Photoinduced Oxygen Transfer using Nitroarenes for the Anaerobic Cleavage of Alkenes

Abstract

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.

Anaerobic Hydroxylation of C(sp3)–H Bonds Enabled by the Synergistic Nature of Photoexcited Nitroarenes

“…Contemporaneous reports from our laboratory 6 and Leonori's group 7 illustrate that visible-light excitation of nitroarenes leads to a triplet biradical intermediate, which enables the cleavage of alkenes into carbonyl derivatives. Mechanistic studies by Dopp, 8 our group, 6 and others 9 have showcased that the aforementioned triplet biradical intermediate is capable of C−H bond activation via intramolecular hydrogen atom transfer (HAT) with o-alkyl groups of nitroarenes. Seminal works from the groups of Hamilton, Severin, 11 and Berman 12 provide evidence that C−H oxidation can be achieved via oxygen atom transfer (OAT) from nitroarenes under harsh UV irradiation.…”

Anaerobic Hydroxylation of C(sp3)–H Bonds Enabled by the Synergistic Nature of Photoexcited Nitroarenes

“…Next, radical probe 5 was subjected to the reaction conditions to verify the Scheme 2: Mechanistic Studies formation of radical intermediates (Scheme 2B). 22 The formation of naphthalene derivative 8 was observed as a product with concomitant formation of the direct anaerobic oxidation products (6,7). The former likely occurs via the radical-ring opening of 5 and subsequent aromatization via hydroxylation/dehydration (see SI), thus verifying the intermediacy of carbon-centered radicals.…”

“…Contemporaneous reports from our laboratory 6 and Leonori's group 7 illustrate that visible light excitation of nitroarenes leads to a triplet biradical intermediate, which enables the cleavage of alkenes into carbonyl derivatives. Mechanistic studies by Döpp, 8 our Scheme 1: C-H Hydroxylation Approaches group, 6 and others 9 have showcased that the aforementioned triplet biradical intermediate is capable of C-H bond activation via intramolecular hydrogen atom transfer (HAT) with ortho-alkyl groups of nitroarenes. Seminal works from the groups of Hamilton 10 and Serverin 11 provide evidence that C-H oxidation can be achieved via oxygen atom transfer (OAT) from nitroarenes under harsh UV irradiation.…”

Anaerobic Hydroxylation of C(sp3)–H Bonds Enabled by the Synergistic Nature of Photoexcited Nitroarenes

“…Next, radical probe 5 was subjected to the reaction conditions to verify the formation of radical intermediates (Scheme 2B). 24 The formation of naphthalene derivative 8 was observed as a product with concomitant formation of the direct anaerobic oxidation products (6,7). The former likely occurs via the radical-ring opening of 5 and subsequent aromatization via hydroxylation/dehydration (see SI), thus verifying the intermediacy of carbon-centered radicals.…”