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. In general, aryl radicals, vinyl radicals, alkyl radicals, and nitrogen-centered radicals can react with sulfur dioxide, giving rise to sulfonyl radical intermediates. The resulting sulfonyl radicals can be captured by nucleophiles, olefins, and alkynes. The sources of aryl radicals from aryldiazonium salts, aryl halides, and diaryliodonium salts are demonstrated. It is believed that more amazing SO₂-based achievements will be developed in the near future.

Sulfonyl-derived functional groups can be found in a broad range of natural products, pharmaceuticals, and materials. Among the methods for the introduction of the sulfonyl group into small molecules, the approach using sulfur dioxide is the most promising and attractive one. In the past several years, the insertion of sulfur dioxide into small molecules under transition metal catalysis or metal-free conditions via a radical process has been developed. In this review, recent advances in the insertion of sulfur dioxide are presented.