Visible-Light-Mediated α-Arylation of Enol Acetates Using Aryl Diazonium Salts

Abstract

Visible light mediates efficiently the α-arylation of enol acetates by aryl diazonium salts under mild conditions using [Ru(bpy)(3)]Cl(2) as a photoredox catalyst. The broad scope of the reaction toward various diazonium salts and enol acetates was explored. The application of this reaction in the concise synthesis of 2-substituted indoles was demonstrated.

On the mechanism of photocatalytic reactions with eosin Y

“…The charged forms EY3 and EY4 are catalytically active, which in turn also means that solutions containing commercial “eosin Y, spirit soluble” would per se not trigger efficient photoredox catalysis. In such cases, catalytic activity as observed in recent publications by König et al is dependent on the operation of acid–base equilibration (e.g., in DMSO solution) so that the employed eosin Y is converted in situ to the active species EY3 or EY4 [15–18]. Efforts to reproduce the photoredox synthesis of arylboron pinacolates in acetonitrile have so far failed in our hands when irradiating at 535 nm [21].…”

“…Photoredox reactions with arenediazonium salts are often more selective than traditional methods such as copper(II)-mediated Meerwein arylations [11] or protocols employing stoichiometric iron(II) or titanium(III) reductants in aqueous media [12–14]. This renaissance of arene diazonium chemistry has recently led to various applications of eosin Y to visible light-driven syntheses of biaryls [15], stilbenes [16], benzothiophenes [17], α-phenylketones [18], phenanthrenes [19], arylsulfides [20] and arylboron pinacolates [21] (Scheme 1). …”

Visible Light Photoredox Catalysis with Transition Metal Complexes: Applications in Organic Synthesis

“…Aryl radicals may also add to enol acetates to afford α -aryl ketones. 138 These reactions proceed via the intermediacy of α -acetoxy radicals such as 252 , which upon single-electron oxidation and loss of acetate yields α -aryl ketone 253 (Scheme 54). 139 …”

Gold‐katalysierte 1,2‐Difunktionalisierung von Alkinen mit sichtbarem Licht ohne zusätzlichen Photosensibilisator

“…Als Alternative wurden die durch sichtbares Licht oder Basen vermittelte Kupplung präfunktionalisierter Enoletherderivate mit Aryldiazoniumsalzen als Quelle von Arylradikalen entwickelt. [2] Tr otz dieses Fortschritts besteht nach wie vor Bedarf an der Entwicklung neuer effizienter Synthesemethoden, die leicht verfügbare Edukte zur Herstellung der gewünschten Produkte unter milden Bedingungen nutzen (Schema 1).…”