Intermolecular Metal‐Catalyzed Carbenoid Cyclopropanations

Abstract The metal‐catalyzed decomposition of diazo compounds in the presence of alkenes is a well‐established reaction. Since the original Organic Reactions review on the reaction of ethyl diazoacetate with alkenes and aromatic compounds in 1970, several new developments have revolutionized this area of chemistry. Most notably, major advances have been made in catalyst design such that highly chemoselective, diastereoselective and enantioselective carbenoid transformations can now be achieved. Furthermore, it has been recognized that a wide array of carbenoid structures can be utilized in this chemistry, leading to a broad range of synthetic applications. This chapter comprises coverage of the metal‐catalyzed intermolecular cyclopropanations of diazo compounds containing at least one adjacent electron‐withdrawing group. The coverage of diazoacetate chemistry will be limited to material since 1970 because the previous Organic Reactions review covers the earlier literature. The alkene component is limited to alkenes, dienes, furans, and pyrroles because these are the systems that have resulted in the greatest developments since the 1970 review. Metal‐carbenoid intermediates derived from diazo compounds undergo a variety of useful reactions, including cyclopropanation, insertion, and ylide formation. In recent years several excellent reviews have appeared on various aspects of this chemistry. Three recent reviews have focused on asymmetric intermolecular cyclopropanations. Several books and reviews on carbenoid chemistry have major sections on intermolecular cyclopropanations. Because of the historical central prominence of carbenoids derived from diazoacetates, most reviews have tended to focus on this class of carbenoids. In this chapter, a comparison is presented of the chemical differences that exist among the major classes of carbenoids that contain adjacent electron‐withdrawing groups. The extensive nature of the topic precludes coverage of related reactions such as the metal‐catalyzed decomposition of diazoalkanes, phenyldiazoalkanes, or vinyldiazoalkanes that lack adjacent electron‐withdrawing functionality. Other cyclopropanation reactions such as the Simmons‐Smith reaction, photochemical or thermal decomposition of diazo compounds in the presence of alkenes, and cyclopropanation using stoichiometric metal carbenes are not covered.