Evaluation and understanding the performances of various derivatives of carbonyl-stabilized phosphonium ylides in CO2 transformation to cyclic carbonates†
The kinetic and mechanism evaluations of the formation of cyclic carbonates by carbonyl-stabilized phosphonium ylides as an efficient and new class of organocatalysts are the main purposes of this research. Recently, it has been reported that tetraarylphosphonium salts play the role of organocatalysts in carbon dioxide conversion to cyclic carbonates. However, in this research, the oxygen atom of the carbonyl-stabilized phosphonium ylides was treated as the nucleophilic atom for the carbon dioxide activation. Two probable mechanisms were considered and analyzed by the energetic span model. The kinetic behavior of the carbonyl-stabilized phosphonium ylides in the carbon dioxide or ethylene oxide activation was justified by the molecular electrostatic potential (ESP) analysis at the nuclear position. However, it was confirmed that the activation strain model (ASM) was a more efficient tool in explaining the kinetic behaviors in the carbon dioxide or ethylene oxide activation. A change in the ESP value of the donor–acceptor interacting system (ΔΔVn) and distortion energy at the transition states (ΔEstrain(ζ)) were the outcomes of the ESP and ASM models, respectively, which showed a linear correlation. The electron localization function (ELF) concept was used to justify the kinetic behavior of the second step of the preferred mechanism, revealing that the electron-donating/withdrawing groups substituted on the organocatalysts have a remarkable effect on the electron density of the involved basin at the transition states. On the basis of different analyses, it was proposed that carbonyl-stabilized phosphonium ylides having electron-donating substituents are the best candidates for carbon dioxide conversion to cyclic carbonates.