text: Cyclopropanation of olefins with diazocarbonyl compounds is commonly accomplished using rhodium carboxylate complexes, although copper was originally used. The scope of the olefin is generally quite broad—electron-rich, neutral, and electron-poor olefins have all been cyclopropanated efficiently using rhodium-based catalyst systems. This section describes the various classes of diazocarbonyl compounds that react with olefins under rhodium catalysis to afford cyclopropanes.
  Diazoacetates that possess a single carbonyl substituent attached to the diazo carbon, have been used for the cyclopropanation of a wide array of olefins. Diastereoselectivity for the (E) cyclopropane increases as the size of the ester group increases. In addition, adding electron density to the catalyst (for instance by replacing acetate ligands with acetamide, acam) increases the diastereoselectivity of the reaction. 
  Diazocarbonyl compounds substituted with two electron-withdrawing groups, such as diazomalonates, are prone to experience side reactions under cyclopropanation conditions. [3+2] Cycloaddition and C-H insertion side products have been observed.
  Diazoacetates substituted with a vinyl or aryl group on the diazo carbon are unreactive towards trans-alkenes. This result has been explained by invoking the transition state model in Eq. (2). Reactions of these substrates are highly selective for the (E) cyclopropane isomer.
  Vinyl diazoacetates react with dienes to afford divinyl cyclopropanes, which undergo Cope rearrangement to afford cycloheptadienes. The more substituted double bond of the diene reacts preferentially.
  Furans react similarly with vinyl diazoacetates, although the intermediate cyclopropane may transform either into the Cope rearrangement product or an opened unsaturated carbonyl compound. The distribution of these products is highly dependent on the substitution pattern of the furan.
  Pyrroles react with vinyl diazoacetates to form nitrogen-bridged cycloheptadienes. The use of methyl lactate as a chiral auxiliary on the vinyl diazoacetate led to moderate diastereoselectivity in the tandem cyclopropanation/Cope rearrangement of Boc-protected pyrrole.
  The enantioselectivity of asymmetric cyclopropanations may depend profoundly on the solvent.