In the presence of an appropriate transition metal (typically copper or rhodium), α-diazocarbonyl compounds are converted to transition metal carbenes, which undergo addition reactions in the presence of carbon–carbon double bonds to form cyclopropanes. Insertion into carbon–carbon or carbon–hydrogen bonds is possible in substrates lacking a double bond. The intramolecular version of this reaction forms fused carbocycles, although yields of reactions mediated by copper are typically moderate. For enantioselective cyclopropanations and insertions, both copper- and rhodium-based catalysts are employed, although the latter have been more heavily studied in recent years.