Pyridinium salts are valuable building blocks, which have been widely applied in various organic transformations during the past few decades. In particular, N-functionalized pyridinium salts have been explored as convenient radical precursors, which would go through reductive single-electron transfer. As a result, the chemistry of such pyridinium compounds for generating carbon-, nitrogen-, and oxygen-centered radicals has been witnessed, and a remarkable progress has been achieved, making it a hot topic over the last five years. This Review describes recent advances in the area of pyridinium salts as radical precursors, concerning the development of radical reactions involving pyridinium salts in organic synthesis.

Aryl N-aminosulfonamides could be easily produced via a palladium-catalyzed coupling of aryl halides, potassium metabisulfite, and hydrazines. Potassium metabisulfite is an excellent equivalent of sulfur dioxide in the reaction of palladium-catalyzed aminosulfonylation.

Recent advances in the sulfonylation reactions by using inorganic sulfites as the source of sulfonyl group are reported. The approaches employing inorganic sulfites as the sulfur dioxide surrogates are attractive and promising for the synthesis of sulfonyl compounds since inorganic sulfites are abundant, easily available and cheap.

Recently, sulfonylation reactions using potassium/sodium metabisulfite as the sulfur dioxide surrogate have been developed rapidly. In most cases, the transformations go through radical processes with the insertion of sulfur dioxide under mild conditions. Additionally, transition metal catalysis is applied in the reactions for the synthesis of sulfonyl-containing compounds.

A Pd-catalyzed, meta-selective C−H arylation of nosyl-protected phenethylamines, and benzylamines is disclosed using a combination of norbornene and pyridine-based ligands. Subjecting 2-aryl anilines to this protocol lead to meta-C–H arylation at the remote aryl ring. A diverse range of aryl iodides are tolerated in this reaction, along with select heteroaryl iodides. Select aryl bromides bearing ortho-coordinating groups can also be utilized as effective coupling partners in this reaction. The use of pyridine ligands has allowed the palladium loading to be reduced to 2.5 mol%. Furthermore, a catalytic amount of 2-norbornene (20 mol%) to mediate this meta-C–H activation process is demonstrated for the first time. Utilization of a common protecting group as the directing group for meta-C–H activation of amines is an important feature of this reaction in terms of practical applications.