Visible-Light-Enabled Allylic C–H Oxidation: Metal-free Photocatalytic Generation of Enones

Abstract

A practical and efficient method has been established for the direct oxidation of allylic C−H bonds catalyzed by visible-light-enabled photoredox agents. This protocol uses oxygen as the sole oxidant under metal-free conditions at room temperature and produces functionalized enones. When combined with acid-promoted dehydration, this benign and mild method enables the production of meta-functionalized phenols from simple cyclohexenols. A key to this reaction is the choice of tetrabutylammonium bromide as a hydrogen atom transfer cocatalyst.

Metal-free and site-selective α-C–H functionalization of tetrahydrofuran enabled by the photocatalytic generation of bromine radicals

“…When the reaction was conducted under an O 2 atmosphere, only a trace amount of the desired product was observed. The main reason we speculated is probably that, with excess O 2 in this photocatalytic system, the C–H bond activation for the oxidation reaction 22 is preferred to proceed rather than cross-coupling reaction. Our speculation could be further confirmed by the fact that, the benzylic C(sp 3 )–H could be efficiently oxidized to the corresponding ketones using this photocatalytic platform with an O 2 atmosphere (Scheme 2b).…”

High Nitrile Yields of Aerobic Ammoxidation of Alcohols Achieved by Generating •O2– and Br• Radicals over Iron-Modified TiO2 Photocatalysts

“…The oxidation of alcohols to aldehydes is a selective oxidation reaction, and reaction mechanisms involving • O 2 – as the reactive oxygen species have been well documented. ,,, Control experiments showed that NH 4 Br significantly promoted the oxidation of benzyl alcohol to benzaldehyde in the absence of NH 3 (reaction 2, Scheme a). Thus, it is concluded that the in situ formed Br • radicals promoted the generation of the carbon-centered radicals (II) via the cleavage of the C α –H bond, a critical step in the photocatalytic oxidation of alcohols into aldehydes. , As excellent HAT agents, bromine anions have been successfully applied in photo-redox catalysis. ,,, For example, the selective alkylation and amination of inactive C­(sp 3 )–H bonds over a photo-redox catalyst with HBr as the HAT agent was reported, in which the in situ generated Br • is responsible for the abstraction of the hydrogen atom from the C­(sp 3 )–H partner to produce a tertiary radical instead of bromine substitution . It is consistent with our experimental result that no bromine-substituted products were detected.…”

“…39,49 As excellent HAT agents, bromine anions have been successfully applied in photo-redox catalysis. 32,45,51,52 For example, the selective alkylation and amination of inactive C(sp 3 )−H bonds over a photo-redox catalyst with HBr as the HAT agent was reported, in which the in situ generated Br • is responsible for the abstraction of the hydrogen atom from the C(sp 3 )−H partner to produce a tertiary radical instead of bromine substitution. 32 It is consistent with our experimental result that no brominesubstituted products were detected.…”

Lewis Acid-Relayed Singlet Oxygen Reaction with Enamines: Selective Dimerization of Enamines to Pyrrolin-4-ones

“…Nonetheless, most enamines are stable with molecular oxygen . Instead, the activation of enamine with highly active oxygen species, 1 O 2 generated from photosensitization of air, at room temperature is attractive in reaction operation, functional group tolerance, and even late-stage application, which has been proved in the 1 O 2 reaction with indole, furan, and so forth . At first glance, the photo-oxidation of enamine seems rather similar to these reactions, and indeed, analogous [2 + 2] (or “ene”) process intermediates have been proposed. , However, singlet oxygen 1 O 2 reaction of enamine, especially for useful hetero-compound synthesis, is rather unsuccessful. , By analyzing the structural difference in enamine and possible intermediates with 1 O 2 (Scheme a), we envisioned that a Lewis acid could capture the possible intermediate for subsequent conversion, for example, to couple with a C nucleophile or N nucleophile to produce new C–C or C–N bonds (Scheme b).…”