text: Simmons-Smith cyclopropanation, which employs carbenes derived from diiodomethane, is a popular alternative to rhodium-catalyzed cyclopropanation. In the presence of a chiral diamine, Simmons-Smith cyclopropanation is enantioselective; however, selectivities are not as high as the corresponding rhodium-catalyzed reactions.
  Substituted zinc carbenoids can be prepared from the corresponding ketones or aldehydes through a sequence analogous to the mechanism of the Clemmensen reduction. Cyclopropanation of olefins with these intermediates occurs with moderate diastereoselectivity and yield.
  Other diazo compounds besides diazocarbonyl compounds have been used for rhodium-catalyzed cyclopropanations; however, these substrates are much more difficult to handle and unstable than diazocarbonyl compounds. Thus, they have not been extensively adopted for organic synthesis.