Recent Applications of Photoexcited Nitroarenes in Organic Synthesis

The photochemical activity of nitroarenes was first reported by Ciamician and Silber in 1901 when the unusual rearrangement of 2-nitrobenzaldehyde to 2-nitrosobenzoic acid was disclosed.¹ The cyclization of photoexcited nitroarenes to ortho alkene and alkyne systems was described not long thereafter.^2,3 Subsequent work by Ward⁴ and others⁵ defined the reactivity of photoexcited nitroarenes as an excitation, analogous to that of carbonyl groups, ultimately leading to a reactive triplet state biradical intermediate. This intermediate was shown to be capable of preforming HAT reactions and, as later shown by Buchi and Ayer,⁶ reacting with alkenes to form oxidative cleavage products.  This reactivity went largely unexplored for the next 50 years, however recent advancements in photochemistry have renewed interest in photoexcited nitroarenes. Utilizing the previously described chemistry, near simultaneous disclosures by the Leonori⁷ and Parasram⁸ groups were published in which they describe a photoexcited nitroarene driven oxidative cleavage of alkenes, offering a unique metal free anerobic alternative to ozonolysis. Inspired by this reactivity with -systems, the Sarpong group utilized nitroarenes in the mild oxidative cleavage of oximes to ketones,⁹ a transformation which typically requires harshly acidic conditions. Besides their reactivity in -systems and HAT reactions, an early report by Weller and Hamilton¹⁰ showed that excited nitroarenes could oxidize alkanes, and thus act as suitable OAT reagents. Elaborating on this, the Parasram group leveraged the bifunctional nature of nitroarenes as HAT and OAT reagents and preformed mild C(sp³)-H oxidation on a broad range of benzylic and aliphatic substrates.¹¹ In subsequent work, they further developed this method by the oxidation of alcohols and amines to form the corresponding ketone or imine, as well as aldehydes and imines to form the carboxylic acid and amide respectively.¹² Overall, photoexcited nitroarenes are effective oxidation reagents whose multifunctional nature makes them applicable to a wide range of transformations. The mild conditions and operational simplicity of these reactions allow them to function for broad range of substrates and achieve traditionally challenging oxidations, making nitroarenes an appealing and useful synthetic tool. 

References

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