We were intrigued by the formation of cyclopropane 3 from dimethyl malonate and investigated the mechanism of this reaction further. We postulated that the deprotonated dimethyl malonate reacts with [Co III (TPP)Cl] first (Scheme 1), before Figure . a) X-band EPR spectra obtained after irradiation (370 nm) of in situ formed complex 1 and benzophenone (black), and [Co II (TPP)] (grey) in toluene-d8. Simulated profile of cobalt(III) carbene radical complex 2 (pink). b) X-band EPR spectra obtained after irradiation (370 nm) of in situ formed complex 1 with benzophenone (black) and without benzophenone (blue) (see supporting information for experimental and simulation parameters). c) Schematic structure of cobalt(III) carbene radical complex 2.