Aryl Cation Chemistry as an Emerging Versatile Tool for Metal-Free Arylations

Abstract

The development of transition-metal-free arylation procedures represents a key goal in modern organic synthesis. A green arylation can be obtained by having recourse to light as a 'clean' reagent in place of toxic metals. In particular, a new method based upon the photoheterolytic fragmentation of substituted aryl halides or esters, with the formation of aryl cations has recently emerged. The peculiar reactivity of these cations with carbon nucleophiles allows several straightforward photo-arylations, which compare favorably with other thermal or photochemical metal-free analogues.

Photochemical synthesis: Using light to build C–C bonds under mild conditions

“…Thus, singlet states behave as unselective electrophiles, often reacting with the (polar/ protic) solvent in which they are generated, to result a formal solvolysis process. On the other hand, the diradical nature of triplet states causes a high selectivity towards pbond nucleophiles (such as alkenes, alkynes and (hetero) aromatics) when present in the reaction mixture, thus leading to the formation of a new aryleC bond, an appealing synthetic target [10].…”

Silylium Ion-Catalyzed α-Arylation of Carboxylic Acids, Amides, and Esters: Efficient Synthesis of Anesthetic and Anti-inflammatory Drugs

“…[7] The high cost, stringent reaction conditions, and potential contamination with heavy metals pose challenges for industrial and pharmaceutical applications, leading to developing "metal-free" arylation methods. [8] Despite being one of the effortless and ubiquitous carbonyl derivatives, the direct α-arylation of free aliphatic carboxylic acids has not been accomplished. Similarly, the α-arylation of common secondary amides bearing an acidic N-H bond has not been documented.…”

Visible-Light-Promoted Aryl Cation Formation: Aromatic SN1 Reactions of Areneazo-2-(2-nitro)propanes

“…Arenes with a suitable leaving group can undergo heterolytic bond cleavage (C sp 2 –LG) to give the aryl cations that could participate in the cross-coupling reactions without need for transition metal catalysis (Scheme a) . With ever-growing synthetic utility of aryl diazonium salts in metal-catalyzed and photoredox-catalyzed cross-coupling reactions, it is not surprising to find that the aryl diazonium salts can also generate the aryl cationic species under acidic and UV irradiation conditions .…”