Iron porphryin complexes are active catalysts for the cyclopropanation of alkenes by ethyl diazoacetate, Fe(TTP) (TTP = meso-tetra-p-tolylporphyrin), an isolated iron(II) porphyrin complex, can be used as the catalyst, or the iron(III) complexes of several porphyrins can be reduced in situ. The reactions produce synthetically useful excesses of the trans cyclopropyl ester products. This stereoselectivity exhibits a modest solvent dependence, with donor solvents giving higher ratios of the trans cyclopropane products, The diastereoselectivity exhibits only a modest dependence on the steric bulk of the porphyrin. The reactions are selective for 1-alkenes and 1,1-disubstituted alkenes, Conjugated substrates and enol ethers react more rapidly than simple aliphatic alkenes, A mechanistic model for the iron-mediated reactions is proposed which is consistent with the data presented herein.