A detailed mechanistic study of the cyclopropanation of electron-deficient alkenes, namely methyl acrylate with ethyl diazoacetate (EDA), was carried out, aiming at understanding both the superior activity and the higher sensitivity of the cobalt(II) tetramethyldibenzotetraazal[14]annulene [Co(MeTAA)] catalyst as compared with cobalt(II) tetraphenylporphyrin [Co(TPP)]. Cobalt(lll)-carbene radicals were demonstrated to be present as key intermediates in the reaction, using a combination of kinetic studies, experimental EPR spin-trapping experiments, and supporting DFT studies. Reaction progress was monitored in real time by observing N-2 formation and measuring its partial pressure under isothermal conditions. Reaction progress kinetic analysis (RPKA) was used to analyze the experimental data. Results showed that the reaction is first-order in both [catalyst] and [EDA] and zero-order in [methyl acrylate], in agreement with the DFT-calculated mechanism. The calculated activation parameters corresponding to the rate-determining step of the reaction are in agreement with the experimental values, thus providing strong support for the proposed metalloradical mechanism. (C) 2018 The Authors. Published by Elsevier Inc.