Aromatic substitutions of arenediazonium salts via metal catalysis, single electron transfer, and weak base mediation.

The re-discovery of arenediazonium salts as stable and easily accessible electrophiles for radical aromatic substitutions initiated by single-electron transfer has led to the development of numerous protocols. The diverse set of methods involving metal-mediated, transition metal-catalyzed, photoredox-catalytic and electrochemical electron-transfer mechanisms has recently been complemented by reactions that operate under the mild conditions of very weak inorganic bases. This method provides great advantages with regard to operational simplicity, price, hazard potential, and scalability. The scope of weak base-mediated radical aromatic substitutions has been greatly expanded to include various C-C and C-heteroatom bond forming reactions, cyclizations and rearrangements, and even reactions that preserve the reduction-labile dinitrogen functionality within the product structure. The recent reports of highly fruitful synthetic applications have impressively documented that non-hazardous, inexpensive, and easily accessible inorganic bases can mimic the reducing ability of metal salts, complex reductants, or photo/electrochemical setups. This review covers a concise summary of the most important development in the field and provides detailed analyses of reaction scopes and mechanisms.

Metal-free radical aromatic carbonylations mediated by weak bases.

We report a new method of metal-free alkoxycarbonylation. This reaction involves the generation of aryl radicals from arenediazonium salts by a very weak base (HCO2Na) under mild conditions. Subsequent radical trapping with carbon monoxide and alcohols gives alkyl benzoates. The conditions (metal-free, 1 equiv. base, MeCN, r.t., 3 h) tolerate various functional groups (I, Br, Cl, CF3, SF5, NO2, ester). Mechanistic studies indicate the operation of a radical aromatic substitution mechanism.

Metal-free radical thiolations mediated by very weak bases.

Aromatic thioethers and analogous heavier chalcogenides were prepared by reaction of arene-diazonium salts with disulfides in the presence of the cheap and weak base NaOAc. The mild and practical reaction conditions (equimolar reagents, DMSO, r.t., 8 h) tolerate various functional groups (e.g. Br, Cl, NO2, CO2R, OH, SCF3, furans). Mechanistic studies indicate the operation of a radical aromatic substitution mechanism via aryl, acetyloxyl, thiyl, and dimsyl radicals.