Photoinduced Oxygen Transfer Using Nitroarenes for the Anaerobic Cleavage of Alkenes

Abstract

Herein we report the anaerobic cleavage of alkenes into carbonyl compounds using nitroarenes as oxygen transfer reagents under visible light. This approach serves as a safe and practical alternative to mainstream oxidative cleavage protocols, such as ozonolysis and the Lemieux−Johnson reaction. A wide range of alkenes possessing oxidatively sensitive functionalities underwent anaerobic cleavage to generate carbonyl derivatives with high efficiency and regioselectivity. Mechanistic studies support that the transformation occurs via direct photoexcitation of the nitroarene followed by a nonstereospecific radical cycloaddition event with alkenes. This leads to 1,3,2-and 1,4,2-dioxazolidine intermediates that fragment to give the carbonyl products. A combination of radical clock experiments and in situ photoNMR spectroscopy revealed the identities of the key radical species and the putative aryl dioxazolidine intermediates, respectively.

A switchable redox annulation of 2-nitroarylethanols affording N-heterocycles: photoexcited nitro as a multifunctional handle

“…Based on the above mechanistic experiments and previous reports, 19,20,34 the plausible mechanisms were proposed (Scheme 7). If ground-state nitroarene ( 1a ) is exposed to appropriate irradiation, it could be photoexcited to the long-lived triplet state (T), which has a biradical character.…”

“…According to the previous reports concerning the reduction of nitroarenes to nitroso compounds, 19,19 a ,20 we assume that nitroso and carbonyl might be the outcomes of the d -HAA process. Therefore, we designed the less fast intermolecular HAA reaction to verify this hypothesis and try to detect the active nitroso species.…”

“…If ground-state nitroarene ( 1a ) is exposed to appropriate irradiation, it could be photoexcited to the long-lived triplet state (T), which has a biradical character. 19 Then the active biradical participates in an intramolecular double hydrogen atom abstraction ( d -HAA) from the adjacent hydroxyalkyl, affording the nitroso intermediate 6 (as illustrated in Scheme 7, top). Thereafter the key species 6 could switch two paths dependent on a diboron reagent.…”

“…Besides, the proton-coupled electron transfer (PCET) strategy for activating the O–H bond of alcohols is heavily dependent on the combination of a Brønsted base with an oxidant to remove a proton and an electron from the O–H bond, respectively 18 (Scheme 1B). Inspired by these profound strategies for radical generation, we envisaged that the photoexcited nitro might act as a potential dual HAT-functionality for the activation of both α-C–H and O–H bonds of hydroxyl owing to its nature as a biradical in the triplet state, 19 and we describe this process as double hydrogen atom abstraction ( d -HAA). As a preliminary attempt for this hypothesis, our previous work achieved a tandem reduction of nitroarenes to anilines triggered by a d -HAA process with isopropanol 20 (Scheme 1C).…”

Oxidative Cleavage of Alkenes by Photosensitized Nitroarenes

“…The recent independent reports by the research groups of Parasram and Leonori have now addressed this longstanding problem by exploring the excited state reactivities of nitroarenes with alkenes under mild purple light photosensitization conditions. [9,10] Unlike powerful mercury lamps, mild photosensitization conditions allowed formation of the "N-doped" ozonide and a controlled cleavage of the alkenyl CÀ C bond to generate the corresponding carbonyl compounds in high yields (Scheme 2C). Parasram and co-workers identified 4-cyanonitrobenzene as an effective one step oxygen transfer agent for styrenes at room temperature.…”