Visible‐Light Promoted Catalyst‐Free Imidation of Arenes and Heteroarenes

Abstract

We described herein a catalyst-free visible-light photolytic protocol for the imidation of arenes and heteroarenes. N-Bromosaccharin was identified as a viable and chemoselective nitrogen radical precursor that undergoes controllable homolytic cleavage under ambient light irradiation. The reaction can be applied to a number of arenes and heteroarenes with good chemo- and regioselectivity. Mechanistic studies revealed that radical chain termination by electron transfer-proton transfer (ET-PT) is the leading productive pathway for the reaction.

O2‐mediated dehydrogenative amination of phenols

“…He was the first to isolate phenol in pure crystalline form and he used it to carry out simple nitration experiments. [1] One of the key advantages of the direct aromatic functionalization strategy, whether by electrophilic attack [2] or by the less common homolytic aromatic substitution (HAS), [3] is the absence of pre-activation steps for the aromatic C À H bonds. Molecular complexity can thus be achieved in a single chemical step.…”

Visible‐Light‐Mediated Generation of Nitrogen‐Centered Radicals: Metal‐Free Hydroimination and Iminohydroxylation Cyclization Reactions

“…[1a,2] Thed evelopment of amild, selective,and general method to catalytically generate NCRs from readily available precursors would enable the facile construction of many N-heterocycles,w hich are privileged motifs in natural products and therapeutic agents. [3] Photoredox catalysis has emerged as apowerful technique through which single electron transfer (SET) reactions can be performed under mild conditions.[4] MacMillan [5] and coworkers have developed an asymmetric visible-light-mediated amination of aldehydes by enamine catalysis,a nd the groups of Sanford, [6] Lee, [7] Yu, [8] and Luo [9] have reported the photoredox generation of phthalimidyl and saccharyl radicals and their use in Minisci-type reactions.T he groups of Zheng [10] and Knowles [11] have developed am ethod for the photoredox generation of diaryl and aryl alkyl aminium radical cations and employed them in CÀNb ond-forming reactions.Drawing inspiration from the work of Forrester, [12] Narasaka, [13] and Walton, [14] we speculated that appropriately functionalized O-aryl oximes could serve as general, benchstable NCR precursors that could deliver iminyl radicals upon photoredox activation under mild conditions.[15] Such an approach would clearly benefit from the facile synthesis of aryl oximes,and we hoped that the high structural modularity of the O-aryl hydroxylamines would allow us to identify substrates that do not require the use of transition-metalbased photocatalysts.[16] Herein, we describe the successful implementation of this approach and the development of novel, transition-metal-free,v isible-light-mediated hydroimination and iminohydroxylation cyclization reactions (Scheme 1B).Theg uiding principle of our photoredox NCR synthesis capitalized on the evidence that electron-poor aromatic compounds have reduction potentials compatible with SET reduction by visible-light-excited photocatalysts, [17] as shown by MacMillan and co-workers.[18] Our envisaged photoredox iminyl NCR generation was initiated by the visible-lightpromoted excitation of ap hotocatalyst (PC! *PC) [19] followed by SET reduction of the aryl unit of oxime A to give radical anion B (Scheme 2A).…”

O2‐vermittelte dehydrierende Aminierung von Phenolen

“…Angesichts der relativen Stabilität des N-radikalischen Phenotiazins verläuft die abschließende Kupplung aller Voraussicht nach über einen klassischen, homolytischen aromatischen Substitutionsschritt (Schema 3). [3] Ein klassischer elektrophiler Aminierungsschritt ist jedoch zu diesem Zeitpunkt nicht auszuschließen.…”

“…Ferner gelang ihm als erstem, Phenole in ihrer reinen kristallinen Form zu isolieren und Nitrierungsexperimente mit ihnen durchzufüh-ren. [1] Einer der grçßten Vorteile der direkten aromatischen Funktionalisierung,o bn un durch elektrophilen Angriff [2] oder die weniger gebräuchliche homolytische aromatische Substitution (HAS), [3] besteht darin, dass keine Präfunktio-nalisierungsschritte an der C-H-Position vorghenommen werden müssen. Folglich wird durch einen einzigen chemischen Schritt eine hçhere molekulare Komplexität erzielt.…”

“…Ein großer Nachteil -n eben der Problematik der C-H-Regioselektivität -ist jedoch die Notwendigkeit einer Präaktivierung bzw.P räoxidation der Kupplungspartner,u me ine ausreichende Reaktivität für die Kondensation sicherzustellen. Unseres Wissens existiert keine dehydrierende aromatische Aminierungsreakion, sei es durch elektrophilen Angriff (Schema 1) [4,5] oder einen homolytischen Subtitutionsverlauf, [3] in der das oxidierte Aminierungssubtrat in situ generiert wird und somit Präaktivierungsschritte beider Kupplungspartner entfallen und darüber hinaus die Reaktion ohne Metall- [6] bzw.H alogenidzusatz auskommt. Eine solche Reaktion ist erstrebenswert und wäre aus synthetischer Sicht nicht nur praktisch, sondern auch nachhaltig,i nsofern O 2 als einziges terminales Oxidans eingesetzt werden kann.…”