric synthesis.…”

“…Photocatalyzed hydrogen atom transfer (HAT) processes have shown considerable potential in the activation of inert C−H bonds [15–19] . The reactions can proceed through either an indirect HAT ( i ‐HAT) or a direct HAT ( d ‐HAT) pathway to cleave C(sp 3 )−H bonds, in either case generating high‐energy C ‐centered radicals [20] .…”

“…[7][8][9][10][11][12][13][14] Photocatalyzed hydrogen atom transfer (HAT) processes have shown considerable potential in the activation of inert CÀ H bonds. [15][16][17][18][19] The reactions can proceed through either an indirect HAT (i-HAT) or a direct HAT (d-HAT) pathway to cleave C(sp 3 )À H bonds, in either case generating high-energy Ccentered radicals. [20] Mechanistically, an i-HAT photocatalytic reaction involves the cooperation of a photocatalyst and a HAT co-catalyst or reagent to generate species capable of abstracting a hydrogen atom.…”