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Synthetic applications of aryl diazonium salts enabled by visible light photoredox catalysis
*Corresponding author
Universität Regensburg, Institut für Organische Chemie, Universitätsstraße 31, D-93053 Regensburg, Germany

KEYWORDS: Visible light; aryl diazonium salts; photocatalysis; aryl radicals; Meerwein arylation
ABSTRACT: The application of aryl radicals in organic synthesis is challenging, but very useful. Recently their generation from aryl diazonium salts via visible light photocatalysis has been developed as an efficient alternative to established methods in chemical synthesis. We summarize here our contributions to this fast developing area of research.

Aryl diazonium salts are an important class of intermediates and have found - many applications in organic synthesis (1). In addition, aryl diazonium salts serve as an alternative to aryl halides, which have been widely used in transition metal mediated cross coupling reactions for carbon-carbon and carbon-hetero bond formation (2). The electrophilic nature of diazonium salts arises from N2 being a good leaving group, which does not interfere with the reaction mixture and allows the use of ambient reaction conditions. The chemistry of diazonium salts has been studied for long starting around 1858. Several name reactions associated with aryl diazonium salts including the Sandmeyer reaction (1884), the Pschorr cyclization (1896), the Gomberg–Bachmann reaction (1924), and the Meerwein arylation (1939) were developed (3).
Meerwein first reported the arylation of coumarin, cinnamic acid, and acrylic acid with aryl diazonium salts catalyzed by copper (II) salts (4). Later on, the scope of this reaction was further extended to electron rich olefins (5). Even though the Meerwein arylation has its own disadvantages, such as many side products, low yields, and a limited substrate scope, it serves as the foundation of modern cross coupling reactions. New improved variants of Meerwein and Pschorr reactions have been developed by several research groups for the synthesis of complex organic molecules (6). The synthesis of benzothiophenes from the corresponding o-methylthio aryl diazonium salts has been disclosed by Zanradi in the presence of freshly prepared FeSO4 (7). Heinrich and coworkers reported different types of Meerwein arylation reactions by employing TiCl3 and FeSO4 as chemical reductants (5, 8-10). Schiesser reported a synthesis of benzoselenophene and benzothiophene through a radical cyclization process involving the addition of aryl radicals to alkynes (11). Alternatively aryl radicals can also be generated from diazonium salts using organic reducing agents such as tetrakis(dimethylamino)ethylene (TDAE) (12).
The aryl diazonium salt generates an aryl radical and dinitrogen by taking up an electron from the reducing agent; in the classical reactions a catalytic or stoichiometric amount of transition-metal salts has been used. Visible light can also provide the required redox energy and it is an ideal reagent for organic synthesis (13, 14). The photolysis of diazonium salts on exposure to sunlight by loss of nitrogen is an important industrial technique, which has been used for diazo copying, photolithography, and printing on silk or cotton. Typical diazonium salts absorb light in the UV region of the spectrum and direct photolysis of diazonium salts gives phenol in aqueous media via heterolytic bond cleavage. The reducing agent, counter ions, nucleophilic additives, and the solvent are the main factors that influence the cleavage reaction of the diazonium group (14). In order to continue reading this article please register to our website – registration is for free and no fees will be applied afterwards to download contents.

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