As valuable C1 building blocks, isocyanides represent an important class of reactive species and synthons. During the past decades, exhaustive efforts have been devoted to the discovery of highly efficient reactions involving isocyanide on the basis of the development of the Passerini and Ugi reactions. Several types of reactions involving isocyanides have been reported, such as nucleophilic attack, electrophilic addition, imidoylation reactions, and oxidation etc. In this review, recent progress in isocyanide insertion chemistry is presented. Among all isocyanide insertions, two catalytic systems have been developed, that is, Lewis (Brønsted) acid-catalyzed isocyanide insertions and transition-metal-enabled isocyanide insertions, respectively. This review is hence written in the sequence of Lewis (Brønsted) acid-catalyzed isocyanide insertion and transitional metal-enabled isocyanide insertion, where isocyanide insertion into heteroatom-hydrogen bonds, carbon-halogen bonds, carbon-hydrogen bonds, and metal carbenes are summarized.

This review is focused on the recent advances in the chemistry of sulfur dioxide fixation through a radical process. Diverse sulfonyl compounds can be obtained efficiently under mild conditions.

A novel copper-catalyzed direct trifluoromethylation of internal alkynes was developed, obtaining a series of trifluoromethylated coumarins in good yields. The cyclization was proposed to proceed via a radical mechanism under copper-catalyzed conditions with good functional group tolerance.

Recent developments in the photochemical carbon-carbon and carbon-heteroatom bond formation via photoinduced Ar-X bond dissociation are summarized. The transformations through the Ar-X bond dissociation enabled by photoenergy include nucleophilic substitution, arylation, alkylation, aminocarbonylation, aminosulfonylation and decarboxylative coupling reactions. Usually, the reactions undergo oneelectron-transfer couplings and aryl radicals or cations are involved as the key intermediates. Under ultraviolet irradiation or visible light, the reactions of aryl halides with reactive partners proceed smoothly under mild conditions. In some cases, the transformations can proceed without any metals or photo-redox catalysts. Moreover, the broad reaction scope is demonstrated with good functional group tolerance.Scheme 1 Cleavage of Ar-X bond under UV irradiation.Scheme 2 Formation of an aryl radical by a photoinduced redox process in the presence of a metal catalyst or organocatalyst.

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.