The product ratio of intermolecular insertion and cyclopropanation in transition metal-catalyzed diazo decompositions depends strongly upon the metal, its ligands and upon the substituents of the diazo compound. Ethyl diazoacetate (2a) reacts with cyclohexene (1) almost exclusively by cyclopropanation. However, diazomalonate (2d) and methyl 2-diazophenylacetate (2e) in the presence of Rh^II catalysts exhibit a marked tendency towards allylic CH insertion. With 1,4-cyclohexadiene (6), methyl 2-diazophenylacetate (2e) in the presence of chiral Rh^II catalysts affords the allylic insertion product 7 in almost quantitative yield and with up to 74% ee.