Mechanistic investigations into the cyclopropanation of electron-deficient alkenes with ethyl diazoacetate using [Co(MeTAA)

Abstract

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) tetramethyldibenzotetraaza[14]annulene [Co(MeTAA)] catalyst as compared with cobalt(II) tetraphenylporphyrin [Co(TPP)]. Cobalt(III)-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 N2 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.