We started performing the cyclopropanation reaction between dimethyl malonate and styrene to investigate whether dimethyl malonate can be used as a carbene precursor. Previous studies showed that cobalt porphyrin complexes have excellent reactivity for this reaction. Hence, we selected [Co II (TPP)] and [Co III (TPP)Cl] as potential catalysts. Furthermore, KOtBu was used as a non-nucleophilic base to deprotonate dimethyl malonate and benzophenone as HAT reagent, which was activated with a 370 nm light source. After irradiating the reaction mixture with [Co III (TPP)Cl] (10 mol%) overnight, cyclopropane 3 was observed with an average yield of 11% (Table , entry 1), which is a stoichiometric (~100%) conversion with respect to the cobalt catalyst. However, when performing the reaction with [Co II (TPP)], no product was formed (entry 2), showing that the oxidation state of cobalt plays a crucial role in the reaction. Control experiments showed no product formation in absence of [Co III (TPP)Cl], KOtBu, and light (entry 3-5). Only in absence of benzophenone, traces of product were observed (entry 6), suggesting that the product can also be formed without benzophenone as HAT reagent. These results confirm our hypothesis that dimethyl malonate can be used as carbene precursor in the cyclopropanation reaction. If the reaction proceeds via a cobalt(III) carbene radical intermediate that reacts with styrene, then cobalt(III) is known to be reduced back to cobalt(II). In contrast to [Co III (TPP)Cl], [Co II (TPP)] is unlikely to bind and activate dimethyl malonate due to the lower oxidation state. Thus, when the cobalt catalyst is not regenerated back to its formal 3+ oxidation state after the reaction, cyclopropane 3 is only formed stoichiometrically (entry 1). Attempts to oxidize cobalt(II) back to cobalt(III) under the applied reaction conditions in presence of all substrates were unfortunately unsuccessful (see supporting information, Table ).