No 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 photoexcited-nitroarene-mediated anaerobic C−H hydroxylation of aliphatic systems is reported. The success of this reaction is due to the bifunctional nature of the photoexcited nitroarene, which serves as the C−H bond activator and the oxygen atom source. Compared to previous methods, this approach is cost-and atom-economical due to the commercial availability of the nitroarene, the sole mediator of the reaction. Because of the anaerobic conditions of the transformation, a noteworthy expansion in substrate scope can be obtained compared to prior reports. Mechanistic studies support that the photoexcited nitroarenes engage in successive hydrogen atom transfer and radical recombination events with hydrocarbons, leading to Narylhydroxylamine ether intermediates. Spontaneous fragmentation of these intermediates leads to the key oxygen atom transfer products.

Applications of photoexcited nitroarenes have been underdeveloped in organic synthesis. Since early reports on the direct excitation of nitroaromatics with harsh UV light, these synthetically useful reagents have not been tamed for use in modern synthetic chemistry. We have developed practical synthetic protocols for the anaerobic oxidation of hydrocarbon substrates using commercially available nitroarenes as photochemically activated oxidants under visible light. Using this approach, a wide variety of olefins are anaerobically cleaved to their corresponding carbonyls, and aliphatic C–H bonds are hydroxylated to give alcohols. The anaerobic reaction conditions enable oxidatively sensitive functional groups to be tolerated and the employment of visible light makes this method highly sustainable. Mechanistic studies support that the photoexcited nitroarene biradical intermediate is responsible for the oxygen atom transfer events.1 Introduction2 Alkene Cleavage Promoted by Photoexcited Nitroarenes3 Photoinduced Nitroarene-Mediated C–H Hydroxylation4 Conclusions

Herein we report a visible light promoted method for the anaerobic cleavage of alkenes into carbonyl compounds using nitroarenes as oxygen transfer reagents. 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 a solvent-separated ion-pair complex and the nitroarene is the sole photoabsorbing species. Direct photoexcitation of the nitroarenes empowers a radical cycloaddition event with alkenes leading to a 1,3,2-dioxazolidine intermediate, which fragments 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 1,3,2-dioxazolidine intermediate, respectively.

We report a photoinduced phenanthrene synthesis from aryl iodides and styrenes through an arylation/cyclization cascade. Compared to prior methods, this approach obviates the need for hazardous reagents and provides access to unsymmetrical phenanthrenes with good functional group tolerance. Mechanistic studies revealed that photoexcitation of aryl iodides leads to homolytic C−I bond cleavage. Arylation of styrenes with the formed aryl radical species furnishes stilbene derivatives, which undergo photoinduced cyclization promoted by iodine generated in situ to yield phenanthrene products.

A photoexcited nitroarene-mediated, anaerobic C–H hydroxylation of aliphatic systems is reported. The success of this reaction is due to the bifunctional nature of the photoexcited nitroarenes, which serve as the C–H bond activator and the oxygen atom source. Compared to previous methods, this approach is cost and atom economical due to the commercial availability of the nitroarene, the sole mediator of the reaction. Owing to the anaerobic conditions of the transformation, a noteworthy expansion in substrate scope can be obtained compared to prior reports. Mechanistic studies support that the photoexcited nitroarenes engage in successive hydrogen atom transfer and radical recombination events with hydrocarbons, leading to N-arylhydroxylamine ether intermediates. Spontaneous fragmentation of these intermediates leads to the key oxygen atom transfer products.

A series of symmetric and non-symmetric PCP-pincer complexes have been prepared and an evaluation of their activity in catalytic allylic alkylation carried out. The X-ray crystallography and NMR spectroscopy data suggest that metallacycles containing one (or two) 6-membered rings are inherently more rigid than initially anticipated. The observed catalyst activity is found to increase as the metallacycle size increases from 5 to 6.