Diazo compounds as cross-coupling partners in C(sp2)-H bond functionalization

Diazo compounds are undoubtedly among the most versatile classes of organic compounds. Thanks to their ability to serve as either carbene precursors or 1,3-dipoles, they have traditionally been used in a broad array of useful transformations including cyclopropanations, carbon−hydrogen, and heteroatom-hydrogen insertions, ylide formations, 1,2-migrations, and 1,3-dipolar cycloadditions. Although more than 120 years old diazo compounds continue to find new applications in organic synthesis. Over the last two decades, carbon–hydrogen bond functionalization has been emerging as a cutting-edge research area. According to this approach, the reaction is initiated by a first step of metal-catalyzed carbon–hydrogen bond activation, followed by a cross-coupling reaction of the nucleophilic metal-carbon species, thus generated, with an appropriate electrophilic partner. Since electrophilic metal carbenes can be generated starting from diazo compounds, that is what carbon–hydrogen bond activation and diazo compound decomposition can be realized within the same catalytic cycle by choosing the appropriate catalytic system. This article deals with the metal-catalyzed intermolecular functionalization of carbon(sp2)–hydrogen bonds by the use of diazo compounds as electrophilic cross-coupling partners.